Java Basics Book
w w w
ZDU Student Manual
JAVA
PROGRAMMING:
PART 1
Java Programming: Part 1
ISBN:
0-73725-349-5
Part number:
ZDU56705
ACKNOWLEDGMENTS
Content Development
The content of this self-study guide is based on the training course "Java Programming," developed by Instruction Set, Inc. for its curriculum of instructor-led technical training.
This guide was designed and developed by an Instruction Set team of instructional designers, course developers, and editors.
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Vice President and General Manager of ZD University: Ed Passarella
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Director, ZD University: George Kane
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CONTENTS
Java Programming: Part 1
LESSON 1: FUNDAMENTAL ASPECTS OF PROGRAMMING
Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
What Is Programming? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
A Brief Overview of Software Development . . . . . . . . . . . . . . . . .3
Conceptual Differences Between Software and Hardware. . . . . . .4
Conceptual Differences Between the Developer and the User . . . .4
Programming Languages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
Syntax vs. Semantics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
Types of Programming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Console Versus Windows-Based Applications . . . . . . . . . . . . . . .6
Procedural Programming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
Object-Oriented Programming . . . . . . . . . . . . . . . . . . . . . . . . . . .7
Event-Driven Programming . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
C/C++/Java Language Family . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
The Common Language Core . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
Identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Data Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Expressions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Input and Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Program Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
Control Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16
Scoping. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
Lesson Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
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JAVA PROGRAMMING: PART 1
LESSON 2: THE JAVA ENVIRONMENT
Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Uses for Java . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29
The World Wide Web (WWW) . . . . . . . . . . . . . . . . . . . . . . . . .30
HTML . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30
Competing Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31
Java Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Java Portability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33
Advantages and Disadvantages. . . . . . . . . . . . . . . . . . . . . . . . . . .34
Applications vs. Applets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34
Application/Applet Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Writing Source Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35
Compiling an Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37
Running the Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38
Tools and Packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
The javadoc and jdb Tools . . . . . . . . . . . . . . . . . . . . . . . . . .41
Using packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Searching for Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42
The import Statement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43
Packages, Classes, Files, Directories. . . . . . . . . . . . . . . . . . . . . . .44
Lesson Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
LESSON 3: JAVA BASICS
Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
Language Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50
Data Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .51
Declaring Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .53
Literals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54
Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
Java Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .56
Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .57
Precedence and Associativity . . . . . . . . . . . . . . . . . . . . . . . . . . . .61
Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
Control Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .65
CONTENTS
Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
Lesson Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
LESSON 4: CLASSES IN JAVA
Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
Java Is Object-Oriented . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
Classes, Objects and Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
Instantiating a Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
Class-Type Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
Operations on Class-Type Variables. . . . . . . . . . . . . . . . . . . . . . . . . . 81
The null Value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
Member Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
Invoking a Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
Class Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
Declaring Instance Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
Declaring Instance Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
The this Variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
Class Definitions and Source Files. . . . . . . . . . . . . . . . . . . . . . . . 89
Lesson Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
LESSON 5: CLASSES IN JAVA—II
Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
Method Overloading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
Constructors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
Encapsulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
Access Specifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
Comparing Objects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
Class Variables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
Class Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 final Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
Class Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
Finalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
Lesson Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
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JAVA PROGRAMMING: PART 1
Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
LESSON 6: ARRAYS AND STRINGS
Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
Java Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
Array Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
Using Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
Copying Array Elements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
String Objects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
String Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
String Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .110
String Searching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .112
Other String Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .113
String Concatenation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
Converting Objects to Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
Converting Strings to Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
Lesson Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
LESSON 7: INHERITANCE
Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
Introduction to Inheritance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
Example of Inheritance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .121
Derivation Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .122
Effects of Inheritance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .122
Protected Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
Overriding Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
Dynamic Method Dispatching. . . . . . . . . . . . . . . . . . . . . . . . . .125
Polymorphism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .126
The super Keyword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .127
Final Methods and Final Classes . . . . . . . . . . . . . . . . . . . .128
Constructor Chaining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
Inheritance and Finalization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
Abstract Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
Interfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
CONTENTS
The implements Declaration . . . . . . . . . . . . . . . . . . . . . . . . 133
Casting Between Class Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
Lesson Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
LESSON 8: WRITING JAVA APPLETS
Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
What Is an Applet? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
A "Hello, World" Applet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
The Applet Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
Invoking an Applet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
Getting Applet Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
Specifying Applet Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 146
The Delegation Event Model—Action Events . . . . . . . . . . . . . . . . 146
The ActionEvent Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
Adjustment Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
The paint() Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
The Graphics Class. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
Java Fonts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
Selecting a Font . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
Drawing Lines and Shapes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
Drawing with Color . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
The Color Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
Foreground and Background Colors . . . . . . . . . . . . . . . . . . . . . 157
Lesson Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
APPENDIX A: HYPERTEXT MARKUP LANGUAGE (HTML)
HTML History and SGML . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Head Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Formatting: Blocks and Separators . . . . . . . . . . . . . . . . . . . . . . . . . .
Formatting: Physical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Netscape List Extensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
164
165
165
166
166
167
167
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JAVA PROGRAMMING: PART 1
Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
Images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
Tables (HTML 3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
Miscellaneous Netscape Extensions. . . . . . . . . . . . . . . . . . . . . . . . . . 170
Java Applets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
Java Script . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
APPENDIX B: JAVA SAMPLE
Usage and Copyright Notification . . . . . . . . . . . . . . . . . . . . . . . . . . . 174
The XYZApp.java Source. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
APPENDIX C: JAVA CLASS HIERARCHY
Java Class Hierarchy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
ANSWERS: TO REVIEW QUESTIONS
Lesson 2
Lesson 3
Lesson 4
Lesson 5
Lesson 6
Lesson 7
Lesson 8
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w w w
LESSON 1
Fundamental Aspects of
Programming
OVERVIEW
Learning to use a programming language effectively requires a good background in the principles of software development. The novice programmer must visualize the software development process as a whole, from requirements analysis through testing. The various types of programming and their implications must be understood clearly. To facilitate this, the several basic components of programming and programming languages are explicated in this lesson by applying them to an incrementally developed software example.
LESSON TOPICS q What Is Programming? q Types of Programming q The Common Language Core q Program Structure
2
JAVA PROGRAMMING: PART 1
®®®
OBJECTIVES
By the end of this lesson, you should be able to:
® Explain the nature of software development.
® Differentiate between the different types of programming approaches.
® Understand the similarities and differences between C, C++ and Java.
® Edit, compile, and run simple console based applications.
®®®
WHAT IS PROGRAMMING?
Before tackling Java programming, it is important to understand what programming is and how it is used. People program to create software, which can take many forms such as spreadsheet applications, Web applets, operating systems or corporate transaction databases.
Software consists of a series of instructions that tells hardware (or another piece of software) to perform one or more actions. The programming language is the set of syntax used to create those instructions and the rules associated with their usage.
Like other natural languages, programming languages allow for some freedom and creativity in how a piece of software achieves its purpose. However, unlike in spoken languages, programming must rigorously adhere to the rules and constructs of the programming language to achieve precise results.
Machines cannot guess at meaning like humans can. Therefore, a good programmer must be very good at taking a problem or an idea in real world human terms, and abstracting it into the constructs provided by the programming language.
Programming generally refers to the coding portion of software development.
Good software development, however, requires several other steps in addition to coding.
LESSON 1: Fundamental Aspects of Programming
3
To develop an application, the developer normally follows a series of steps such as the following:
® Requirement Analysis
® Design
® Coding
® Testing
Requirement analysis is the process of determining the requirements of the application. Typical application requirements might be, “to support a spreadsheet-like interface” or “to calculate the precision of a result to 5 significant decimal places.”
The requirement analysis phase is perhaps the most important step in the development process. Application requirements need to be clearly understood and articulated if the software is to be a success. Proper requirements analysis will help avoid unnecessary corrective effort later in the process.
The design phase may occur at a high or low level. This step is responsible for the overall structure of the application and how the parts of the application interact. Coding is the process of converting a design into a series of instructions through a programming language. Source code is written and then put through a code compiler, which translates the code in machine level instructions.
Before putting code into use, it needs to be tested to locate and remove errors. Testing checks the application against the requirements, tests the application functions with sample data, and verifies whether the user will find the application easy-to-use, among other steps. When the application behaves abnormally, corrections are made to the software and the testing process resumes. When testing is complete the application is ready to use.
LESSON 1
A Brief Overview of Software
Development
4
JAVA PROGRAMMING: PART 1
Conceptual Differences Between
Software and Hardware
Software is a series of instructions to a machine. The computer takes the instructions and executes them to produce the intended results. Software can be readily changed. It can be modified to make changes in the behavior of the application. These changes are changes in the application itself.
The hardware that makes up the computer does not readily change. At the heart of the computer is a central processing unit that executes machine level instructions. These instructions are very low level and are not easy for the average programmer to use. The actual set of instructions for any language is fixed. However, they are recombined in different sequences that result in different behavior for different programs.
Conceptual Differences Between the
Developer and the User
The end user is the individual that executes an application for the purpose of performing a task. The developer is the individual that creates the application.
There are different types of developers. Some developers specialize in the requirement analysis or design phase of a project. There are others who primarily code and test an application. For small to medium size projects, one developer or a small group of developers may perform all of these tasks.
In addition, there are different types of applications that require different types of development expertise. Database intensive applications will require the skills of a developer proficient in the architecture and mechanics of databases, usually a specific database such as Oracle. Other developers will specialize in real-time applications that require carefully timed responses to system inputs.
Programming Languages
A variety of different programming languages have evolved over time. Each of these languages has its own unique syntax and semantics. Languages are often categorized as either high level or low level, though this simple classification is not always adequate.
LESSON 1: Fundamental Aspects of Programming
5
Certain high-level languages are specialized. For example, there are languages designed to work directly with databases. Others are targeted towards developing graphics applications.
Low-level languages have a one-to-one correspondence between a statement in the language and a machine instruction. These languages are normally called assembly languages and are unique to a specific computer. They provide a great deal of control over what can be done by a program but are very difficult and tedious to use.
Some languages rely upon an interpreter instead of a compiler. The interpreter is an application that takes high level instructions for a given language and simulates the execution of the instructions. The interpreted application will produce the same results as if it were compiled. Interpreted code executes slower than a compiled version of the application but interpreters provide several advantages over compilers, including better portability and more powerful development features. Java is an example of a language that is frequently interpreted rather than compiled. Any language may be compiled or interpreted if a compiler or interpreter has been written for that language.
Syntax vs. Semantics
There are syntactic rules for each language that determine whether a given command is in the proper form. Like rules that govern the form of an
English sentence, syntax rules of a programming language determine whether the program is valid or invalid.
When a program violates the syntax rules of the language, the compiler will generate an error message and will usually stop the compilation process. If no errors are generated, the program is said to have compiled cleanly. The program will run, but that does not necessarily mean that the program will do what it is intended to do. The program must order instructions in such a
LESSON 1
Most developers work with high-level languages such as COBOL, C or
C++, Java, and FORTRAN. High-level languages do not use instructions that the machine understands directly; instead, they utilize commands that are more intelligible for humans. Naturally, high-level languages are easier to use. Few high-level language instructions are required to perform the same task in a low-level language. While programmers using a high-level language lose direct control of instructions to the machine, this is not a concern for most developers.
6
JAVA PROGRAMMING: PART 1
way that they produce the correct result. In other words, the program must be semantically correct.
The semantics of an instruction is the meaning of the instruction i.e., what the instruction does. Understanding this is fundamental to learning programming languages. Fortunately, this is normally a straight-forward process. High-level programming languages are designed with logical and intuitive semantic structures.
®®®
TYPES OF PROGRAMMING
There are several types of programming. This section, by no means exhaustive, will discuss some of the key types:
® Console-based
® Windows-based
® Procedural
® Object-oriented
® Event-driven
Console Versus Windows-Based
Applications
Console based applications use an MS-DOS type user interface (often called character driven) for input and output. The application is limited to character-based displays of information. Frequently, the order of data entry is dictated by the application. There are rarely windows of any nature present in these.
Windows programming use extensive amount of graphics programming to provide a more diverse range of input and output features. This graphics programming may be hidden from the application developer but is supported by various operating system libraries.
Windows programming offers the developer freedom to use different
Graphical User Interface (GUI) controls to display or obtain information from the user. Text Boxes can be used for textual information, lists can be
LESSON 1: Fundamental Aspects of Programming
7
Console-based programming is easier than windows programming and does not use as many system resources. Windows programming is more difficult to learn and is resource intensive. However, given that most people expect windows based applications these days, creating windows is often an application requirement. The demand for windows has spurred significant advances in computer processing power and windows development tools.
Procedural Programming
Procedural programming is used by almost all programming languages. Procedural programming focuses the step-by-step sequence of instructions that are needed to perform a task. This approach has been needed because applications and computers are not capable of automatically deciding the appropriate sequence to use.
Non-procedural programming is characterized by the developer specifying what needs to be done rather than how the task is to be performed. Procedural programming involves instructing the system as to how to achieve a task. While non-procedural languages exist, these languages are normally limited to a small application domain and are limited in the range of tasks that can perform.
Procedural programming is tricky for large applications. Large procedural programs can become unwieldy and difficult to manage. There is often little opportunity for abstraction. By abstracting a problem, it can be simplified.
The interfaces between parts of an application can become difficult to manage also. Programmers tend to spend a lot of time learning how to program to these interfaces.
Most commonly used languages are procedural, including C, C++, Java and
FORTRAN. These languages have evolved over time but they all have procedural aspects to them.
Object-Oriented Programming
The evolution of object oriented programming brought about a paradigm shift in how application problem sets are modeled for development. In a traditional procedural language like C, things represented in a banking application like account number, customer, and balance dissolve into a series of
LESSON 1
displayed in List Boxes, and mutually exclusive options can be provided through Radio Buttons.
8
JAVA PROGRAMMING: PART 1
functions and variables. In object-oriented programming, objects are defined and maintained that represent these real things that are part of a problem statement. Just as things in real life have characteristics and actions associated with them, objects possess properties, methods, and events. Each of these is directly associated with the object. For example, a bank account object would have the properties of account number and balance. These properties are tied directly to an instance of Bank Account, and do not exist in the program, other than as a part of Bank Account.
Methods are actions performed against the object. For example, the bank account object may have a deposit method that is used to add money to an account. Events are actions that take place under specific conditions. The bank account might have a negative balance event that would occur whenever the balance became negative. The programmer would code this event to take appropriate action when this event occurs.
Object-oriented languages support the definition of a class that represents an object and the actual allocation of memory for the object. Classes are the generalization of a specific object: Customer is a class, and Kelly Smith is an object that is an instance of the class Customer. C++ and Java are examples of object-oriented languages (note that they are also procedural languages).
The coding used in support of methods and events is procedural.
Object-oriented languages offer a number of benefits. Systems modeled in objects are easier to maintain and manipulate, because properties and methods can be altered for a class without affecting the rest of the system.
Object-oriented programming also supports reuse of code: once developed, a class can be utilized as often as necessary. For instance, a programmer could use an existing Button c lass in an application without necessarily understanding all of the code that supports the class. In addition, classes may inherit properties from more general classes, so that the properties do not have to be redefined for each sub class.
An object-based language, as opposed to an object-oriented one, is a language that supports the creation of classes but does not support inheritance, a crucial programming feature which facilitates the creation of related types from existing types. Visual Basic 5.0 and its earlier versions are an example of an object-based language.
LESSON 1: Fundamental Aspects of Programming
9
Event driven programming is frequently used to support Windows programming. The operating system will monitor the system for events and then send these events to the appropriate application. The application’s reaction is based on the nature of the event. Keyboard input and mouse movements are the most common events.
An event-driven application is structured around the process of receiving and reacting to events. Unlike in procedural programming, where the programmer dictates the order of information received, event-driven applications generally allow the user to choose the order in which information is entered. For example, an application may have an entry field for Zip Code. In an event driven application, the Zip Code field will be one in a series of address fields in a dialog box. The user can use the mouse or tab key to jump around to different fields and fill them in any order. The application’s structure must account for the fact that it can not anticipate when Zip Code will be filled, but rather must react when the event occurs.
Object-oriented programming is well suited for event driven applications.
Objects can have events assigned to them, which the application can monitor. Traditional procedural languages such as C, do not support event driven programming very well. Java, in particular, is designed to address events explicitly and has a well defined event model to create and handle events.
C/C++/Java Language Family
C was developed to meet general system programming needs. It was quickly adapted for general use and found widespread acceptance. C is a high-level procedural language that has many low-level features. These features help to make it versatile and efficient. However, many of these features give it a reputation for being cryptic and hard to maintain.
C++ extends the C language to provide object-oriented features. The language is backward compatible with C, and code from the two languages can be used with each other with little difficulty. C++ has found quick acceptance and is supported by a number of pre-built specialized classes.
Java can be considered the third generation of the C/C++ family. It is not backward compatible with C/C++ but was designed to be very similar to these languages. The creators of Java intentionally left out some of the fea-
LESSON 1
Event-Driven Programming
10
JAVA PROGRAMMING: PART 1
tures of C/C++ that have been problematic for programmers. Java is strongly object-oriented. In fact, one cannot create Java code that is not object-oriented. Java’s portability is a key advantage and is the reason why Java is often used for Web development.
®®®
THE COMMON LANGUAGE CORE
There are several programming features common to most programming languages. These include the definition of identifiers, expressions, and condition control statements. These common core features are discussed and illustrated below with sample code where appropriate.
Comments
Comments are sections of a program that are ignored by the compiler and are used to document the program. Documentation is important in that it provides information about the program that is useful in the maintenance of the program.
Software applications generally require periodic revision—the application requirements change, errors are corrected or new features are added. Good documentation aids the programmer in identifying the structure and purpose of each section of code. Often, the person modifying the code is not the same person that wrote the code, making the use of good comments critical.
The only comment that is common to C, C++ and Java is the multi-line comment. The comment begins with a /* and ends with a */. Everything between these two sets of symbols is treated as a comment and is ignored by the compiler.
/* This is a comment spread across several lines */
/* This comment occurs on a single line */ i = i + 1; /* This comment begins on the same line as a statement */
LESSON 1: Fundamental Aspects of Programming
11
i = i + 1;
// Alternate comment notation
Identifiers
Identifiers are used to temporarily hold information. For example, to compute the pay for an individual, the numbers of hours worked is multiplied by the pay rate. Identifiers (the name of the variable listed on the left) are needed for these values. hours = 40; payrate = 12.50; pay = hours * payrate;
Identifiers have certain rules for their creation. Generally, the identifier must begin with a letter and is followed by letters, digits, and perhaps another character, such as the underscore character.
The assignment operator, which is the equal sign (=), is used to modify the value assigned to an identifier. Simply using the identifier as part of some operation will retrieve its current value. The value stored in an identifier can change. Data Types
There are different types of identifiers based on different needs. If simple counting numbers are needed then there is the integer data type. If a number with decimal points is required then the float data type is used. The common data types are listed in Table A.
TABLE A. Data Types
Data Type
Value
Examples
int
Positive and negative integers
127
-6
float
Single precision numbers
6.42
LESSON 1
An alternative style available in C++ and Java uses two forward slashes (//) to indicate that everything from that point until the end of the line is a comment.
12
JAVA PROGRAMMING: PART 1
TABLE A. Data Types (continued)
Data Type
Value
Examples
double
Double precision numbers
(numbers with a very large value or # of digits)
137
.5426246321
5.42 x 1045
char
Single characters
h
char* or String
String data (any series of characters) widget
42XXTALL
Note that very large numbers or very precise numbers use a different data type than normal real numbers. The boundary between single and double precision numbers is defined differently for each language.
String data is handled differently between C/C++ and Java. C and C++ use a pointer to the address of a string of characters that is terminated by a zero.
Java uses a built-in data type String to hold strings.
Before an identifier can be used, it must be declared. Specifying the type of data followed by the identifier’s name does this: int float int double
i; pay; hours; radius; Most languages provide the developer with the ability to define more complex data structures based on primitive data types. C and C++ do this using the struct statement. struct Employee { char*name; int age; double payrate;
};
An actual identifier is declared as other primitive types are declared. struct Employee boss;
LESSON 1: Fundamental Aspects of Programming
13
Expressions are needed to perform computations. An expression consists of an operator and one or more operands. Operators specify the action to be performed, such as addition or multiplication. Operands are the data acted upon. Statements are separated by semicolons and consist of one or more expressions. In the following example, the assignment operator, equal sign, and the addition operator are used to add two numbers together and assign the sum to a third identifier. a = b + c;
There is a predefined order in which expressions are evaluated. This is known as the order of precedence. For example, addition always occurs before assignment (+ happens before =). There also exists rules of associativity that specify the order in which two expressions of equal precedence are to be evaluated. These rules can be overridden through the use of parentheses. For addition, evaluation is done left to right. The following two statements are equivalent: a = b + c + d;
a = b + (c + d);
Input and Output
Getting information into a program and then writing it out are important aspects of programming. Data can be read from a number of sources: keyboard, mouse, files, databases, etc. Data can be outputted to the monitor, to various files, and to a database, among other places.
The syntax of Input/Output (I/O) is mostly language specific. C and C++ rely upon include files that contain specialized code to perform I/O. Java uses a series of classes to do similar I/O actions. Input statements are not addressed here, however, output examples are provided.
LESSON 1
Expressions
14
JAVA PROGRAMMING: PART 1
In C, the printf statement is used. It has two major parts: a format specification that is enclosed with double quotes and a list of data values to be printed. The following shows printing out pay information: printf(“Hours worked: %f Payrate: %f hours, payrate, pay);
Pay: %f\n”,
The %f fields within the format string are “place holders” for the output values to be printed. The text printed will be that which is inside of the double quotes, with the %f field replaced by the values stored in the variables in the parameter list. The three %f fields are replaced with the values stored in hours, payrate and pay. The \n is used to cause a carriage return and a new line to be printed. Output goes to standard output, which is normally the monitor. For floating values, a %f file is used. For other data types, different place holders are used.
In C++, insertion operators (>
integer
L
right shift with zero extension
=
arithmetic
L
less than, greater than, or equal to
instanceof
object, type
L
type comparison
==, !=
native
L
have same or different values
==, !=
object
L
refer to the same or different object
7
&
integer or boolean
L
bitwise AND or boolean AND
8
^
integer or boolean
L
bitwise XOR or boolean XOR
9
|
integer or boolean
L
bitwise OR or boolean OR
10
&&
boolean
L
conditional boolean AND
11
||
boolean
L
conditional boolean OR
12
?:
boolean, any, any
R
conditional operator (ternary)
13
=
variable, any
R
assignment
*=, /=, %=,
+=, -=, =, >>>=,
&=, ^=, |=
variable, any
R
assignment after operation
4
5
6
Associativity
When two operators have the same precedence, associativity determines evaluation order.
For example, the multiplicative operators (*, /) have the same precedence and associate left to right. Therefore, the example below has the value 9, not
1.
6/2*3
LESSON 3
TABLE D. Operator Precedence
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JAVA PROGRAMMING: PART 1
For the assignment operators, right to left association is more appropriate.
The two examples below are equivalent. a=b=3 a=(b=3)
This means, “set b’s value to 3; copy its value to a.”
Assignment: This is an operator with right to left associativity which changes the left-hand argument’s value and produces a value for the next operation. The above example assigns 3 into b and produces the value of 3.The next assignment operator assigns 3 into a. This also produces a value of 3 (which is thrown away).
This reverse associativity makes it easy to assign several variables at once: a=b=c=d=e=0 ®®®
STATEMENTS
Java, like C, is an expression-based language. Expressions must always appear in the context of statements. A statement consists of an expression followed by a terminating semicolon (;).
Like in C, the Java format of text on a line has no significance to the compiler except for:
® C++ style comments which end at the end of a line
® quoted string literals (e.g., “Hello World!”) which cannot span more than one line
® white space that the compiler recognizes as the separator between keywords, identifier names, operators, and other symbols
Statements can be made up of zero or more expressions, provided that their combination makes syntactic sense. Expressions (except for the return value
LESSON 3: Java Basics
65
The following are expressions:
®a
®i * j
®z = x * y
Adding semicolons to these expressions turns them into statements:
® a;
® i * j;
® z = x * y;
The first two are not legal and return errors with the 1.0.2 Java compiler.
The third calculates a product and stores it into z.
Control Structures
Flow control in Java uses similar syntax as in C. The syntax for the most commonly used control structures may be summarized as follows:
® if
• if ( boolean-expression ) statement
• if ( boolean-expression ) statement else statement if (i > 0) x = y * z; else // else part is optional x = 0;
LESSON 3
of void methods) have a value and a type. Many involve executable code, though this is not necessary. A singe value (e.g., 27) is a legal expression. A terminating semicolon (e.g., 27;) turns it into a legal statement even though it does not do anything useful. A simple statement consists of an expression and a semicolon. The semicolon is a required terminator.
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JAVA PROGRAMMING: PART 1
® while
NOTE
The difference between while and do-while refers to when the condition check is done— before or after executing the loop.
while (boolean-expression ) statement
while (getMember (i)) i++; do statement while (boolean-expression)
do a += a; while (a > ?
75
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JAVA PROGRAMMING: PART 1
EXERCISE
1. Go to the exercise subdirectory of the Java directory. Edit the file
DivTest.java. In the area provided, insert Java code that prints one of the following two messages:
® x is evenly divisible by y
® x is not evenly divisible by y depending on whether the value of variable y evenly divides the value of variable x in the DivTest class.
Modify your program to print the values of x and y instead of “x” and
“y”. Test your program by setting various values for x and y, recompiling and running your program. What happens if you set y to zero and run?
2. Still in the exercise subdirectory, look for a source file named Digits.java. Edit the file, which contains the beginnings of a program to take the value of the variable x and print an English word for each of its digits. For example, if the value of x is 496, the program prints: four nine six
Finish the program. (Hint: use a switch statement to translate a digit to a string.) Test your program. Make sure that it works for a value of zero. w w w
LESSON 4
Classes in Java
OVERVIEW
A class is a user defined type created by grouping together pertinent methods and data members. The class is a key component of Java language, as it facilitates modularity and code reuse. Once created, a new type can be used in expressions in the same way as a built-in type (such as int or char). While Java classes are, on the whole, similar to C++ classes, there are still small but crucial differences.
LESSON TOPICS q Java Is Object-Oriented q Instantiating a Class q Class-Type Variables q Operations on Class-Type Variables q The null Value q Member Access q Class Definitions
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JAVA PROGRAMMING: PART 1
®®®
OBJECTIVES
By the end of this lesson, you should be able to:
® Work with Java class instances.
® Declare and initialize instance variables.
® Access data members and methods of an instance.
®®®
JAVA IS OBJECT-ORIENTED
Java is an object-oriented (OO) programming language. A Java program may be thought of as a system of interacting objects in which each object has its own state and behavior. The form and behavior of each object is defined by the object’s class. A class is a data type and a code module. Designing a
Java program means designing the classes that make up the program.
Object-oriented program design builds from the bottom up as much as it does from the top down. Objects are constructed from other objects, and they may, in turn, become the components of larger objects. The developer abstracts the behavior of similar objects into class definitions. Similar classes are further abstracted into superclasses, resulting in hierarchies of related classes. Viewed from the opposite perspective, the programmer defines classes that are highly abstract, derives more specialized classes from them, then creates the actual objects as instances of those classes.
Object-oriented programming results in programs whose structures more accurately map to the problem domain and are thus are more likely to meet users’ needs. The object model carries modularity to a much higher level of abstraction than the structured model. The cross linking that makes both development and maintenance a painful exercise in tracing subtle sideeffects is greatly reduced. This modularity, together with the more flexible structure of OO programs, makes software easier to develop, debug, enhance, and modify.
The generalization of highly modular objects into classes and classes into families of related types also permits much greater reuse of code than has been achieved hitherto by way of function libraries, database systems, and other traditional means. Reusability combines with streamlined development and maintenance to make programmers far more productive.
LESSON 4: Classes in Java
79
A Java class definition establishes the interface and implementation of a category of objects.
Using a syntax rooted in that of C structures, the programmer defines data members and function members of a class. Data members specify the internal data representation. Function members serve two purposes: their headers
(names, parameters, and return types) specify the interface visible to code outside the class, and their bodies specify the methods by which objects of the class perform the work they are designed to do.
User code takes advantage of a class specification by instantiating the class,
i.e., by creating instances of it. A variable of a programmer defined type is declared with the same syntax that is used to define variables of built-in types. Each instance of a class is an object. The object’s data members can take on values unique to that object, while its member functions implement the behavior common to all objects of the same class. There can be any number of instances of a given class at any one time.
A Java class-type variable identifies an instance. It does not contain an instance, as is the case in some other object-oriented programming languages. There can be any number of variables referring to a given object at any one time.
®®®
WARNING
Unlike C++, a declaration like
Box myBox; does not create an instance of the Box class. The instance is created only after new is called on Box.
INSTANTIATING A CLASS
With the exception of array objects, class-type objects may be instantiated only through the new operator. The keyword new must be followed by the name of the class to be instantiated, and then by a list of initialization parameters. The new operation allocates space for a new object, initializes the new object, and returns it. For example, the creation of an instance of
Box (object) is shown in Figure 3:
LESSON 4
Classes, Objects and Variables
80
JAVA PROGRAMMING: PART 1
FIGURE 3
Creation of an instrance of Box
Box mybox = new Box (2.0);
class name
constructor keyword object name
initialization parameter
The new operator allocates space for the new object, calls a class constructor and returns a reference to the object.
It should be noted that the class and its constructor have the same name.
®®®
CLASS-TYPE VARIABLES
Creating the variable looks just like a native-data type definition. However, the variable is simply a reference. It does not actually instantiate the object.
Class-type variables are references, not objects themselves. Instantiation of an object with the keyword new and declaration of a class-type variable are separate operations. These two steps may be combined in one statement as an initialization. For example:
Box myBox = new Box (2.0);
A class-type variable refers to (points to) an instance; it does not contain an instance. There can be any number of variables referring to a given instance at any one time. The difference between a class-type variable and a variable of native type which contains a value of its type is to be noted.
LESSON 4: Classes in Java
OPERATIONS ON CLASS-TYPE
VARIABLES
All class-type variables support the following operations:
® assignment
® ==
® !=
The assignment operator reassigns the object to which the variable refers.
Assignment does not modify the value of any object.
Suppose there is a class named Box:
Box box1;
Box box2 = new Box (2.0, 3.0);
Box box3 = new Box (box2); box1 = box2; box3 = new Box ();
When a class-object variable is used in code, it does not imply the value stored in the variable. Rather, it implies the object variable references. In the example above, box1 and box2 are set to reference the same object. Then box3 is given a brand new object. This is very different from the behavior of native-data type variables. In this sense, a class variable is more like a class pointer from C++ than a class object.
LESSON 4
®®®
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JAVA PROGRAMMING: PART 1
Below is an illustration of reference semantics:
® two variables initialized to refer to two separate objects:
Box box1 = new Box (1.0, 1.0);
Box box2 = new Box (2.0, 2.0);
® assignment: box1 = box2;
® the effect of the assignment:
• box1 and box2 refer to the same object.
• The object box1 referred to previously is available for garbage collection.
• Neither object changed in value.
®®®
THE null VALUE
Another way in which Java class-type variables are similar to C and C++ pointers is that Java class-type variables may be null. A Java class-type variable with the special value null refers to no object. The value null is compatible with any class type, i.e., null may appear wherever a class-type value is expected. Attempting to access a member of a null variable results in a run-time error.
LESSON 4: Classes in Java
NOTE
If radius were a class variable, it could be accessed as
Circle.radius.
Unlike C++, you don't need to use :: after class name in
Java.
MEMBER ACCESS
Once a class has been instantiated, variables for that instance are accessed by the “.” symbol (dot).
For example, suppose that myCircle is a variable of class Circle, and class Circle has a data member named radius of type double; then the following statements are legal: myCircle.radius = 42.6; double rad = myCircle.radius;
Programmers familiar with C have to note that there is no -> operator because object variables are not pointers.
Invoking a Method
Objects are usually thought of as self-contained entities whose characteristics are defined by, among other things, the set of actions they are intended to perform. A central concept of OO programming is that objects pass messages to each other. Objects can be seem as being self-contained and responsive, like living organisms reacting to signals from the world around them.
External code triggers actions from, and gets information about, objects only by sending messages to them and interpreting the responses. A call to an object’s method is a message to that object requesting that it perform some specified action.
A message has three components: a target, a selector, and a parameter list. Java reflects its derivation from C by implementing messages as function calls, using a syntax like that used to access members of structures. The user code references the object (the target), then invokes a function which is a member of that object (the selector), and supplies the arguments that function requires (the parameter list).
LESSON 4
®®®
83
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JAVA PROGRAMMING: PART 1
For example, suppose the object myCircle contains the following methods:
® getArea, which returns a double
® moveTo, which takes two double arguments double area = myCircle.getArea (); myCircle.moveTo (0, 0);
®®®
CLASS DEFINITIONS
Every data item or method in a Java program, even the starting function main(), must be declared as a component of a class.
A class definition consists of the keyword class followed by the class name, then a set of curly brackets {} that designate the body of the class.
Variables and methods declared here are called class members. Class member declarations, and the class itself, can be preceded by an access specifier. Public classes can be instantiated by any other code.
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85
public class Box { double height, width;
// Constructors public Box(double h, double w) { height = h; width = w;
}
public Box(double s) { height = width = s;
}
public double getHeight() { return height;
}
public double getWidth() { return width;
}
public double getArea() { return height * width;
}
}
Figure 4 illustrates what the header communicates.
FIGURE 4
Meaning of headers
public class Box
Any other class can use this class.
This class is named Box.
This is a class type.
The class body (enclosed in {...}) defines the members of the class: variables, or data members, methods, or function members. Members can be
LESSON 4
The following is an example of a Java class definition:
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JAVA PROGRAMMING: PART 1
declared public—they are not public by default. A private member can only be accessed by another member of that class. A public member can be accessed directly from outside the object. The principle of encapsulation infers that data members should be private and that at least some of the function members should be public. The public methods define an object’s interface. Declaring Instance Variables
All variables, unless declared static (discussed later), are replicated for every instance of the class and therefore are referred to as instance variables.
For example, in the class definition for Box, the following code defines two instance variables: double height, width;
Every instance of type Box receives its own height and width.
When a class is instantiated, an object of that type is created; the new instance obtains its own copy of every member variable including the methods. For example, consider the following class: class Color { int r, g, b; public setColor(int red, int grn, int blu) { r = red; g = grn; b = blu;
}
}
If background was the name of an instance of Color, then the color could be defined by calling its setColor method: background.setColor(255, 100, 0);
Note that the color could not be set by assigning values to the data members directly because they are private and not public.
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® background.r = 255;
® background.g = 100;
® background.b = 0;
Declaring Instance Methods
Methods defined inside a class are instance methods by default. An instance method must be applied to an instance of the class of which it is a member.
For example, the class Box defines the methods:
® public double getHeight() { ... }
® public double getWidth() { ... }
® public double getArea() { ... }
These methods can be applied to any instance of the class Box.
Each method is defined fully within the class definition: the body of the method is enclosed within curly braces ({...}).
Methods can refer to instance variables of the target object directly: public double getArea() { return height * width;
}
TRIVIA
This conforms to the concept of encapsulation in object-oriented programming in which class data is held privately within the class and outside code uses functions rather than directly accessing class data.
Suppose we have a variable of type Color named background. We might choose to “blacken” the background color by applying the method setColor() to the instance background, by calling: background.setColor(255, 255, 255);
This applies the method setColor() to the instance background by using the member access symbol “.” (dot). Inside the method, the variables are used by name and they are taken as applying to this instance’s copy of each. Methods like this are called instance methods because they can be applied to individual instances.
LESSON 4
The following statements would all generate compilation errors:
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JAVA PROGRAMMING: PART 1
The this Variable
Java provides a keyword called this which allows an object to refer to itself.
Unlike the this pointer in C++, the Java version is simply another name for the object. For example, if an object has a data member width, then the following statements are equivalent when used within one of the object’s function members: width = w;
this.width = w;
WARNING
If a parameter name is the same as a class member name, an unqualified name will refer to the parameter and not to the class member. this must be used to refer to the class member.
It is a good practice to avoid having the same name for parameters and members. The this keyword is not often needed, but there are times when it is useful. Whenever an instance method must refer to the object of which it is a member (or one of its constructors), it can use this to do so. It can also be used to distinguish between parameter variables and data members having the same name. public class Box { double height, width; public Box(double height, double width) {
// Note that the parameter variables have the same
// name as the data members, so we use the this
// keyword when referring to the data members. this.height = height; this.width = width;
}
public Box(double s) {
// Here we use the this keyword to invoke the
// two-parameter constructor with the single
// parameter,saving us the trouble of
// rewriting the assignment statements. this(s, s);
}
// Other methods...
}
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Each class is defined entirely within its source file. Unlike C and C++, there are no header files in Java. While compiling a class, the Java compiler examines other compiled classes to check references and types across class boundaries.
There can be at most, one public class per Java source file. A Java source file may contain any number of non-public classes, but those classes are limited to use within the same package and are best used as “helper” classes for the main public class in the same source file. If a Java source file contains a public class, the name of the source file must match the name of the class, i.e., the source file must be named .java, where is the name of the public class defined within.
LESSON 4
Class Definitions and Source Files
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JAVA PROGRAMMING: PART 1
®®®
LESSON SUMMARY
In this lesson, you have learned:
® Java classes are user defined types that facilitate code reuse and modularity.
® A user defined type is used in the same way as a built-in type.
® Class members are accessed using the dot (.) operator.
® The this keyword allows an object to refer to itself.
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91
1. How would you declare an object of type Animal named Lion that takes a weight of 500 and length of 45 as parameters?
2. What code would change the weight to 250 and the length to 35 in the above Lion instance? Assume that weight and length are the names of the Animal data members.
3. How many public classes are allowed in a Java source file?
Answers on page 197
LESSON 4
REVIEW QUESTIONS
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JAVA PROGRAMMING: PART 1
EXERCISE
1. Create a new class named Clock. (You may put your new class in any package you like.) Class Clock has the following attributes
• Hour (integer 1..12)
• Minutes (integer 0..59)
• Seconds (integer 0..59)
•
isAM (boolean):
These attributes are represented by data fields with default access protection. Class Clock supports the following public extractor methods:
• getHours()
• getMinutes()
• getSeconds()
• getIsAM()
Class Clock may be constructed in any of the following ways:
• no parameters (sets clock to midnight)
• hour and isAM specified (sets minutes and seconds to zero)
• all attributes specified
Class Clock also supports a setTime() method that sets all four attributes from argument values. Compile the new class.
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LESSON 5
Classes in Java—II
OVERVIEW
Java classes have some additional features that aid program development. For example, class methods may be called with varying numbers and types of parameters by using overloading. Two keywords, static and final, can be used to further adjust the behavior of class members. Another useful mechanism is the finalize method, which performs clean-up work when a class instance is destroyed.
LESSON TOPICS q Method Overloading q Constructors q Encapsulation q Access Specifiers q Comparing Objects q Class Variables q Class Methods q Finalization
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JAVA PROGRAMMING: PART 1
®®®
OBJECTIVES
By the end of this lesson, you should be able to:
® Apply overloading mechanisms.
® Employ the static and final keywords.
® Use initialization syntax.
® Understand finalization methods.
® Write complete Java classes.
®®®
METHOD OVERLOADING
Overloading means declaring a method with the same name, more than once. To distinguish between the different versions, the compiler uses the supplied parameter list. In other words, a method is identified by its signature, made up of both the name and the types of its parameters. The return type of a method is not a part of its signature. An overloaded method must differ from other methods of the same name in number and/or types of arguments: ® public double dothis (Circle cx) {...}
® public void dothis (double x) {...}
® public void dothis () {...}
A method’s return type does not distinguish it: public double dothis (Circle cx) {...} public void dothis (Circle cx) {...}
//
// ERROR: these methods conflict
//
When a call to an overloaded method is made, the compiler must perform overload resolution to determine which method is being called. The compiler considers all the methods of the same name and number of arguments
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®®®
WARNING
While there may be multiple constructors for a single class, they must differ somehow in the number or types of arguments. There cannot be two constructors for one class with exactly the same number and types of arguments.
CONSTRUCTORS
A constructor is a special method that initializes a newly instantiated object.
Constructors have the same name as the class and no return type, not even a void. They can take any number of arguments. They are called whenever a class is instantiated, and are therefore used to initialize that instance. Constructors can be overloaded, so a class programmer can offer various ways for a class user to instantiate and initialize the class.
The following example gives the Box class two constructors: public class Box { double height, width; public Box(double hw) { height = width = hw;
}
public Box(double h, double w){ height = h; width = w;
}
...
}
WARNING
A different constructor from within one constructor may not be called with the usual new method, since that would create a brand new object instead of calling a different constructor as part of building the current object. If a class does not provide any constructor explicitly, the compiler generates one that uses no parameters and is sometimes called the default constructor. If a class provides any constructor, the compiler does not provide this noparameter version.
A constructor can use the keyword this() as a method to call a different overload of that class’s constructor. This can help avoid redundant coding: public Box(double s) { this(s, s);
}
Instance variables are already initialized by the time the constructor executes.
The constructor need only perform additional initialization.
LESSON 5
as the call, compares the actual types of arguments of the call with the formal argument types of the methods, and chooses the appropriate method.
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A new operation always calls a constructor to fully initialize a new object.
Remember that the choice of constructor to call depends on normal overload resolution rules based on the initialization parameters:
Box b = new Box (1.0, 2.0);
// calls: Box (double h, double w)
®®®
ENCAPSULATION
Encapsulation is combining of behavior and data into the discrete program units called objects. The programmer hides the data behind a specific interface (usually a purely functional one) that controls access to the data. Encapsulation prevents external code from modifying the data directly avoiding the possibility of unwanted side effects.
By hiding the underlying implementation details, the programmer hides the way in which an object is represented and manipulated from the rest of the program. In this respect, encapsulation contributes to program modularity, as it groups code and data components in discrete, self-contained, and selfprotecting objects. By defining an interface made up only of methods, the programmer provides a clean interface to the object and limits the actions that can be performed on it.
A well-defined, fully encapsulated object provides a clear, comprehensive interface, and takes care of its own security. External code cannot access its internals—and has no reason to do so.
®®®
ACCESS SPECIFIERS
Classes can be declared public. A public class is accessible by any other
Java class. A class that is not declared public has package access, which means that only classes within the same package may access it.
Class members can be declared public or private to enforce proper encapsulation. Certain data elements or methods that are necessary for the class’s internal behavior should be protected from access by outside classes.
These should be declared private. Class members labelled public are accessible to all other classes; members labelled private are accessible only to the code of the class itself. There is also a protected access speci-
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®®®
COMPARING OBJECTS
Through the mechanism of inheritance (discussed in a later lesson), every
Java object has a method called equals() that takes another object as its argument and returns true if the target object is equal to the argument object. This sort of equality is not necessarily the same as the one implemented by the == operator, which returns true if and only if its operands refer to exactly the same object. Types such as String, in which equality depends on the value of an object and not simply the object’s identity, redefine the equals() method to compare values instead of references.
To compare the values of two objects, use the equals() method: if (r2.equals (r1)) {
...
}
The behavior of equals() depends on the class. The default behavior is to compare object identities just as the == operator does. Where appropriate, it compares values.
®®®
CLASS VARIABLES
Normally, variables declared in a class are instance variables, which appear once per instance of the class. It is sometimes useful to define variables that, like global variables in other programming languages, are unique and persist throughout the lifetime of the program.
A Java class can have data members that appear once per class, rather than once per instance, and persist for the duration of the program. These members are called class variables.
LESSON 5
fier that will be discussed in the lesson on inheritance. A member with no declared access specifier has package access.
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NOTE
This is similar to the use of static for class members in
C++.
To declare a variable of this sort, add the modifier static to a class variable declaration: public class Box { double height, width; static int BoxCount = 0; public Box(double h, double w) { height = h; width = w;
BoxCount += 1;
}
...
}
Within the class, a class variable may be referred to by just its name and no qualifier. Outside the class, the variable name must be qualified by either an instance of the class or the name of the class. Qualifying a class variable by an instance of the class is identical to accessing an instance data member.
Since class variables are associated with a whole class and not any particular instance, accessing a class variable does not require an instance of the class.
A class variable may be qualified by applying the dot operator to the class name, as if the class name were a class instance.
Class Initialization
TIP
Static initialization blocks can be used to assign the results of a complex computation to be the initial value of a certain static variable. You are not limited to using constant values for initialization.
Class variables may be initialized in the class definition. The required syntax is identical to initialization of instance variables. Any variable not explicitly initialized receives the default value for its type.
The initialization takes place when the interpreter first loads the class. Additionally, a class may specify arbitrary initialization code to be run when the class is loaded. This is accomplished by using a static initialization block. The syntax for defining a static initialization block is the keyword static followed by the body of a method. A class may have more than one static initialization block. When a class is loaded, all its static initialization blocks and static variable initializations execute in the order in which they appear in the source file.
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NOTE
Java does not have any preprocessor and thus does not support constants in the sense of C’s
#define directive.
public class Box { static int num_of_Boxs = 0; static float peculiar; static // a static initializer
{
peculiar = 2.7f + 15;
}
...
}
final Variables
A class-member variable can be declared with the modifier final meaning that its value cannot be changed in other parts of the code once the variable is initialized. Java has reserved C’s keyword const though it currently has no meaning.
TRIVIA
Java designers reserved the const keyword to avoid potential confusion from C/C++ programmers inadvertently using const in Java.
A class variable declared final is a class-wide constant. Constants are often declared public to make them available program-wide. class Trig { public static final float PI = 3.14159;
... more stuff ...
}
float zz; zz = 2.0f * Trig.PI;
LESSON 5
The following is an example of static initializer blocks:
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Variables declared static and final are true program constants:
WARNING
If a variable is declared final but not static, it could have different values in different instances of the class. This can be confusing if the programmer expects this variable to be a true constant. It seldom makes sense to use final without static. ®®®
WARNING
If a static method tries to access an instance variable, there will be a compiler error.
// create constant suit public class Suit { public final static public final static public final static public final static
}
values int int int int
DIAMONDS = 0;
CLUBS = 1;
HEARTS = 2;
SPADES = 3;
// create a mathematical constant public class Trig { public static final float PI=3.1416f;
}
CLASS METHODS
As discussed in the preceding section, class variables (as opposed to instance variables) exist before any instances are created. Sometimes the class also needs methods that refer to those variables, and those methods need to be callable by outside code before any instances have been created. Such methods can be declared with the static modifier and are called class methods, because they apply to class variables and not to instances of the class. The following code gives an example of a class method: public class Thing { static int value = 3; public static void resetValue (int val) { value = val;
}
...
}
Outside of a class, class methods can be called either by class name or through an instance:
Thing.resetValue(9);
Thing th = new Thing(6); th.resetValue(9); LESSON 5: Classes in Java—II
101
It is a common practice to define classes that contain only static members.
Since such a class has no instance data or methods, instantiating it is useless.
Such a class is nothing but a wrapper for a related set of global methods and/ or data—a wrapper made necessary by the absence of true globals in Java.
®®®
NOTE
Notice this important distinction in how constructors are declared and finalized. Constructors have no return type, but finalize must have void as a return type.
Unlike constructors, finalize has only one option for arguments—no arguments.
FINALIZATION
When class objects are created, the appropriate constructor is called. What happens when objects go away? In C++, classes can implement a destructor that releases memory when the object is destroyed. Unlike C++, Java does not require a destructor, because memory is not directly allocatable. However, class objects may still create conditions that must be manually undone, such as closing files opened for I/O.
In Java, classes may define a method called finalize(). If finalize() exists for a class, it is called just before an instance of that class is destroyed. The compiler generates a default finalize() method if a class does not provide one. Note that the exact time that finalize() is called can never be predicted, simply because the exact time of garbage collection (i.e., actual destruction of objects) cannot be predicted. Under some circumstances, such as when the interpreter exits at the end of a program, it is possible that finalize() might never be called. Thus, the programmer of a class should never rely on finalize() to do critical tasks. finalize() is only useful to free system resources, such as open files and network connections. A finalizer method is declared by naming it finalize()with no parameters and a void return value.
LESSON 5
Class methods can access class variables, but cannot access instance variables.
Because class methods may be called with no instances available, they are not given a this reference like instance methods.
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®®®
LESSON SUMMARY
In this lesson, you have learned:
® Overloading allows a method to have more than one signature.
® Variables declared static apply to the whole class not to specific instances. ® Variables declared final cannot be changed after they are initialized.
® Finalization carries out various tasks when a class instance is no longer needed. LESSON 5: Classes in Java—II
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1. If a class is not declared public, then who can instantiate the class?
2. What is the keyword that declares a variable to be constant?
3. What is the keyword that declares a method to be a class method rather than an instance method?
Answers on page 197
LESSON 5
REVIEW QUESTIONS
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JAVA PROGRAMMING: PART 1
EXERCISE
1. Expand the Clock c lass to support a tick() method. The tick() method increments the current time by one second. Add an empty main() method to class Clock, so that you can run the class (see
FirstApp.java for an example of a runnable main() method).
Write test code that instantiates and uses class Clock in main().
Test the clock’s transitions from hour to hour, minute to minute, AM to
PM.
Compile and run the test.
2. Create a class called MathTechniques that supports the following constant and methods:
• pi - 3.14159
• areaOfRectangle that is passed a height and a width
• perimeterOfRectangle that is passed a height and a width
• areaOfCircle that is passed a radius
• perimeterOfCircle that is radius
3. Create a separate class to test out the MathTechniques class.
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LESSON 6
Arrays and Strings
OVERVIEW
The array is a useful construct that stores an indexed collection of identical objects. Perhaps the most useful application of arrays in Java is the character string. A string is an object and may be readily assigned, copied, and accessed. A string cannot, however, be modified.
LESSON TOPICS q Java Arrays q Array Constants q Using Arrays q Copying Array Elements q String Objects q String Methods q String Concatenation q Converting Objects to Strings q Converting Strings to Numbers
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®®®
OBJECTIVES
By the end of this lesson, you should be able to:
® Work with Java arrays.
® Work with Java string objects.
®®®
NOTE
Arrays are neither a true class (there is no class called Array), nor a built-in type
(Arrays have members like a class).
Arrays in Java are in between a class and a data type although they are implemented as a class internally by Java.
JAVA ARRAYS
Arrays are ordered collections of identical objects that can be accessed via an index. In C and C++, an array is implemented as a contiguous block of memory accessed via pointers to the elements. In Java, arrays are first-class objects. Java arrays have their own behavior that is encapsulated into their definition when the compiler creates them. Since arrays are objects, array variables behave like class-type variables. It is important to distinguish between the array variable and the array instance to which it refers. Declaring an array only declares the array variable. To instantiate the array, the new operator must be used with the [] operator to enclose the array size. The array size can be any integer expression.
The following code declares an array variable and initializes it to null: char data[] = null;
This can also be written as: char[] data = null;
To declare an array variable and initialize it to refer to a new instance, use the following syntax: int length = 60; char[] data = new char [length];
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107
TABLE E. Array Initialization
Type
integers and floating point
zero
characters
null
booleans
false
class-type objects
®®®
Initialization
null
ARRAY CONSTANTS
The declaration of an array must include its type, but not its size. Arrays may be initialized with conventional initialization syntax. An array initializer is either an expression (such as a new expression) that evaluates to the correct array type, or a list of initial element values enclosed in { }. This latter notation is borrowed from C/C++. In Java, this is called an array constant.
Any expression can be used for the initial array element values. (C requires initializers be compiler-time calculable; both C++ and Java allow initializers to be run-time calculable.)
The following array constant provides initial values for its elements: int[] int_array = {1, 3, 4, 15, 0};
Each element of an array of class-type objects behaves like a class-type variable:
® Elements may be allocated by new, making them initially null:
Box[] boxes = new Box [100];
® Elements may be initialized in an array constant:
Box b = new Box (2.2, 3.3);
Box[] boxes = { b, new Box (1.1,2.2) };
LESSON 6
Array elements are always initialized according to their type as shown in
Table E.
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®®®
USING ARRAYS
Array elements can be accessed using C notation (an index in square brackets, [n]), where the index must be an integer. Like arrays in C, arrays start with index number 0, not 1. The index can be thought of as an offset from the beginning of the array.
The index may not be less than zero or greater than the declared size. If the array is declared size n, the index must be in the range 0 to n-1. Unlike C, arrays are not implemented as pointers, so programming techniques like negative array indexing are not allowed. Any attempt to index an array with an illegal value will cause an exception to be thrown.
NOTE
The length() in
String is a method while the length in array is a data member.
All arrays have a data field called length that indicates its size. This value is read-only, and contains the number of elements in the array: int ia[] = new int[100]; for (int i = 0; i < ia.length; i++)
{
ia[i] = i * i;
}
Since length is read-only, its value cannot be changed manually: ia.length = 20; // error
®®®
COPYING ARRAY ELEMENTS
The library method System.arraycopy() is useful for copying a number of elements from one array to another. The method can be used on any type of array and is declared as follows: public static void arraycopy (Object src, int src_position,
Object dst, int dst_position, int length)
The method copies elements from the given source array, beginning at the specified position to the destination array at the specified position. It copies
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109
Any type of array may be copied. If range exceeds bounds of either array, a run-time error results.
®®®
STRING OBJECTS
The Java String c lass (java.lang.String) is a class of object that represents a character array of arbitrary length. While this external class can be used to handle string objects, Java integrates internal, built-in strings into the language. This string literal is one way these strings can be used:
“This is a quoted string literal”
This is actually an object of type String having the value of the quoted character array, which is the most common way to instantiate a string object.
However, the String c lass does contain several constructors, listed in
Table F.
TABLE F String Constructors
.
Description
Syntax
construct an empty string
String ()
copy a string
String (String str)
initialize with a char array
String (char[] chars)
initialize with an ASCII byte array
String (byte[] bytes, int hibyte)
Strings can be used like any other type of Java object: void printMsg (String userName)
{
String msg1 = “Hello, “;
String msg2 = userName; if (msg2 == null) msg2 = “anonymous user”;
System.out.println (msg1 + msg2);
}
An important attribute of the String c lass is that once a string object is constructed, its value cannot change (note that it is the value of an object that
LESSON 6
the number of elements specified by the length argument. The destination array must already be allocated.
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JAVA PROGRAMMING: PART 1
cannot change, not that of a string variable, which is just a reference to a string object). All String data members are private, and no string method modifies the string’s value.
®®®
STRING METHODS
Although a string represents an array, standard array syntax cannot be used to inquire into it. Instead, the String class provides equivalent methods that allow access to a string’s value or any part of it. These are detailed in
Table G.
TABLE G. String Methods
Method
Description
Example
int length()
Returns the number of characters in the string
String str = “Another string”; int len = str.length
();
char charAt (int index)
Extracts one character from the string
char first = str.charAt
(0)
char last = str.charAt
(len - 1)
void getChars (int srcBegin, int srcEnd, char[] dest, int destBegin) Extracts a range of characters from the string into a character array
getChars(3, 6, word,
0);
String substring (int begin)
Return the substring of this string delimited by character position begin and the end of the string
String s3 = str.substring (8);
String substring (int begin, int end)
Returns the substring of this string delimited by character position begin
(inclusive) and character position end
(exclusive)
String s1 = str.substring (0, 7);
String Comparison
Since string variables are Java class-type variables, the equality (==) operator compares whether two strings are the same object, not whether they have
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111
TABLE H. More String Methods
Method
Description
Example
boolean equalsIgnoreCase (String otherString) Returns true if this string value equals the other, considering letters equivalent regardless of case
if (s1.equalsIgnoreCase
(s2)) { ... }
int compareTo (String otherString)
Returns zero if the string values are equal, a negative value if this string is less than the other, or a positive value if this string is greater than the other
if (s1.compareTo (s2) >
0) {
String temp = s1; s1 = s2; s2 = temp;
}
boolean startsWith (String prefix)
Returns true if this string starts with the other string if (s1.startsWith (s2))
{ ... }
boolean endsWith (String suffix)
Returns true if this string ends with the other string
if (s1.endsWith (s2)) {
... }
boolean regionMatches (int thisBegin,
String otherString, int otherBegin, int length)
General string region comparison if (s1.regionMatches(0, s2, 5, 10)
{...}
boolean regionMatches (boolean ignoreCase, int thisBegin, String otherString, int otherBegin, int length)
General string region comparison, with option of case insensitivity
if (s1.regionMatches(true, 0, s2, 5,
10) {...}
LESSON 6
the same value. To compare string values, use the equals() method common to all objects, or one of the other methods listed in Table H.
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String Searching
The String class also provides methods that search a string for the occurrence of a single character or substring. These return the index of the matching substring or character if found, or -1 if not found.
® int indexOf (char ch)
® int indexOf (char ch, int begin)
® int lastIndexOf (char ch)
® int lastIndexOf (char ch, int fromIndex)
® int indexOf (String str)
® int indexOf (String str, int begin)
® int lastIndexOf (String str)
® int lastIndexOf (String str, int fromIndex)
The following example shows the usage of these functions: if (s1.indexOf (’:’) >= 0) { ... }
String suffix = s1.substring (s1.lastIndexOf (’.’)); int spaceCount = 0; int index = s1.indexOf (’ ’); while (index >= 0)
{
++spaceCount; index = s1.indexOf (’ ’, index + 1);
}
int index = s1.indexOf (“that”);
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The String class provides miscellaneous methods for manipulating string data. Each of the methods listed below returns a new string instead of modifying its given string. (string objects are immutable by design).
String replace (char oldChar, char newChar)
// Replaces all occurrences of oldChar with newChar.
// For example: s2 = s1.replace (’ ’, ’_’);
String toLowerCase ()
// Converts all upper case characters to lower case.
// For example: s2 = s1.toLowerCase ();
String toUpperCase ()
// Converts all lower case characters to upper case
// For example: s2 = s1.toUpperCase ();
String trim ()
// Removes leading and trailing white space
// For example: s2 = s1.trim ();
®®®
STRING CONCATENATION
The String class provides a method for concatenating two strings:
NOTE
C++ programers should note that the
+ and += String operators are built into the Java language; and that Java does not allow programmer- defined operators. String concat (String otherString)
The + and += String operators are more commonly used:
String fname = s1 + “.txt”;
String path = s1 + “/” + s2 + “.txt”;
The Java compiler recognizes the + and += operators as String operators.
For each + expression, the compiler generates calls to methods that carry out the concatentation. For each += expression, the compiler generates calls to methods that carry out the concatenation and assignment.
LESSON 6
Other String Methods
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JAVA PROGRAMMING: PART 1
®®®
CONVERTING OBJECTS TO STRINGS
The String + operator accepts a non-string operand, provided the other operand is a string. The action of the + operator on non-string operands is to convert the non-string to a string, then to do the concatenation. Operands of native types are converted to string by formatting their values. Operands of class types are converted to a string by the method toString() that is defined for all classes. Any object or value can be converted to a string by explicitly using one of the static valueOf() methods defined in class
String:
String str = String.valueOf (obj);
If the argument to valueOf() is of class type, then valueOf() calls that object’s toString() method. Any class can define its own toString() method, or just rely on the default. The output produced by toString() is suitable for debugging and diagnostics. It is not meant to be an elaborate text representation of the object, nor is it meant to be parsed.
These conversion rules also apply to the right-hand side of the String += operator. ®®®
CONVERTING STRINGS TO NUMBERS
Methods from the various wrapper classes, such as Integer and Double, can be used to convert numerical strings to numbers. This is often necessary for command line arguments where the parameter list is available to the main () method as a string array.
The wrapper classes contain static methods ( such as parseInt() ) which convert a string to its own internal data type. These can be used with class names rather than creating a separate object. Be aware that these par-
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115
import java.io.PrintWriter; public class ArgTest { public static void main(String[] args) { int i, j;
// Create an stdout object
PrintWriter stdout = new PrintWriter(System.out, true);
// Make sure we have parameters if (args.length == 0) { stdout.println("No arguments");
System.exit(1);
}
// Loop through the args array for (i = 0; i < args.length; i++) { try { j = Integer.parseInt(args[i]); stdout.println(j + " is an integer");
}
catch (NumberFormatException e) { stdout.println(args[i] + " is not an integer");
}
}
}
}
LESSON 6
ticular conversion methods throw a NumberFormatException and must be used in the context of a try and catch block combination.
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®®®
LESSON SUMMARY
In this lesson, you have learned:
® Arrays are fixed-size, indexed collections of data elements, all of the same type.
® The Java run-time system catches out-of-bounds array indexes.
® Array variables behave like class-type variables.
® If an array contains class-type elements, then each element behaves like a class-type variable.
® Java strings are objects of class String.
® Java strings are immutable.
® Java recognizes + and += concatenation operators.
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1. What is wrong with the following code: int arr[] = new int[50]; arr.length = 25; for (int i = 0; i < arr.length; i++)
{
arr[i] = 0;
}
2. Using two different methods, add the text “ball” onto the end of a
String sport already containing the text “base.” Store the result in
String new_sport.
Answers on page 197
LESSON 6
REVIEW QUESTIONS
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JAVA PROGRAMMING: PART 1
EXERCISE
1. Write a class named FloatArray that represents an array of doubleprecision floating point values. Class FloatArray ’s first constructor takes one argument of type integer that specifies the size of the array.
The initial value of the array elements is zero. Do not try to handle invalid size values.
Class FloatArray ’s second constructor takes two arguments: the array size and a double that specifies the initial value of all the array elements.
Write a getValue() method that returns a value from the array, given the index of the element.
Write a setValue() method that sets a value in the array, given the index of the element.
Write a getAverage() method that returns the average of all values in the array.
Write a runnable main() method that tests the class floatarray.
Compile and run the class.
2. Write a static method called removeString. (You may create a new class for your method, or work in one you have already written.) removeString has the following signature: public static String removeString
(String string, String substring)
The function of removeString is to remove all occurrences of the given substring in the given string. For instance, the following call results in the string dated:
String result = removeString (“undaunted”,"un")
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LESSON 7
Inheritance
OVERVIEW
Inheritance is a powerful feature that allows new classes to be derived from existing ones. The derived class inherits the interface of the base class and can add new methods or change existing ones to suit its purpose.
Any number of classes can be added to an inheritance scheme to produce a hierarchy of related types. The Java implementation of inheritance supports polymorphism, which allows objects of different types to be treated as if they were of the same type.
LESSON TOPICS q Introduction to Inheritance q Protected Access q Overriding Methods q Constructor Chaining q Inheritance and Finalization q Abstract Classes q Interfaces q Casting Between Class Types
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®®®
OBJECTIVES
By the end of this lesson, you should be able to:
® Read and understand source code for derived classes.
® Derive a new class.
® Define a set of classes that behave polymorphically.
® Take advantage of Java’s run-time typing.
®®®
INTRODUCTION TO INHERITANCE
Developers working with object-oriented languages visualize classes as being related to each other in hierarchies of “kind of” relationships. Just as programmers generalize the common features of objects in an abstract data type, they also identify and abstract common features of similar types in a superclass. Once the superclass is defined, each subclass is defined as a “kind of” the superclass, so that it can then inherit the common features. The programmer need only specify the behavior that sets the subclass apart from its parent and its siblings. Java programmers say that the subclasses are derived from the superclass, and refer to them as derived types and base types, respectively.
Programmers conceptualize programs both upward and downward, through levels of abstraction during the design and implementation stages of a development cycle. However, it is observed that they usually tend to think predominantly from the bottom up during the design phase, and predominantly top down during the implementation phase. Once the characteristics of a superclass are well described, its interface can be written before the subclasses are developed. Each subclass inherits both its superclass’s interface and its implementation. It modifies and extends the former, or extends only the latter, as needed. The use of inheritance produces a hierarchy of classes.
For example, it is relatively easy to proceed downward from a base class like bank-account to more specialized classes of accounts, provided that the analysis and design phases are complete. By contrast, it is much more difficult proceeding upward, i.e., to try and code many different kinds of bank accounts first, and then coordinating all their differing interfaces into a superclass. LESSON 7: Inheritance
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An example of inheritance where a programmer might identify a need for two types of text objects would be CompactText and WordProcText.
CompactText emphasizes storage efficiency rather than functionality and
WordProcText is used for word processing that contains font information. While the differences between the text objects are significant enough to warrant implementing them as different classes, the similarities that exist between them suggest the presence of certain shared properties and the possibility of their belonging to one superclass.
In this case, rather than creating two largely similar class definitions, a superclass called TextObject can be first defined. This establishes a common interface for the two. From this base class, the classes CompactText and WordProcText are derived. Each derived type inherits its interface and implementation from the base type. For example, all of the classes have a length() method. The base class might or might not provide a default implementation for this method. If the base class does provide a default implementation, the derived classes can inherit the implementation as their own. A derived class can override or extend the implementation of an inherited method as appropriate. A derived class may not change the interface of a method. CompactText might use the implementation of the length() method defined in TextObject, while WordProcText might override this implementation with its own.
Perhaps it is only later that an EncodedText type is needed. If an inheritance relationship exists, then implementing the new type becomes much simpler. Whether or not derived classes are created at the same time as their base class, each of the derived types extends the interface or implementation of the base class.
Establishing a sensible hierarchy of related types is one of the most challenging aspects of object-oriented program design. It is not easy to generalize a base type in a manner that not only abstracts the common behavior of the types already known, but also provides a good basis for the later derivation of new types. An important reward for utilizing inheritance is a dramatic increase in code reuse, which can save a programmer time and valuable resources. LESSON 7
Example of Inheritance
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JAVA PROGRAMMING: PART 1
Derivation Syntax
The keyword extends in the subclass definition below declares one class as a subclass of another. A Java class may derive from at most one base class. public class TextObject
{
public int length () { ... }
}
public class CompactText extends TextObject
{
} public class WordProcText extends TextObject
{
}
Any user-defined class that does not explicitly derive from another class derives from the library class java.lang.Object. The Object c lass is therefore the ultimate base class of all other Java classes. The Object c lass provides the equals() method as well as other features common to all
Java objects.
Effects of Inheritance
In Java, private methods of the base class are not inherited (the derived class cannot call them anyway), but everything else is inherited.
A derived-type object can be said to have a dual identity. It is a type of both its new class and its parent type. For example, if class Bass is derived from
Fish, an object of type Bass is both a Bass and a Fish. However, the reverse is not necessarily true. The programmer, in this case, must force a conversion from base class type to derived class type with an explicit cast:
BaseClass Obj1; /*...*/
DerivedClass Obj2=(DerivedClass) Obj1;
LESSON 7: Inheritance
NOTE
Since Java has the concept of a package,
(a grouping of classes), Java offers a few additional combinations of access options as compared with C++.
PROTECTED ACCESS
In addition to public, private and default (package-friendly) member access, there are two other access specifiers that apply only to derived classes.
Any member that is declared protected is accessible only by a class that is derived from the current class or is in the same package as the containing class. Any member that is declared private protected is accessible only by a class that is derived from the containing class.
Consider the following example: public class A
{
protected int x; private protected int y;
}
Member x is accessible to any class that derives from class A and to any class in the same package as class A. Member y is only accessible to any class that derives from class A.
Figure 5 below shows the relationship between classes with different access specifiers. LESSON 7
®®®
123
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JAVA PROGRAMMING: PART 1
FIGURE 5
Accessibiliy in an inheritance relationship Package Things
Package Stuff
Class A
Class E
Class C
(derived)
Class B
Class D
Table I details access to class A’s members.
TABLE I. Access to class A’s members
Access specifier
Accessible by classes:
public
A, B, C, D, E
protected
A, B, C
private protected
A, B, D
private
®®®
A, B, C, D
default
A
OVERRIDING METHODS
The process of a derived class redefining a method contained in the parent class (with the same parameter types) is called overriding the method.
Whenever a method is called for a derived-type object, the compiler calls the overriding version instead of the inherited version. The method is thus
LESSON 7: Inheritance
125
public class TextObject
{
public int length ()
{
// TextObject’s implementation
}
} public class CompactText extends TextObject
{
public int length ()
{
// CompactText’s implementation
}
}
This allows a programmer to use a parent type variable to point to a derived type object, call a method for that object, and have the compiler call the specialized version. This is how polymorphism is implemented. C++ programmers will recognize this as a virtual function. In Java, all methods are virtual unless specified otherwise.
Dynamic Method Dispatching
Consider the following example: class Fish { void eat(), other items... } class Bass extends Fish { void eat()...} class Shark extends Fish { void eat()...}
Fish redfish, bluefish; redfish = new Bass(); bluefish = new Shark(); redfish.eat(); bluefish.eat();
The compiler cannot tell at compile-time which method to call here. The solution entails waiting until run-time to determine which function is the
LESSON 7
implemented in a way that is unique to the derived class. The following is a syntax example:
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JAVA PROGRAMMING: PART 1
correct one. In other words, the run-time code must check the actual type rather than the apparent type of an object based on the name of the variable used in the code. This is known as dynamic method dispatching.
Polymorphism
Dynamic method dispatching is a major component of polymorphism. Polymorphism is a very powerful feature of object-oriented languages. It allows a program to treat objects of different types as if they were all of the same type. TRIVIA
The key difference here, compared to a traditional non-polymorphic approach, is that differences in behavior are internalized in different derived classes. This differs from an external body of code selecting a certain behavior from a list of options. The creator of a group of related data types designs them to respond to the same message in ways that are nominally identical, no matter how different they may be in detail. The code which defines and uses objects of these types can then send the same message to all of them and get an appropriate response from each.
A user program can create heterogeneous lists of objects of related types and invoke a virtual function for each one of them. At run time, the program will branch to the version of the function that is appropriate to that object.
This is accomplished by the following manner. When a class declares a member function, the compiler includes a type code in every object of that class. Wherever the function is invoked, the Java run-time system checks the type of each object and invokes the function appropriate to that object’s specific class. By automating type checking, polymorphism eliminates the need for many of the large switch statements that tend to infest C programs.
Coding these is notoriously tedious, mechanical, and error-prone. Such statements also require maintenance whenever a new class of data is introduced, even if that class is closely related to others.
In contrast, the addition of a new data type typically has much less effect on programs that use polymorphism. Since code that requests a string’s length pays no attention to the particular string type, the compiler neither knows nor cares that some of the string types were introduced after the code was written. Such code therefore requires less initial development effort as well as less maintenance.
LESSON 7: Inheritance
127
TextObject[] tarr =
{
new EncodedText (), new WordProcText (), new CompactText (), new TextObject ()
};
int totalLen = 0; for (int i = 0; i < tarr.length; ++i)
{
totalLen += tarr[i].length ();
}
The code that invokes the length() method ignores type differences.
New types of TextObjects can be introduced without forcing changes in calling code.
The super Keyword
It is often desirable for an overriding implementation of a method to call the inherited implementation of the method. Often, the inherited code need not be completely replaced, only augmented to add some new actions. Any method in a derived class that overrides a method in the base class can call the corresponding base class method by using the super keyword as shown below: public class WordProcText extends TextObject
{
public int length ()
{
int len = super.length ();
...
return len;
}
}
LESSON 7
The following polymorphism example invokes the length() method of each element of an array of TextObjects:
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JAVA PROGRAMMING: PART 1
Note that there is no way to access a method overridden more than once.
The following code is illegal: super.super.f() Final Methods and Final Classes
NOTE
final in Java is the counterpart of virtual in C++. In
C++ you need to specify virtual to get dynamic binding.
In Java you need to specify final to get static binding.
®®®
Dynamic lookup takes time. Therefore, there are some situations where it should be disabled. Also, there are some methods for which the compiler must be able to resolve a required action, or for which an overriding version would be too complicated to implement.
Methods labeled final, private, or static are not subject to dynamic lookup because they may not be overridden. private methods are simply not inherited because they would never be callable anyway. static methods apply to a particular class’s static data and thus make no sense in a derivation. final methods are those designated as not-overrideable for reasons of complexity or safety.
CONSTRUCTOR CHAINING
When a derived-type object is instantiated, it is sometimes desirable to have its constructor call a particular overload of its parent class. This is done with what looks like a call to the method super(). Often this is used to pass a particular variable up into a parent class’s constructor. This must be the first statement in a constructor.
If the programmer does not include a super() call, the compiler will provide one. The only exception to this occurs when a constructor calls another constructor via the form this(). In this case, the compiler knows that the first constructor will call super().
Constructors do not use dynamic lookup simply because each class has its own constructor.
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129
public class A
{
A(int ix){...}
A(float fx){...}// overloaded
}
public class B extends A
{
B(int ix){...}
}
The keyword super can be used to specify which of A’s constructors to call from B’s constructor: class B extends A {
{
B(int ix)
{
super(ix);// calls A(int ix)
}
}
This use of super() is legal only as the first statement of a constructor. If a constructor calls neither super() nor this(), the compiler implicitly calls the base class’s no-parameter constructor as seen in Figure 6.
LESSON 7
Consider the following two classes:
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JAVA PROGRAMMING: PART 1
FIGURE 6
Constructor Chaining
class A
{
A(){...}
A(float xx){...}
}
C om piler calls
class B extends A
{
A() h ere
B(){...}
B(float fx)
B(int ix)
{
this();
...
}
{super(fx);...}
b u t n o t h e re
}
One common mistake is to define a particular constructor in a parent class, and then derive from it without calling super(). In this case, the compiler would call the parent class’s no-parameter constructor, but would not have defined one. This can produce some mystifying error messages.
The appropriate overload must exist in the parent class. Recall that if any constructor is written, the compiler will not provide the default no-parameter version.
LESSON 7: Inheritance
131
class A { int value;
A(int input) { value = input; }
}
class B extends A { int myvalue;
}
®®®
NOTE
in C++, a parent class’s destructor is called automatically.
This does not happen in Java.
®®®
INHERITANCE AND FINALIZATION
If a parent class contains a finalize() method, it must be called explicitly by the derived class’s finalize() method. Unlike C++, Java destructors are not called automatically. If it is needed, the derived class’s finalize() method must call it manually with: super.finalize(); ABSTRACT CLASSES
Methods declared with the keyword abstract define a skeleton for a method but do not implement it. This requires a derived class to provide the code for this class. A class with one or more abstract methods must itself be declared abstract and cannot be instantiated. It can only be used for derivation.
C++ programmers will recognize this as a pure virtual function that makes an abstract class. A method can be declared abstract with no brackets to enclose any code.
LESSON 7
The following will produce a compiler error:
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JAVA PROGRAMMING: PART 1
FIGURE 7
The relationship between an abstract class and its derived classes abstract float area(); class Shape
(derived)
Circle
Square
Triangle
A generic shape cannot exist. Each subtype implements its own area() method. ®®®
TIP
The rule of thumb in choosing between an abstract class and an interface is as follows: if you want any code in the base class to be reused by derived classes, use an abstract class.
Otherwise use an interface. INTERFACES
Some languages support multiple inheritance in which a child class may derive some of its features from one parent class and other features from another (or others). There is a continuing controversy over whether the benefits promised by this feature justify the extra complexity that comes with it.
Java does not support multiple inheritance. However, it does provide a mechanism called an interface whereby the properties of one class can be used by another without formal inheritance. This enables Java to avoid most of the syntactical complications that multiple inheritance requires. When a parent class is to be defined as abstract, instead of defining one or more methods as abstract, the class can be defined with the keyword interface instead of class. This defines the minimum set of methods a class must contain. An interface type is named for derivation with the keyword implements instead of extends. All of its methods are implicitly public and abstract. If desired, they can be declared that way for clarity. Also, any data defined in an interface must be constant (i.e., final static).
LESSON 7: Inheritance
133
A derived class cannot have two parent classes, as multiple inheritance leads to problems of how to decide which method to use for inherited methods.
Yet multiple inheritance is desirable because it lets an object be assigned into, and thus used as, variables of different types. For example, a programmer might instantiate a Circle as a class derived from Shape, yet wish to pass it into a method requiring a Graphic object parameter.
The implements declaration in Figure 7, seen as a way of deriving from an interface, also means that the new class can be treated as being of that new type. Though a class can have only one parent class, it can implement any number of classes, and thus behave as more than one type of object. For example: interface Shape{...} class Circle extends Drawable implements Shape{...} class Square extends Drawable implements Shape{...}
Shape sp1 = new Circle(), sp2 = new Square(); sp1.area(); sp2.area():
The classes Circle and Square are derived from class Drawable, but by implementing Shape, they also inherit Shape’s interface and are enabled to act as polymorphic instances of Shape.
With interface and implements, Java supports the polymorphic features of multiple inheritance, while yet avoiding the complications of methods implemented in multiple ways.
®®®
CASTING BETWEEN CLASS TYPES
The Java run-time system keeps track of the type of each object. While this implies some memory space overhead for each object, the programmer can achieve a type-safe run-time environment. The Java language makes some of
LESSON 7
The implements Declaration
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JAVA PROGRAMMING: PART 1
its run-time typing power available to the programmer through the instanceof operator. The form of an instanceof expression is as follows: object-expression instanceof class-name
The left-hand term is any expression that evaluates to a class type. The right-hand term is the name of a class or interface. An instanceof expression returns true if the actual type of the object on the left-hand side is of the type named by the right-hand side, and false otherwise. The object is of the named type if it is of exactly the named type or is of a type that directly or indirectly extends the identified type.
The instanceof operator can be use for safe casting between class types: if (tobj instanceof CompactText)
{
CompactText ct = (CompactText) tobj;
}
LESSON 7: Inheritance
LESSON SUMMARY
In this lesson, you have learned:
® Inheritance involves defining a class as a specific type of another class.
® Inheritance increases code reuse and allows for polymorphism.
® Java supports dynamic method dispatch at run-time.
® Methods and classes may be declared final.
® Abstract methods have no implementation.
® Interface types support limited multiple inheritance.
® Java’s run-time typing allows safe casting between class types.
LESSON 7
®®®
135
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JAVA PROGRAMMING: PART 1
REVIEW QUESTIONS
1. Write the class declarations for the following relationship, assuming that all classes are public: a Bulldog is a kind of Dog, and a Dog is a kind of
Animal.
2. What is the difference between the protected and private protected access methods?
3. What is purpose of the super keyword?
Answers on page 198
LESSON 7: Inheritance
137
1. Create an interface called SecondObserver that defines a single method, tick(). Go back to your Clock c lass and make class Clock implement the SecondObserver interface.
Create a new class called PrintClock that extends your Clock c lass.
PrintClock provides a method called toString() that produces a string representation of the current time. Test your class.
2. Create a new class called TickingPrintClock that extends your
PrintClock class. TickingPrintClock extends the tick() method to print the new time on each tick, in addition to the normal function of the method.
Test your new class. You can insert the following code block to cause your program to sleep for one second: try {
Thread.sleep (1000);
}
catch (InterruptedException ie)
{
}
LESSON 7
EXERCISE
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LESSON 8
Writing Java Applets
OVERVIEW
Java applets are graphical mini-programs that run in the context of another program. Thanks to the growth of the Internet, applets have become popular additions to World Wide Web pages. This lesson introduces basic applet methods and the means to embed applets in
HTML documents.
LESSON TOPICS q What Is an Applet? q The Applet Class q The Delegation Event Model—Action Events q The paint() Method q The Graphics Class q Java Fonts q Drawing Lines and Shapes q Drawing with Color q The Color Class
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JAVA PROGRAMMING: PART 1
®®®
OBJECTIVES
By the end of this lesson, you should be able to:
® Create a Java applet.
® Embed an applet in an HTML document.
® Use graphics library methods to draw in applets.
® Use the event model to capture click type events.
®®®
TRIVIA
The applets make static documents come alive. This feature of Java was a major reason for its quick popularity as an internet language.
WHAT IS AN APPLET?
An applet is not a standalone application. It is a mini program invoked within a larger program, such as an applet viewer or a Java-enabled World
Wide Web (WWW) browser. An applet depends on the viewer or browser to provide a context in which to run. This means that the viewer or browser implements common applet functions and provides those functions to the applet by way of a Java Application Program Interface (Java API). The viewer or browser also controls the execution of the applet.
An applet is graphical by nature. Each applet occupies a rectangular panel in its viewer's window. Applets are meant to be embedded within a graphical document, such as an HTML document.
A "Hello, World" Applet
The following is a simple applet named HelloWorldApplet.java:
NOTE
Without the import statement, use java.applet.App let instead of
Applet; and java.awt.Graphics instead of
Graphics.
import java.applet.*; import java.awt.*; public class HelloWorldApplet extends Applet
{
public void paint (Graphics g)
{
g.drawString (“Hello World”, 25, 50);
}
}
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141
® java.applet.Applet
® java.awt.Graphics
Without those import statements, the Java compiler would not recognize the classes Applet and Graphics, which the applet class refers to.
®®®
THE Applet CLASS
Every applet is an extension of the java.applet.Applet c lass. The base Applet c lass provides methods that a derived Applet class may call to obtain information and services from the browser context. These include methods that do the following:
® get applet parameters
® get the network location of the HTML file that contains the applet
® get the network location of the applet class directory
® print a status message in the browser
® fetch an image
® fetch an audio clip
® play an audio clip
® resize the applet
LESSON 8
These import statements bring the classes into the scope of our applet class:
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Additionally, the Applet c lass provides an interface by which the viewer or browser obtains information about the applet and controls the applet's execution. The viewer may:
® request information about the author, version and copyright of the applet ® request a description of the parameters the applet recognizes
® initialize the applet
® destroy the applet
® start the applet's execution
® stop the applet's execution
The Applet class provides default implementations of each of these methods. Those implementations may be overridden as necessary.
The “Hello, World” applet is complete as it stands. The only method overridden is the paint method.
Invoking an Applet
An applet may be invoked by embedding directives in an HTML file and viewing the file through an applet viewer or Java-enabled browser. The tag is the basis for embedding an applet in an HTML file.
Below is an example that invokes the “Hello, World” applet:
The Hello, World Applet
If your browser was Java-enabled, a “Hello, World” message would appear here.
The code attribute of the tag is required. It specifies the
Applet class to run. Width and height are also required to specify the initial size of the panel in which an applet runs. Note that the size of the applet
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143
If an applet takes parameters, values may be passed for the parameters by adding tags between and . The browser ignores text and other tags between the applet tags. Non-Java-enabled browsers do not process and . Therefore, anything that appears between the tags, not related to the applet, is visible in nonJava-enabled browsers. The programmer can take advantage of this feature to provide filler content for users with older browsers.
The viewer or browser looks for the compiled Java code at the location of the document. To specify otherwise, use the codebase attribute of the tag as shown:
Whether a code base is specified or the default is used, it is important to keep in mind that the code base is the root directory of the package tree. If the code is organized into packages, the Java class loader expects to find the class files in like-named subdirectories of the code base directory. The code base directory itself is the home of the default package.
If an applet resides in a package other than the default, the holding package must be specified in the code attribute using the period character (.) to separate package/class components. For example:
Getting Applet Parameters
The following example demonstrates how to make an applet respond to setup parameters specified in the document. This applet displays a checkerboard pattern of black and a second color. The second color and the size of each square may be specified as parameters to the applet within the document.
LESSON 8
panel is determined in the document, not from within the applet. An applet may call its resize() method, but the browser is not guaranteed to update the display. The applet directive must be closed with a tag.
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CheckerApplet gets its parameters in the init() method. It may also get its parameters in the paint() method. However, getting the values and saving the settings once at the start of the applet, instead of at every refresh, is convenient and efficient. The applet viewer or browser calls the init() method of each applet it runs. The viewer calls init() once, immediately after loading the applet. (Applet.init() is implemented to do nothing.) Override the default implementation to insert custom initialization code.
The Applet.getParameter() method fetches a parameter given the parameter's name (the value of a parameter is always a string). If the value is numeric or other non-character data, the string must be parsed.
The following is a skeleton of CheckerApplet.java: import java.applet.*; import java.awt.*; public class CheckerApplet extends Applet
{
int squareSize = 50;// initialized to default size public void init () {} private void parseSquareSize (String param) {} private Color parseColor (String param) {} public void paint (Graphics g) {}
}
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145
public void init ()
{
String squareSizeParam = getParameter (“squareSize”); parseSquareSize (squareSizeParam);
String colorParam = getParameter (“color”);
Color fg = parseColor (colorParam); setBackground (Color.black); setForeground (fg);
}
private void parseSquareSize (String param)
{
if (param == null) return; try { squareSize = Integer.parseInt (param);
}
catch (Exception e) {
// Let default value remain
}
}
The applet calls parseSquareSize() to parse the squareSize parameter. parseSquareSize() calls the library method Integer.parseInt(), which parses a string and returns an integer. Integer.parseInt() throws an exception whenever its argument is invalid.
Therefore, parseSquareSize() catches exceptions, rather than allowing the applet to fail on bad input.
The applet calls parseColor() to parse the color parameter into a Color value. parseColor() does a series of string comparisons to match the parameter value to the name of a predefined color. For more information concerning parseColor(), refer to the sample library.
LESSON 8
Here are CheckerApplet ’s init() and private parseSquareSize() methods:
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JAVA PROGRAMMING: PART 1
Specifying Applet Parameters
The following is an example of an HTML file with a CheckerApplet embedded in it. The HTML file specifies both parameters to the applet by means of the tag.
Checkerboard Applet
Parameter names are not case sensitive.
®®®
THE DELEGATION EVENT MODEL—
ACTION EVENTS
Events, such as the clicking of a button or the movement of a scroll bar, can be intercepted and processed by an applet. The technique used by Java to do this is called the Delegation Event Model. This model uses the concept that an object (e.g., a command button) is the source of events; and another object
(e.g., an applet) listens to a source for a specific type of event.
A listener informs the source that it is interested in listening to that source.
The source adds that listener to its list of objects. When an event occurs, the source informs all interested listeners, and passes information regarding the event to the listener.
If a listener is interested in an event it must implement a set of methods that the source can call when the event occurs. This set of methods is called an interface. The concept of an interface will be discussed in greater depth later in the course. Having implemented the interface, the source is then able to call the listener. When a method of the interface is called, a single argument or an event object is passed that conveys information about the event.
The essential steps required for handling events include:
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147
2. Adding the listener to the source's event list.
3. Implementing the appropriate methods.
4. Using the event object to process the event.
Step 1: Implement the listener:
...
import java.awt.event.*; public class MyApplet extends Applet implements
ActionListener
{
The implements keyword is used in the class definition to inform the java compiler that this class supports a specific type of interface. There are several types of event interfaces supported by Java. The ActionListener and
AdjustmentListener interfaces are described in this section. The
ActionListener interface deals with high level events such as the pressing of a button. The AdjustmentListener is used with controls such as a scroll bar. Other high and low level interfaces exist.
Step 2: Add the listener to the source's event list: public void init ()
{
Button b = new Button (“Help”);
b.addActionListener(this);
add (b);
}
The addActionListener method is used to add the source (the current applet) to the list of listeners interested in listening to the button. The this keyword identifies the current applet as the listener. The method is applied to the button thus making the button the source object.
Step 3: Implement the appropriate method:a public void actionPerformed(ActionEvent evt)
{
LESSON 8
1. Use of the implements keyword by the listener.
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JAVA PROGRAMMING: PART 1
The ActionListener interface has only a single method, actionPerformed. When an interface is implemented by a class, all of its methods must be overridden by that class. The actionPerformed method is passed as a single argument, ActionEvent. This argument contains information about that event.
Step 4: Process the event: if (evt.getSource() instanceof Button)
{
Button source = (Button)evt.getSource(); if (source.getLabel() == “Help”)
/ * ... handle “Help” button ... */
The actionPerformed method may be invoked by more than one source if the listener has added itself to other sources. One of the primary purposes of the ActionEvent object is to differentiate between different sources. The getSource method returns a reference to the object that caused the event. It is used with such methods as getLabel, that returns the button label, to determine the source of the event.
The ActionEvent Class
The ActionEvent c lass contains the event type ACTION_PERFORMED.
An ActionEvent can be fired by:
® clicking a button
® double clicking a list
® choosing a menu item
® pressing when in a text field
The getSource() method is used to determine the object in which the event originated.
An action command is associated with each ActionEvent. This command is a simple String that is assigned to a source object. It is assigned when the object is created using the setActionCommand method. Later, the getActionCommand method can be used within such methods as actionPerformed, to uniquely identify the source of the event. This
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149
The user may hold down a modifier key such as or when an event is fired. The getModifiers() method can be used to determine which key was held down. The following masks are provided with the
ActionEvent c lass for comparison with the modifier constants:
® ALT_MASK
® CTRL_MASK
® META_MASK
® SHIFT_MASK
Adjustment Events
Adjustment events are fired when the user manipulates an Adjustable item, such as a scrollbar or slider control. They are handled similarly to
Action events.
LESSON 8
technique avoids having to use a label, or some other attribute of the source, that may change with the use of the object.
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JAVA PROGRAMMING: PART 1
To respond to a scrollbar manipulation, implement the adjustmentValueChanged method of the AdjustmentListener interface, and query the AdjustmentEvent object:
...
import java.awt.event.*; public class MyApplet extends Applet implements
AdjustmentListener
{ public void init ()
{
Scrollbar sb = new Scrollbar ( ... ); sb.addAdjustmentListener(this); add (sb);
}
public void adjustmentValueChanged (AdjustmentEvent evt)
{
switch (evt.getAdjustmentType ())
{
case AdjustmentEvent.UNIT_INCREMENT: case AdjustmentEvent.UNIT_DECREMENT: case AdjustmentEvent.BLOCK_INCREMENT: case AdjustmentEvent.BLOCK_DECREMENT: case AdjustmentEvent.TRACK:
Scrollbar sb = (Scrollbar)evt.getAdjustable(); int orientation = sb.getOrientation (); int value = sb.getValue ();
/* ...update display to match value */
}
}
}
The AdjustmentEvent Class
The AdjustmentEvent c lass contains the event type
ADJUSTMENT_VALUE_CHANGED. This event can be fired by:
® manipulating a scrollbar
® manipulating a custom component (such as a slider)
The getAdjustable() method returns a reference to the component that fired the event. If that type of adjustment is needed, the getAdjust-
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151
® BLOCK_DECREMENT
® BLOCK_INCREMENT
® TRACK
® UNIT_DECREMENT
® UNIT_INCREMENT
®®®
THE paint() METHOD
The Applet c lass derives a class within Java's Abstract Windowing Toolkit
(AWT) library, java.awt.Panel. The Java AWT forms a layer between the viewer or browser and the applet running within it. Every graphical component in the AWT library, including Panel, derives from java.awt.Component. The Component c lass supports a set of methods for drawing, positioning, sizing, etc. The paint() method is one of those common methods.
The Java AWT calls the paint() method to completely render the component. For example, when the window containing the component is resized, it may be necessary to repaint certain window components. The default implementation of paint() does nothing. To draw custom graphical content in your applet, you must override the paint() method. Its signature is: public void paint (Graphics g);
®®®
THE Graphics CLASS
The paint() method takes one parameter, an object of class Graphics.
The object holds a context in which drawing operations may be performed.
The Graphics class provides the methods for drawing; including drawLine() (the method that draws a line), and drawString() (the method that draws text). The paint() method in the “Hello, World” applet calls
LESSON 8
ableType() method will return one of the following values which are defined in the AdjustableEvent c lass:
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Graphics.drawString() to draw the text string “Hello, World” at position (25, 50) within the applet's panel:
FIGURE 8
String placement in Graphics
Class
25
50
Hello, World
The Graphics object clips all graphics operations to within the current draw region.
As shown in the diagram above, the x coordinate of an AWT graphics point represents the number of pixels horizontally from the left of the panel. The y coordinate represents the number of pixels vertically from the top.
®®®
JAVA FONTS
A Java font is represented by an object of class java.awt.Font. A Font object has three attributes:
® family
® style
® size
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153
® “Helvetica”
® “TimesRoman”
® “Courier”
® “Dialog”
® “DialogInput”
There is also a default font family to be used in the event that a font is missing.
Java provides plain, bold, and italic font styles. Bold and italic may be combined, resulting in four styles, listed in Table J.
TABLE J. Java Styles and Their Symbols
Style
Symbol
Plain
Font.PLAIN
Bold
Font.BOLD
Italic
Font.ITALIC
BoldItalic
Font.BOLD | Font.ITALIC
The third attribute of a font is its point size.
Selecting a Font
To select a font, the programmer must:
® construct a Font object
® install a Font object into the Graphics context
The Font constructor takes three arguments:
® the family (a string)
® style
® point size
LESSON 8
Java provides five font families, each of which maps to a true font on the host platform. A font family is identified by a string:
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JAVA PROGRAMMING: PART 1
For example:
Font myFont = new Font ("Helvetica", Font.PLAIN, 18);
To install a font into a Graphics context, use the Graphics.setFont() method: public class NewHelloWorldApplet extends Applet
{
static Font myFont = new Font ("Courier", Font.PLAIN, 18); public void paint (Graphics g)
{
g.setFont (myFont);
g.drawString ("Hello, World", 25, 50);
}
}
®®®
DRAWING LINES AND SHAPES
Below is an example of an applet (called ShapesApplet.java) that demonstrates some of the drawing functions available through class
Graphics.
import java.applet.*; import java.awt.*; public class ShapesApplet extends Applet
{
public void paint (Graphics g)
{
int left = 0; int top = 0; int right = 319; int bottom = 119;
g.drawRect
g.drawLine
g.drawLine
g.drawOval
}
}
(left, top, right - left, bottom - top);
(left, top, right, bottom);
(right, top, left, bottom);
(left, top, right - left, bottom - top);
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155
The arguments to drawRect() are: the x coordinate of the left edge of the rectangle, the y coordinate of the top edge of the rectangle, the width of the rectangle in pixels, and the height of the rectangle in pixels drawOval() draws an oval within the rectangle specified by its parameters, which are similar in meaning to those of drawRect().
®®®
DRAWING WITH COLOR
The following example is another applet that demonstrates the use of color for graphics.
ColorsApplet.java:
import java.applet.*; import java.awt.*; public class ColorsApplet extends Applet
{
public void paint (Graphics g)
{
int left = 10; int top = 50; int width = 50; int height = 50;
g.setColor (Color.red);
g.fillRect (left, top, width, height); left += 100;
g.setColor (Color.yellow);
g.fillRect (left, top, width, height); left += 100;
g.setColor (Color.blue);
g.fillRect (left, top, width, height);
}
}
LESSON 8
The arguments to drawLine() are the (x,y) coordinates of the first endpoint and the (x,y) coordinates of the second endpoint. drawLine() draws a line one pixel wide from the first endpoint to the second endpoint, inclusive. 156
JAVA PROGRAMMING: PART 1
This applet displays three square tiles of various colors. The method
Graphics.setColor() is used to set the current color.
®®®
THE Color CLASS
The method Graphics.setColor() installs a color into a graphics context. The color setting remains until another call to setColor(). At the start of the paint() method, the color installed in the graphics context is the foreground color of the applet, usually black. color.red, color.blue, etc., are constant objects of type color, defined as static data members in class Color. The entire list of predefined colors appears in Table K.
TABLE K. RGB Color Values
Color
Red Value
Green Value
Blue Value
white
255
255
255
lightGray
192
192
192
gray
128
128
128
darkGray
64
64
64
black
0
0
0
red
255
0
0
pink
255
175
175
orange
255
200
0
yellow
255
255
0
green
0
255
0
magenta
255
0
255
cyan
0
255
255
blue
0
0
255
If the desired color is not one of the 13 predefined color objects, a new color object can be constructed. The color constructor constructs a new color object from RGB values in the range (0, 255). For example,
Color c = new Color (255, 128, 0);
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157
c = Color.getHSBColor (0.5, 0.7, 0.2);
HSB values are floating point numbers in the range (0.0, 1.0).
Although these methods allow you to construct millions of different color values, the number of different colors displayable, at one time, is far fewer.
The Java library internals map each color to the closest available color in the current model of the display device.
Foreground and Background Colors
Because an applet is an Abstract Windowing Tookit (AWT) component, it can store two colors: foreground and background. By default, the foreground and background colors of the applet are inherited from the document. The colors are usually black on white or black on gray.
These colors may be accessed through the methods getForeground() and getBackground(), for example:
Color c = getBackground ();
The colors may be set through the methods setForeground() and setBackground(), for example: setForeground (new Color (64, 64, 64));
Before calling the applet's paint() method, the Java AWT fills the panel with the applet's background color. (The viewer or browser calls the init() method before the first call to paint().) At the start of the paint() method, the color installed in the graphics context becomes the applet's foreground color.
CheckerApplet takes advantage of this behavior in the paint() method. Because the Java AWT has already filled the panel with the background color (black), the paint() method does not need to draw the black squares. Because the foreground color is already installed in the graphics
LESSON 8
It is sometimes more convenient to express a color in terms of HSB (hue, saturation, brightness) values. A static method Color.getHSBColor() exists for this purpose:
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JAVA PROGRAMMING: PART 1
context at the start of paint(), the paint method does not need to call
Graphics.setColor(). paint() simply iterates over the visible squares, drawing a foreground square whenever the counter is even: public void paint (Graphics g)
{
Dimension sz = size (); for (int y = 0; y < sz.height; y += squareSize) { int count = y / squareSize; for (int x = 0; x < sz.width; x += squareSize) { if ((count % 2) == 0) {
g.fillRect (x, y, squareSize, squareSize);
}
++count;
}
}
}
The method size() returns an object of type Dimension, giving the current width and height of the applet's pane.
LESSON 8: Writing Java Applets
LESSON SUMMARY
In this lesson, you have learned:
® An applet is a graphical mini-program that runs in the context of another program, such as an applet viewer or a Java-enabled WWW browser. ® The directive is used to embed an applet in HTML documents.
® The Java applet API allow programmers to:
• draw text and graphics into an applet
• work with fonts and colors
LESSON 8
®®®
159
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JAVA PROGRAMMING: PART 1
REVIEW QUESTIONS
1. What is the fully qualified name of the base class of all applets?
2. What is the name of the method that fetches the value of an applet parameter? 3. Write the full HTML applet tag that starts an applet named
Shape.class at a width of 200 and a height of 100. It also takes a color parameter equal to red and a size parameter equal to 10.
Answers on page 198
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161
1. Modify the “Hello, World” applet to take four parameters named text, font, style and size. These specify what text to display, and what family, style and size of the font to use. Valid values for style include: plain, bold, italic, bolditalic. size is an integer, and must be parsed (see
CheckerApplet for an example). If any of the parameters is unspecified, use a default value.
2. Implement an applet that contains a single button. When the user clicks on that button, display your name at location (100, 100) on the applet.
The code used to illustrate the event handler earlier in this chapter can be used as the basis for this application. Add a string identifier to hold your name and initialize it to an empty string in the init method. In the actionPerformed method, assign your name to the string. The repaint method will force redrawing of the applet. Place this method right after your name has been assigned. Display the string in the paint method. Ensure that the following import statements are included at the beginning of the program: import java.awt.event.*; import java.applet.Applet; import java.awt.*;
LESSON 8
EXERCISE
w w w
APPENDIX A
Hypertext Markup
Language (HTML)
OVERVIEW
This appendix contains a brief overview of some common HTML tags, as well as standard Netscape extensions. Many of the Netscape extensions have been submitted for inclusion in the HTML standard.
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JAVA PROGRAMMING: PART 1
®®®
HTML HISTORY AND SGML
HTML is an application of the ISO certified Standard Generalized Markup
Language (SGML) standard. SGML was first published in 1988 as a standard for electronic document exchange, archival and processing. There are other de facto standards that have the same objectives, such as Adobe’s
Acrobat, or Microsoft’s RTF (Rich Text Format).
As a subset of SGML, HTML is much simplified. SGML documents are generally more complex and programming-like than HTML documents.
You can use SGML to define HTML, which allows for greater standardization and interchangeability. HTML’s advantage comes from its combination of SGML tags and contructs and standard English text markup notation.
HTML is therefore easy to interpret, yet powerful enough for its purpose.
The HTML standard is maintained by the Internet Engineering Task Force
(IETF).
The next sections list some common HTML tags, and their usage.
APPENDIX A: Hypertext Markup Language (HTML)
Tag
Description
Comments
Document type
Should surround whole document
Document title
Should be in header
Document header
Body
Comment
(not displayed by the browser)
®®®HEAD ELEMENTS
Tag
Description
Comments
Searchable Document
Adds search prompt, only works if document is setup for search
Document’s base URL
xmax) xmax = x; float y = v[i + 1]; if (y < ymin)
APPENDIX B: Java Sample
} this.xmax this.xmin this.ymax this.ymin this.zmax this.zmin
}
=
=
=
=
=
=
xmax; xmin; ymax; ymin; zmax; zmin; }
/** An applet to put a Cehmical model into a page */ public class XYZApp extends Applet implements Runnable {
XYZChemModel md; boolean painted = true; float xfac; int prevx, prevy; float xtheta, ytheta; float scalefudge = 1;
Matrix3D amat = new Matrix3D(), tmat = new Matrix3D();
String mdname = null;
String message = null;
Image backBuffer;
Graphics backGC;
Dimension backSize; private synchronized void newBackBuffer() { backBuffer = createImage(size().width, size().height); backGC = backBuffer.getGraphics(); backSize = size();
}
public void init() { mdname = getParameter(“model”); try { scalefudge = Float.valueOf(getParameter(“scale”)).floatValue();
} catch(Exception e) {
};
amat.yrot(20); amat.xrot(20); if (mdname == null) mdname = “model.obj”; resize(size().width zmax) zmax = z;
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public void run() {
InputStream is = null; try {
Thread.currentThread().setPriority(Thread.MIN_PRIORITY);
is = new URL(getDocumentBase(), mdname).openStream();
XYZChemModel m = new XYZChemModel (is);
Atom.setApplet(this);
md = m;
m.findBB();
float xw = m.xmax - m.xmin; float yw = m.ymax - m.ymin; float zw = m.zmax - m.zmin; if (yw > xw) xw = yw; if (zw > xw) xw = zw; float f1 = size().width / xw; float f2 = size().height / xw; xfac = 0.7f * (f1 < f2 ? f1 : f2) * scalefudge;
} catch(Exception e) {
e.printStackTrace();
md = null; message = e.toString();
}
try { if (is != null) is.close(); } catch(Exception e) {
}
repaint();
}
public void start() { if (md == null && message == null) new Thread(this).start();
}
public void stop() {
}
public boolean mouseDown(Event e, int x, int y) { prevx = x; prevy = y; return true;
}
public boolean mouseDrag(Event e, int x, int y) { tmat.unit(); float xtheta = (prevy - y) * (360.0f / size().width); float ytheta = (x - prevx) * (360.0f / size().height); tmat.xrot(xtheta); tmat.yrot(ytheta); amat.mult(tmat); if (painted) {
APPENDIX B: Java Sample
} prevx = x; prevy = y; return true;
}
public void update(Graphics g) { if (backBuffer == null)
g.clearRect(0, 0, size().width, size().height); paint(g); } public void paint(Graphics g) { if (md != null) { md.mat.unit(); md.mat.translate(-(md.xmin + md.xmax) / 2,
-(md.ymin + md.ymax) / 2,
-(md.zmin + md.zmax) / 2); md.mat.mult(amat); // md.mat.scale(xfac, -xfac, 8 * xfac / size().width); md.mat.scale(xfac, -xfac, 16 * xfac / size().width); md.mat.translate(size().width / 2, size().height / 2, 8); md.transformed = false; if (backBuffer != null) { if (!backSize.equals(size())) newBackBuffer(); backGC.setColor(getBackground()); backGC.fillRect(0,0,size().width,size().height); md.paint(backGC);
g.drawImage(backBuffer, 0, 0, this);
}
else md.paint(g); setPainted();
} else if (message != null) {
g.drawString(“Error in model:”, 3, 20);
g.drawString(message, 10, 40);
}
} private synchronized void setPainted() { painted = true; notifyAll(); } private synchronized void waitPainted()
{
while (!painted)
{
try
{
wait();
APPENDIX B
painted = false; repaint(); 181
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JAVA PROGRAMMING: PART 1
} catch (InterruptedException e) {}
}
painted = false;
}
}
// end class XYZApp class Atom { private static Applet applet; private static byte[] data; private final static int R = 40; private final static int hx = 15; private final static int hy = 15; private final static int bgGrey = 192; private final static int nBalls = 16; private static int maxr; private int Rl; private int Gl; private int Bl; private Image balls[]; static { data = new byte[R * 2 * R * 2]; int mr = 0; for (int Y = 2 * R; --Y >= 0;) { int x0 = (int) (Math.sqrt(R * R - (Y - R) * (Y - R)) + 0.5); int p = Y * (R * 2) + R - x0; for (int X = -x0; X < x0; X++) { int x = X + hx; int y = Y - R + hy; int r = (int) (Math.sqrt(x * x + y * y) + 0.5); if (r > mr) mr = r; data[p++] = r = 1; --i) { float d = (float) i / maxr; red[i] = (byte) blend(blend(Rl, 255, d), bgGrey, b); green[i] = (byte) blend(blend(Gl, 255, d), bgGrey, b); blue[i] = (byte) blend(blend(Bl, 255, d), bgGrey, b);
}
IndexColorModel model = new IndexColorModel(8, maxr + 1, red, green, blue, 0); balls[r] = applet.createImage( new MemoryImageSource(R*2, R*2, model, data, 0, R*2));
}
} void paint(Graphics gc, int x, int y, int r) {
Image ba[] = balls; if (ba == null) {
Setup();
ba = balls;
}
Image i = ba[r]; int size = 10 + r; gc.drawImage(i, x - (size >> 1), y - (size >> 1), size, size, applet);
}
183
w w w
APPENDIX C
Java Class Hierarchy
OVERVIEW
The Java Class Hierarchy is shown in the next series of pages. Generally, pages are divided by packages, although some packages are too large to fit on a single page. The boxes that are not part of the class library with the centered text serve as keys to the packages represented on that particular page. 186
JAVA PROGRAMMING: PART 1
®®®
JAVA CLASS HIERARCHY
FIGURE 9
KEY
Class
Interface
Extends
Implements
* - Duplicated
APPENDIX C: Java Class Hierarchy
java.lang
Boolean
Character
Class
ClassLoader
Compiler
Math
Number
Double
Float
Integer
Long
Process
Runtime
SecurityManager
String
StringBuffer
System
Thread
ThreadGroup
Throwable
Runnable
APPENDIX C
Object
187
188
JAVA PROGRAMMING: PART 1
O bject (java.lang)
java.io j ava.lang
File
FileDescriptor
InputStream
ByteArraryInputStream
FileInputStream
FilterInputStream
BufferedInputStream
DataInputStream
DataInput*
LineNumberInputStream
PushbackInputStream
PipedInputStream
E xception (java.lang)
SequenceInputStream
IOException
StringBufferInputStream
EOFException
OutputStream
ByteArrayOutputStream
FileNotFoundException
FileOutputStream
InterruptedIOException
FilterOutputStream
BufferedOutputStream
DataOutputStream
DataOutput*
PrintStream
PipedOutputStream
DataInput*
RandomAccessFile
DataInput*
StreamTokenizer
FileNameFilter
APPENDIX C: Java Class Hierarchy
j ava.lang
StreamTokenizer
j ava.io
ContentHandler
E xception (java.lang)
DatagramPacket
DatagramSocket
InetAddress
ServerSocket
Socket
SocketImpl
URL
I OException (java.io)
MalformedURLException
ProtocolException
SocketException
UnknownHostException
UnknowenServiceException
URLConnection
SocketImpFactory
URLEncoder
URLStreamHandlerFactory
URLStreamHandler
ContentHandlerFactory
APPENDIX C
java.net
189
190
JAVA PROGRAMMING: PART 1
java.lang
T hrowable
j ava.awt
Error
A WTError
LinkageError
ClassCircularityError
ClassFormatError
IncompatableClassChangeError
AbstractMethodError
IllegalAccessError
InstantiationError
NoSuchFieldError
NoSuchMethodError
NoClassDefFoundError
UnsatisfiedLinkError
VerifyError
ThreadDeath
VirtualMachineError
InternalError
OutOfMemoryError
StackOverflowError
UnknownError
APPENDIX C: Java Class Hierarchy
j ava.lang
ColorModel
DirectColorModel
IndexColorModel
ImageConsumer*
ImageFilter
C loneable*
CropImageFilter
RGBImageFilter
FilteredImageSource
ImageProducer*
MemoryImageSource
ImageProducer*
PixelGrabber
ImageConsumer*
APPENDIX C
java.awt.image
O bject (java.lang)
191
192
JAVA PROGRAMMING: PART 1
O bject (java.lang)
BorderLayout
LayoutManager*
CardLayout
LayoutManager*
CheckBoxGroup
Color
Component
I mageObserver*
Button
Canvas
java.awt
Checkbox
j ava.awt.image
Choice
j ava.applet
Container
Panel
A pplet
Window
Dialog
FileDialog
Frame
MenuContainer*
Label
List
A ppletContext
Scrollbar
TextComponent
TextArea
TextField
A ppletStub
A udioClip
APPENDIX C: Java Class Hierarchy
j ava.lang
Event
FlowLayout
LayoutManager*
Font
FontMetrics
Graphics
GridBagConstraints
C loneable*
GridBagLayout
LayoutManager*
GridLayout
LayoutManager*
Image
Insets
C loneable*
MediaTracker
MenuComponent
MenuBar
MenuContainer*
MenuItem
Menu
CheckBoxMenuItem
Point
Polygon
Rectangle
Toolkit
MenuContainer*
APPENDIX C
java.awt
Dimension
193
194
JAVA PROGRAMMING: PART 1
java.util
O bject (java.lang)
j ava.lang
B itSet
C loneable*
Date
Dictionary
HashTable
C loneable*
Properties
Observable
Random
StringTokenizer
Enumeration
Vector
C loneable*
Stack
Observer
w w w
ANSWERS
To Review Questions
196
JAVA PROGRAMMING: PART 1
®®®
LESSON 2
Review questions, page 46
1. javac
2. Browsers usually run Java applets, though several programs (including appletviewer and HotJava) can also run Java applets.
3.
4. False. Java applications require a main method, but Java applets do not.
5. False. import merely allows programmers to type shorter names when accessing classes or methods.
®®®
LESSON 3
Review questions, page 74
1. a. int
b.int
c. long
d. double
e. double
2. a. The & operator always evaluates both its left and right operands. The
&& operator always evaluates its left operand, then evaluates its right operand only if the result of the left operand is true.
b. The >> operator fills the high bits of the result with the sign bit of the operand, retaining the original sign in the result. (This is an arithmetic right shift, because it is equivalent to division by a power of two.) The
>>> operator fills the high bits of the result with zero. (This is a logical right shift.)
3. The variable named half has gone out of scope at the close of the compound statement within the do..while loop, but it appears in a relational expression afterward.
ANSWERS: To Review Questions
LESSON 4
Review questions, page 91
1. The declaration is as follows:
Animal Lion = new Animal(500, 45);
2. The following two lines are needed:
Lion.weight = 250;
Lion.length = 35;
3. There may be only one public class in any particular source file.
®®®
LESSON 5
Review questions, page 103
1. A class with default protection can be accessed (and instantiated) only by other classes in the same package.
2. final
3. static
®®®
LESSON 6
Review questions, page 117
1. The length data member is read-only, and as a result can not be explicitly modified.
2. The two methods are: new_sport = sport.concat(“ball”);
// or new_sport = sport + “ball”;
ANSWERS
®®®
197
198
JAVA PROGRAMMING: PART 1
3. StringBuffers are needed to modify character arrays, since regular
Strings are immutable.
®®®
LESSON 7
Review questions, page 136
1. The declarations are as follows: public class Animal
{
} public class Dog extends Animal
{
} public class Bulldog extends Dog
{
}
2. Both protected and private protected members are accessible to derived classes. However, a protected member is accessible to any class in the same package, while a private protected member is not.
3. The super keyword is used in a derived class constructor to call the base class’s constructor.
®®®
LESSON 8
Review questions, page 160
1. java.applet.Applet
2. getParameter
ANSWERS: To Review Questions
ANSWERS
3. The HTML code is:
199
ZDU Student Manual
JAVA
PROGRAMMING:
PART 1
Java Programming: Part 1
ISBN:
0-73725-349-5
Part number:
ZDU56705
ACKNOWLEDGMENTS
Content Development
The content of this self-study guide is based on the training course "Java Programming," developed by Instruction Set, Inc. for its curriculum of instructor-led technical training.
This guide was designed and developed by an Instruction Set team of instructional designers, course developers, and editors.
Administration
Vice President and General Manager of ZD University: Ed Passarella
Marketing Director: Risa Edelstein
Director, ZD University: George Kane
Senior Editor, Curriculum: Jennifer Golden
Project Director, Instruction Set: Laurie Poklop
Project Manager, Instruction Set: Sandy Tranfaglia
DISCLAIMER
While Ziff-Davis Education takes great care to ensure the accuracy and quality of these materials, all material is provided without any warranty whatsoever, including, but not limited to, the implied warranties of merchantability or fitness for a particular purpose.
Trademark Notices: ZD University and Ziff-Davis Education are trademarks and service marks of ZiffDavis Inc. Java Programming: Part 1 is a Copyright of Instruction Set, Inc. All other product names and services used throughout this book are trademarks or registered trademarks of their respective companies. The product names and services are used throughout this book in editorial fashion only and for the benefit of such companies. No such use, or the use of any trade name, is intended to convey endorsement or other affiliation with the book.
Copyright © 1998 Instruction Set, Inc. All rights reserved. This publication, or any part thereof, may not be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, storage in an information retrieval system, or otherwise, without the prior written permission of Instruction Set, Inc., 16 Tech Circle, Natick, MA 01760, (508) 651-9085, (800)
874-6738. Instruction Set’s World Wide Web site is located at http://www.InstructionSet.com.
ZD University’s World Wide Web site is located at http://www.zdu.com.
Photocopying any part of this book without the prior written consent of Instruction Set, Inc. is a violation of federal law. If you believe that Instruction Set materials are being photocopied without permission, please call 1-800-874-6738
CONTENTS
Java Programming: Part 1
LESSON 1: FUNDAMENTAL ASPECTS OF PROGRAMMING
Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
What Is Programming? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
A Brief Overview of Software Development . . . . . . . . . . . . . . . . .3
Conceptual Differences Between Software and Hardware. . . . . . .4
Conceptual Differences Between the Developer and the User . . . .4
Programming Languages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
Syntax vs. Semantics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
Types of Programming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Console Versus Windows-Based Applications . . . . . . . . . . . . . . .6
Procedural Programming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
Object-Oriented Programming . . . . . . . . . . . . . . . . . . . . . . . . . . .7
Event-Driven Programming . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
C/C++/Java Language Family . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
The Common Language Core . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
Identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Data Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Expressions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Input and Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Program Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
Control Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16
Scoping. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
Lesson Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
iv
JAVA PROGRAMMING: PART 1
LESSON 2: THE JAVA ENVIRONMENT
Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Uses for Java . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29
The World Wide Web (WWW) . . . . . . . . . . . . . . . . . . . . . . . . .30
HTML . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30
Competing Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31
Java Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Java Portability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33
Advantages and Disadvantages. . . . . . . . . . . . . . . . . . . . . . . . . . .34
Applications vs. Applets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34
Application/Applet Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Writing Source Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35
Compiling an Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37
Running the Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38
Tools and Packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
The javadoc and jdb Tools . . . . . . . . . . . . . . . . . . . . . . . . . .41
Using packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Searching for Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42
The import Statement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43
Packages, Classes, Files, Directories. . . . . . . . . . . . . . . . . . . . . . .44
Lesson Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
LESSON 3: JAVA BASICS
Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
Language Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50
Data Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .51
Declaring Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .53
Literals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54
Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
Java Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .56
Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .57
Precedence and Associativity . . . . . . . . . . . . . . . . . . . . . . . . . . . .61
Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
Control Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .65
CONTENTS
Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
Lesson Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
LESSON 4: CLASSES IN JAVA
Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
Java Is Object-Oriented . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
Classes, Objects and Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
Instantiating a Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
Class-Type Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
Operations on Class-Type Variables. . . . . . . . . . . . . . . . . . . . . . . . . . 81
The null Value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
Member Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
Invoking a Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
Class Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
Declaring Instance Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
Declaring Instance Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
The this Variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
Class Definitions and Source Files. . . . . . . . . . . . . . . . . . . . . . . . 89
Lesson Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
LESSON 5: CLASSES IN JAVA—II
Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
Method Overloading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
Constructors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
Encapsulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
Access Specifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
Comparing Objects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
Class Variables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
Class Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 final Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
Class Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
Finalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
Lesson Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
v
vi
JAVA PROGRAMMING: PART 1
Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
LESSON 6: ARRAYS AND STRINGS
Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
Java Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
Array Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
Using Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
Copying Array Elements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
String Objects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
String Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
String Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .110
String Searching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .112
Other String Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .113
String Concatenation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
Converting Objects to Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
Converting Strings to Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
Lesson Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
LESSON 7: INHERITANCE
Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
Introduction to Inheritance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
Example of Inheritance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .121
Derivation Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .122
Effects of Inheritance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .122
Protected Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
Overriding Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
Dynamic Method Dispatching. . . . . . . . . . . . . . . . . . . . . . . . . .125
Polymorphism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .126
The super Keyword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .127
Final Methods and Final Classes . . . . . . . . . . . . . . . . . . . .128
Constructor Chaining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
Inheritance and Finalization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
Abstract Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
Interfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
CONTENTS
The implements Declaration . . . . . . . . . . . . . . . . . . . . . . . . 133
Casting Between Class Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
Lesson Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
LESSON 8: WRITING JAVA APPLETS
Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
What Is an Applet? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
A "Hello, World" Applet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
The Applet Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
Invoking an Applet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
Getting Applet Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
Specifying Applet Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 146
The Delegation Event Model—Action Events . . . . . . . . . . . . . . . . 146
The ActionEvent Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
Adjustment Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
The paint() Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
The Graphics Class. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
Java Fonts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
Selecting a Font . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
Drawing Lines and Shapes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
Drawing with Color . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
The Color Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
Foreground and Background Colors . . . . . . . . . . . . . . . . . . . . . 157
Lesson Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
APPENDIX A: HYPERTEXT MARKUP LANGUAGE (HTML)
HTML History and SGML . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Head Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Formatting: Blocks and Separators . . . . . . . . . . . . . . . . . . . . . . . . . .
Formatting: Physical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Netscape List Extensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
164
165
165
166
166
167
167
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JAVA PROGRAMMING: PART 1
Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
Images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
Tables (HTML 3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
Miscellaneous Netscape Extensions. . . . . . . . . . . . . . . . . . . . . . . . . . 170
Java Applets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
Java Script . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
APPENDIX B: JAVA SAMPLE
Usage and Copyright Notification . . . . . . . . . . . . . . . . . . . . . . . . . . . 174
The XYZApp.java Source. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
APPENDIX C: JAVA CLASS HIERARCHY
Java Class Hierarchy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
ANSWERS: TO REVIEW QUESTIONS
Lesson 2
Lesson 3
Lesson 4
Lesson 5
Lesson 6
Lesson 7
Lesson 8
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
w w w
LESSON 1
Fundamental Aspects of
Programming
OVERVIEW
Learning to use a programming language effectively requires a good background in the principles of software development. The novice programmer must visualize the software development process as a whole, from requirements analysis through testing. The various types of programming and their implications must be understood clearly. To facilitate this, the several basic components of programming and programming languages are explicated in this lesson by applying them to an incrementally developed software example.
LESSON TOPICS q What Is Programming? q Types of Programming q The Common Language Core q Program Structure
2
JAVA PROGRAMMING: PART 1
®®®
OBJECTIVES
By the end of this lesson, you should be able to:
® Explain the nature of software development.
® Differentiate between the different types of programming approaches.
® Understand the similarities and differences between C, C++ and Java.
® Edit, compile, and run simple console based applications.
®®®
WHAT IS PROGRAMMING?
Before tackling Java programming, it is important to understand what programming is and how it is used. People program to create software, which can take many forms such as spreadsheet applications, Web applets, operating systems or corporate transaction databases.
Software consists of a series of instructions that tells hardware (or another piece of software) to perform one or more actions. The programming language is the set of syntax used to create those instructions and the rules associated with their usage.
Like other natural languages, programming languages allow for some freedom and creativity in how a piece of software achieves its purpose. However, unlike in spoken languages, programming must rigorously adhere to the rules and constructs of the programming language to achieve precise results.
Machines cannot guess at meaning like humans can. Therefore, a good programmer must be very good at taking a problem or an idea in real world human terms, and abstracting it into the constructs provided by the programming language.
Programming generally refers to the coding portion of software development.
Good software development, however, requires several other steps in addition to coding.
LESSON 1: Fundamental Aspects of Programming
3
To develop an application, the developer normally follows a series of steps such as the following:
® Requirement Analysis
® Design
® Coding
® Testing
Requirement analysis is the process of determining the requirements of the application. Typical application requirements might be, “to support a spreadsheet-like interface” or “to calculate the precision of a result to 5 significant decimal places.”
The requirement analysis phase is perhaps the most important step in the development process. Application requirements need to be clearly understood and articulated if the software is to be a success. Proper requirements analysis will help avoid unnecessary corrective effort later in the process.
The design phase may occur at a high or low level. This step is responsible for the overall structure of the application and how the parts of the application interact. Coding is the process of converting a design into a series of instructions through a programming language. Source code is written and then put through a code compiler, which translates the code in machine level instructions.
Before putting code into use, it needs to be tested to locate and remove errors. Testing checks the application against the requirements, tests the application functions with sample data, and verifies whether the user will find the application easy-to-use, among other steps. When the application behaves abnormally, corrections are made to the software and the testing process resumes. When testing is complete the application is ready to use.
LESSON 1
A Brief Overview of Software
Development
4
JAVA PROGRAMMING: PART 1
Conceptual Differences Between
Software and Hardware
Software is a series of instructions to a machine. The computer takes the instructions and executes them to produce the intended results. Software can be readily changed. It can be modified to make changes in the behavior of the application. These changes are changes in the application itself.
The hardware that makes up the computer does not readily change. At the heart of the computer is a central processing unit that executes machine level instructions. These instructions are very low level and are not easy for the average programmer to use. The actual set of instructions for any language is fixed. However, they are recombined in different sequences that result in different behavior for different programs.
Conceptual Differences Between the
Developer and the User
The end user is the individual that executes an application for the purpose of performing a task. The developer is the individual that creates the application.
There are different types of developers. Some developers specialize in the requirement analysis or design phase of a project. There are others who primarily code and test an application. For small to medium size projects, one developer or a small group of developers may perform all of these tasks.
In addition, there are different types of applications that require different types of development expertise. Database intensive applications will require the skills of a developer proficient in the architecture and mechanics of databases, usually a specific database such as Oracle. Other developers will specialize in real-time applications that require carefully timed responses to system inputs.
Programming Languages
A variety of different programming languages have evolved over time. Each of these languages has its own unique syntax and semantics. Languages are often categorized as either high level or low level, though this simple classification is not always adequate.
LESSON 1: Fundamental Aspects of Programming
5
Certain high-level languages are specialized. For example, there are languages designed to work directly with databases. Others are targeted towards developing graphics applications.
Low-level languages have a one-to-one correspondence between a statement in the language and a machine instruction. These languages are normally called assembly languages and are unique to a specific computer. They provide a great deal of control over what can be done by a program but are very difficult and tedious to use.
Some languages rely upon an interpreter instead of a compiler. The interpreter is an application that takes high level instructions for a given language and simulates the execution of the instructions. The interpreted application will produce the same results as if it were compiled. Interpreted code executes slower than a compiled version of the application but interpreters provide several advantages over compilers, including better portability and more powerful development features. Java is an example of a language that is frequently interpreted rather than compiled. Any language may be compiled or interpreted if a compiler or interpreter has been written for that language.
Syntax vs. Semantics
There are syntactic rules for each language that determine whether a given command is in the proper form. Like rules that govern the form of an
English sentence, syntax rules of a programming language determine whether the program is valid or invalid.
When a program violates the syntax rules of the language, the compiler will generate an error message and will usually stop the compilation process. If no errors are generated, the program is said to have compiled cleanly. The program will run, but that does not necessarily mean that the program will do what it is intended to do. The program must order instructions in such a
LESSON 1
Most developers work with high-level languages such as COBOL, C or
C++, Java, and FORTRAN. High-level languages do not use instructions that the machine understands directly; instead, they utilize commands that are more intelligible for humans. Naturally, high-level languages are easier to use. Few high-level language instructions are required to perform the same task in a low-level language. While programmers using a high-level language lose direct control of instructions to the machine, this is not a concern for most developers.
6
JAVA PROGRAMMING: PART 1
way that they produce the correct result. In other words, the program must be semantically correct.
The semantics of an instruction is the meaning of the instruction i.e., what the instruction does. Understanding this is fundamental to learning programming languages. Fortunately, this is normally a straight-forward process. High-level programming languages are designed with logical and intuitive semantic structures.
®®®
TYPES OF PROGRAMMING
There are several types of programming. This section, by no means exhaustive, will discuss some of the key types:
® Console-based
® Windows-based
® Procedural
® Object-oriented
® Event-driven
Console Versus Windows-Based
Applications
Console based applications use an MS-DOS type user interface (often called character driven) for input and output. The application is limited to character-based displays of information. Frequently, the order of data entry is dictated by the application. There are rarely windows of any nature present in these.
Windows programming use extensive amount of graphics programming to provide a more diverse range of input and output features. This graphics programming may be hidden from the application developer but is supported by various operating system libraries.
Windows programming offers the developer freedom to use different
Graphical User Interface (GUI) controls to display or obtain information from the user. Text Boxes can be used for textual information, lists can be
LESSON 1: Fundamental Aspects of Programming
7
Console-based programming is easier than windows programming and does not use as many system resources. Windows programming is more difficult to learn and is resource intensive. However, given that most people expect windows based applications these days, creating windows is often an application requirement. The demand for windows has spurred significant advances in computer processing power and windows development tools.
Procedural Programming
Procedural programming is used by almost all programming languages. Procedural programming focuses the step-by-step sequence of instructions that are needed to perform a task. This approach has been needed because applications and computers are not capable of automatically deciding the appropriate sequence to use.
Non-procedural programming is characterized by the developer specifying what needs to be done rather than how the task is to be performed. Procedural programming involves instructing the system as to how to achieve a task. While non-procedural languages exist, these languages are normally limited to a small application domain and are limited in the range of tasks that can perform.
Procedural programming is tricky for large applications. Large procedural programs can become unwieldy and difficult to manage. There is often little opportunity for abstraction. By abstracting a problem, it can be simplified.
The interfaces between parts of an application can become difficult to manage also. Programmers tend to spend a lot of time learning how to program to these interfaces.
Most commonly used languages are procedural, including C, C++, Java and
FORTRAN. These languages have evolved over time but they all have procedural aspects to them.
Object-Oriented Programming
The evolution of object oriented programming brought about a paradigm shift in how application problem sets are modeled for development. In a traditional procedural language like C, things represented in a banking application like account number, customer, and balance dissolve into a series of
LESSON 1
displayed in List Boxes, and mutually exclusive options can be provided through Radio Buttons.
8
JAVA PROGRAMMING: PART 1
functions and variables. In object-oriented programming, objects are defined and maintained that represent these real things that are part of a problem statement. Just as things in real life have characteristics and actions associated with them, objects possess properties, methods, and events. Each of these is directly associated with the object. For example, a bank account object would have the properties of account number and balance. These properties are tied directly to an instance of Bank Account, and do not exist in the program, other than as a part of Bank Account.
Methods are actions performed against the object. For example, the bank account object may have a deposit method that is used to add money to an account. Events are actions that take place under specific conditions. The bank account might have a negative balance event that would occur whenever the balance became negative. The programmer would code this event to take appropriate action when this event occurs.
Object-oriented languages support the definition of a class that represents an object and the actual allocation of memory for the object. Classes are the generalization of a specific object: Customer is a class, and Kelly Smith is an object that is an instance of the class Customer. C++ and Java are examples of object-oriented languages (note that they are also procedural languages).
The coding used in support of methods and events is procedural.
Object-oriented languages offer a number of benefits. Systems modeled in objects are easier to maintain and manipulate, because properties and methods can be altered for a class without affecting the rest of the system.
Object-oriented programming also supports reuse of code: once developed, a class can be utilized as often as necessary. For instance, a programmer could use an existing Button c lass in an application without necessarily understanding all of the code that supports the class. In addition, classes may inherit properties from more general classes, so that the properties do not have to be redefined for each sub class.
An object-based language, as opposed to an object-oriented one, is a language that supports the creation of classes but does not support inheritance, a crucial programming feature which facilitates the creation of related types from existing types. Visual Basic 5.0 and its earlier versions are an example of an object-based language.
LESSON 1: Fundamental Aspects of Programming
9
Event driven programming is frequently used to support Windows programming. The operating system will monitor the system for events and then send these events to the appropriate application. The application’s reaction is based on the nature of the event. Keyboard input and mouse movements are the most common events.
An event-driven application is structured around the process of receiving and reacting to events. Unlike in procedural programming, where the programmer dictates the order of information received, event-driven applications generally allow the user to choose the order in which information is entered. For example, an application may have an entry field for Zip Code. In an event driven application, the Zip Code field will be one in a series of address fields in a dialog box. The user can use the mouse or tab key to jump around to different fields and fill them in any order. The application’s structure must account for the fact that it can not anticipate when Zip Code will be filled, but rather must react when the event occurs.
Object-oriented programming is well suited for event driven applications.
Objects can have events assigned to them, which the application can monitor. Traditional procedural languages such as C, do not support event driven programming very well. Java, in particular, is designed to address events explicitly and has a well defined event model to create and handle events.
C/C++/Java Language Family
C was developed to meet general system programming needs. It was quickly adapted for general use and found widespread acceptance. C is a high-level procedural language that has many low-level features. These features help to make it versatile and efficient. However, many of these features give it a reputation for being cryptic and hard to maintain.
C++ extends the C language to provide object-oriented features. The language is backward compatible with C, and code from the two languages can be used with each other with little difficulty. C++ has found quick acceptance and is supported by a number of pre-built specialized classes.
Java can be considered the third generation of the C/C++ family. It is not backward compatible with C/C++ but was designed to be very similar to these languages. The creators of Java intentionally left out some of the fea-
LESSON 1
Event-Driven Programming
10
JAVA PROGRAMMING: PART 1
tures of C/C++ that have been problematic for programmers. Java is strongly object-oriented. In fact, one cannot create Java code that is not object-oriented. Java’s portability is a key advantage and is the reason why Java is often used for Web development.
®®®
THE COMMON LANGUAGE CORE
There are several programming features common to most programming languages. These include the definition of identifiers, expressions, and condition control statements. These common core features are discussed and illustrated below with sample code where appropriate.
Comments
Comments are sections of a program that are ignored by the compiler and are used to document the program. Documentation is important in that it provides information about the program that is useful in the maintenance of the program.
Software applications generally require periodic revision—the application requirements change, errors are corrected or new features are added. Good documentation aids the programmer in identifying the structure and purpose of each section of code. Often, the person modifying the code is not the same person that wrote the code, making the use of good comments critical.
The only comment that is common to C, C++ and Java is the multi-line comment. The comment begins with a /* and ends with a */. Everything between these two sets of symbols is treated as a comment and is ignored by the compiler.
/* This is a comment spread across several lines */
/* This comment occurs on a single line */ i = i + 1; /* This comment begins on the same line as a statement */
LESSON 1: Fundamental Aspects of Programming
11
i = i + 1;
// Alternate comment notation
Identifiers
Identifiers are used to temporarily hold information. For example, to compute the pay for an individual, the numbers of hours worked is multiplied by the pay rate. Identifiers (the name of the variable listed on the left) are needed for these values. hours = 40; payrate = 12.50; pay = hours * payrate;
Identifiers have certain rules for their creation. Generally, the identifier must begin with a letter and is followed by letters, digits, and perhaps another character, such as the underscore character.
The assignment operator, which is the equal sign (=), is used to modify the value assigned to an identifier. Simply using the identifier as part of some operation will retrieve its current value. The value stored in an identifier can change. Data Types
There are different types of identifiers based on different needs. If simple counting numbers are needed then there is the integer data type. If a number with decimal points is required then the float data type is used. The common data types are listed in Table A.
TABLE A. Data Types
Data Type
Value
Examples
int
Positive and negative integers
127
-6
float
Single precision numbers
6.42
LESSON 1
An alternative style available in C++ and Java uses two forward slashes (//) to indicate that everything from that point until the end of the line is a comment.
12
JAVA PROGRAMMING: PART 1
TABLE A. Data Types (continued)
Data Type
Value
Examples
double
Double precision numbers
(numbers with a very large value or # of digits)
137
.5426246321
5.42 x 1045
char
Single characters
h
char* or String
String data (any series of characters) widget
42XXTALL
Note that very large numbers or very precise numbers use a different data type than normal real numbers. The boundary between single and double precision numbers is defined differently for each language.
String data is handled differently between C/C++ and Java. C and C++ use a pointer to the address of a string of characters that is terminated by a zero.
Java uses a built-in data type String to hold strings.
Before an identifier can be used, it must be declared. Specifying the type of data followed by the identifier’s name does this: int float int double
i; pay; hours; radius; Most languages provide the developer with the ability to define more complex data structures based on primitive data types. C and C++ do this using the struct statement. struct Employee { char*name; int age; double payrate;
};
An actual identifier is declared as other primitive types are declared. struct Employee boss;
LESSON 1: Fundamental Aspects of Programming
13
Expressions are needed to perform computations. An expression consists of an operator and one or more operands. Operators specify the action to be performed, such as addition or multiplication. Operands are the data acted upon. Statements are separated by semicolons and consist of one or more expressions. In the following example, the assignment operator, equal sign, and the addition operator are used to add two numbers together and assign the sum to a third identifier. a = b + c;
There is a predefined order in which expressions are evaluated. This is known as the order of precedence. For example, addition always occurs before assignment (+ happens before =). There also exists rules of associativity that specify the order in which two expressions of equal precedence are to be evaluated. These rules can be overridden through the use of parentheses. For addition, evaluation is done left to right. The following two statements are equivalent: a = b + c + d;
a = b + (c + d);
Input and Output
Getting information into a program and then writing it out are important aspects of programming. Data can be read from a number of sources: keyboard, mouse, files, databases, etc. Data can be outputted to the monitor, to various files, and to a database, among other places.
The syntax of Input/Output (I/O) is mostly language specific. C and C++ rely upon include files that contain specialized code to perform I/O. Java uses a series of classes to do similar I/O actions. Input statements are not addressed here, however, output examples are provided.
LESSON 1
Expressions
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JAVA PROGRAMMING: PART 1
In C, the printf statement is used. It has two major parts: a format specification that is enclosed with double quotes and a list of data values to be printed. The following shows printing out pay information: printf(“Hours worked: %f Payrate: %f hours, payrate, pay);
Pay: %f\n”,
The %f fields within the format string are “place holders” for the output values to be printed. The text printed will be that which is inside of the double quotes, with the %f field replaced by the values stored in the variables in the parameter list. The three %f fields are replaced with the values stored in hours, payrate and pay. The \n is used to cause a carriage return and a new line to be printed. Output goes to standard output, which is normally the monitor. For floating values, a %f file is used. For other data types, different place holders are used.
In C++, insertion operators (>
integer
L
right shift with zero extension
=
arithmetic
L
less than, greater than, or equal to
instanceof
object, type
L
type comparison
==, !=
native
L
have same or different values
==, !=
object
L
refer to the same or different object
7
&
integer or boolean
L
bitwise AND or boolean AND
8
^
integer or boolean
L
bitwise XOR or boolean XOR
9
|
integer or boolean
L
bitwise OR or boolean OR
10
&&
boolean
L
conditional boolean AND
11
||
boolean
L
conditional boolean OR
12
?:
boolean, any, any
R
conditional operator (ternary)
13
=
variable, any
R
assignment
*=, /=, %=,
+=, -=, =, >>>=,
&=, ^=, |=
variable, any
R
assignment after operation
4
5
6
Associativity
When two operators have the same precedence, associativity determines evaluation order.
For example, the multiplicative operators (*, /) have the same precedence and associate left to right. Therefore, the example below has the value 9, not
1.
6/2*3
LESSON 3
TABLE D. Operator Precedence
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JAVA PROGRAMMING: PART 1
For the assignment operators, right to left association is more appropriate.
The two examples below are equivalent. a=b=3 a=(b=3)
This means, “set b’s value to 3; copy its value to a.”
Assignment: This is an operator with right to left associativity which changes the left-hand argument’s value and produces a value for the next operation. The above example assigns 3 into b and produces the value of 3.The next assignment operator assigns 3 into a. This also produces a value of 3 (which is thrown away).
This reverse associativity makes it easy to assign several variables at once: a=b=c=d=e=0 ®®®
STATEMENTS
Java, like C, is an expression-based language. Expressions must always appear in the context of statements. A statement consists of an expression followed by a terminating semicolon (;).
Like in C, the Java format of text on a line has no significance to the compiler except for:
® C++ style comments which end at the end of a line
® quoted string literals (e.g., “Hello World!”) which cannot span more than one line
® white space that the compiler recognizes as the separator between keywords, identifier names, operators, and other symbols
Statements can be made up of zero or more expressions, provided that their combination makes syntactic sense. Expressions (except for the return value
LESSON 3: Java Basics
65
The following are expressions:
®a
®i * j
®z = x * y
Adding semicolons to these expressions turns them into statements:
® a;
® i * j;
® z = x * y;
The first two are not legal and return errors with the 1.0.2 Java compiler.
The third calculates a product and stores it into z.
Control Structures
Flow control in Java uses similar syntax as in C. The syntax for the most commonly used control structures may be summarized as follows:
® if
• if ( boolean-expression ) statement
• if ( boolean-expression ) statement else statement if (i > 0) x = y * z; else // else part is optional x = 0;
LESSON 3
of void methods) have a value and a type. Many involve executable code, though this is not necessary. A singe value (e.g., 27) is a legal expression. A terminating semicolon (e.g., 27;) turns it into a legal statement even though it does not do anything useful. A simple statement consists of an expression and a semicolon. The semicolon is a required terminator.
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JAVA PROGRAMMING: PART 1
® while
NOTE
The difference between while and do-while refers to when the condition check is done— before or after executing the loop.
while (boolean-expression ) statement
while (getMember (i)) i++; do statement while (boolean-expression)
do a += a; while (a > ?
75
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JAVA PROGRAMMING: PART 1
EXERCISE
1. Go to the exercise subdirectory of the Java directory. Edit the file
DivTest.java. In the area provided, insert Java code that prints one of the following two messages:
® x is evenly divisible by y
® x is not evenly divisible by y depending on whether the value of variable y evenly divides the value of variable x in the DivTest class.
Modify your program to print the values of x and y instead of “x” and
“y”. Test your program by setting various values for x and y, recompiling and running your program. What happens if you set y to zero and run?
2. Still in the exercise subdirectory, look for a source file named Digits.java. Edit the file, which contains the beginnings of a program to take the value of the variable x and print an English word for each of its digits. For example, if the value of x is 496, the program prints: four nine six
Finish the program. (Hint: use a switch statement to translate a digit to a string.) Test your program. Make sure that it works for a value of zero. w w w
LESSON 4
Classes in Java
OVERVIEW
A class is a user defined type created by grouping together pertinent methods and data members. The class is a key component of Java language, as it facilitates modularity and code reuse. Once created, a new type can be used in expressions in the same way as a built-in type (such as int or char). While Java classes are, on the whole, similar to C++ classes, there are still small but crucial differences.
LESSON TOPICS q Java Is Object-Oriented q Instantiating a Class q Class-Type Variables q Operations on Class-Type Variables q The null Value q Member Access q Class Definitions
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JAVA PROGRAMMING: PART 1
®®®
OBJECTIVES
By the end of this lesson, you should be able to:
® Work with Java class instances.
® Declare and initialize instance variables.
® Access data members and methods of an instance.
®®®
JAVA IS OBJECT-ORIENTED
Java is an object-oriented (OO) programming language. A Java program may be thought of as a system of interacting objects in which each object has its own state and behavior. The form and behavior of each object is defined by the object’s class. A class is a data type and a code module. Designing a
Java program means designing the classes that make up the program.
Object-oriented program design builds from the bottom up as much as it does from the top down. Objects are constructed from other objects, and they may, in turn, become the components of larger objects. The developer abstracts the behavior of similar objects into class definitions. Similar classes are further abstracted into superclasses, resulting in hierarchies of related classes. Viewed from the opposite perspective, the programmer defines classes that are highly abstract, derives more specialized classes from them, then creates the actual objects as instances of those classes.
Object-oriented programming results in programs whose structures more accurately map to the problem domain and are thus are more likely to meet users’ needs. The object model carries modularity to a much higher level of abstraction than the structured model. The cross linking that makes both development and maintenance a painful exercise in tracing subtle sideeffects is greatly reduced. This modularity, together with the more flexible structure of OO programs, makes software easier to develop, debug, enhance, and modify.
The generalization of highly modular objects into classes and classes into families of related types also permits much greater reuse of code than has been achieved hitherto by way of function libraries, database systems, and other traditional means. Reusability combines with streamlined development and maintenance to make programmers far more productive.
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A Java class definition establishes the interface and implementation of a category of objects.
Using a syntax rooted in that of C structures, the programmer defines data members and function members of a class. Data members specify the internal data representation. Function members serve two purposes: their headers
(names, parameters, and return types) specify the interface visible to code outside the class, and their bodies specify the methods by which objects of the class perform the work they are designed to do.
User code takes advantage of a class specification by instantiating the class,
i.e., by creating instances of it. A variable of a programmer defined type is declared with the same syntax that is used to define variables of built-in types. Each instance of a class is an object. The object’s data members can take on values unique to that object, while its member functions implement the behavior common to all objects of the same class. There can be any number of instances of a given class at any one time.
A Java class-type variable identifies an instance. It does not contain an instance, as is the case in some other object-oriented programming languages. There can be any number of variables referring to a given object at any one time.
®®®
WARNING
Unlike C++, a declaration like
Box myBox; does not create an instance of the Box class. The instance is created only after new is called on Box.
INSTANTIATING A CLASS
With the exception of array objects, class-type objects may be instantiated only through the new operator. The keyword new must be followed by the name of the class to be instantiated, and then by a list of initialization parameters. The new operation allocates space for a new object, initializes the new object, and returns it. For example, the creation of an instance of
Box (object) is shown in Figure 3:
LESSON 4
Classes, Objects and Variables
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JAVA PROGRAMMING: PART 1
FIGURE 3
Creation of an instrance of Box
Box mybox = new Box (2.0);
class name
constructor keyword object name
initialization parameter
The new operator allocates space for the new object, calls a class constructor and returns a reference to the object.
It should be noted that the class and its constructor have the same name.
®®®
CLASS-TYPE VARIABLES
Creating the variable looks just like a native-data type definition. However, the variable is simply a reference. It does not actually instantiate the object.
Class-type variables are references, not objects themselves. Instantiation of an object with the keyword new and declaration of a class-type variable are separate operations. These two steps may be combined in one statement as an initialization. For example:
Box myBox = new Box (2.0);
A class-type variable refers to (points to) an instance; it does not contain an instance. There can be any number of variables referring to a given instance at any one time. The difference between a class-type variable and a variable of native type which contains a value of its type is to be noted.
LESSON 4: Classes in Java
OPERATIONS ON CLASS-TYPE
VARIABLES
All class-type variables support the following operations:
® assignment
® ==
® !=
The assignment operator reassigns the object to which the variable refers.
Assignment does not modify the value of any object.
Suppose there is a class named Box:
Box box1;
Box box2 = new Box (2.0, 3.0);
Box box3 = new Box (box2); box1 = box2; box3 = new Box ();
When a class-object variable is used in code, it does not imply the value stored in the variable. Rather, it implies the object variable references. In the example above, box1 and box2 are set to reference the same object. Then box3 is given a brand new object. This is very different from the behavior of native-data type variables. In this sense, a class variable is more like a class pointer from C++ than a class object.
LESSON 4
®®®
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JAVA PROGRAMMING: PART 1
Below is an illustration of reference semantics:
® two variables initialized to refer to two separate objects:
Box box1 = new Box (1.0, 1.0);
Box box2 = new Box (2.0, 2.0);
® assignment: box1 = box2;
® the effect of the assignment:
• box1 and box2 refer to the same object.
• The object box1 referred to previously is available for garbage collection.
• Neither object changed in value.
®®®
THE null VALUE
Another way in which Java class-type variables are similar to C and C++ pointers is that Java class-type variables may be null. A Java class-type variable with the special value null refers to no object. The value null is compatible with any class type, i.e., null may appear wherever a class-type value is expected. Attempting to access a member of a null variable results in a run-time error.
LESSON 4: Classes in Java
NOTE
If radius were a class variable, it could be accessed as
Circle.radius.
Unlike C++, you don't need to use :: after class name in
Java.
MEMBER ACCESS
Once a class has been instantiated, variables for that instance are accessed by the “.” symbol (dot).
For example, suppose that myCircle is a variable of class Circle, and class Circle has a data member named radius of type double; then the following statements are legal: myCircle.radius = 42.6; double rad = myCircle.radius;
Programmers familiar with C have to note that there is no -> operator because object variables are not pointers.
Invoking a Method
Objects are usually thought of as self-contained entities whose characteristics are defined by, among other things, the set of actions they are intended to perform. A central concept of OO programming is that objects pass messages to each other. Objects can be seem as being self-contained and responsive, like living organisms reacting to signals from the world around them.
External code triggers actions from, and gets information about, objects only by sending messages to them and interpreting the responses. A call to an object’s method is a message to that object requesting that it perform some specified action.
A message has three components: a target, a selector, and a parameter list. Java reflects its derivation from C by implementing messages as function calls, using a syntax like that used to access members of structures. The user code references the object (the target), then invokes a function which is a member of that object (the selector), and supplies the arguments that function requires (the parameter list).
LESSON 4
®®®
83
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JAVA PROGRAMMING: PART 1
For example, suppose the object myCircle contains the following methods:
® getArea, which returns a double
® moveTo, which takes two double arguments double area = myCircle.getArea (); myCircle.moveTo (0, 0);
®®®
CLASS DEFINITIONS
Every data item or method in a Java program, even the starting function main(), must be declared as a component of a class.
A class definition consists of the keyword class followed by the class name, then a set of curly brackets {} that designate the body of the class.
Variables and methods declared here are called class members. Class member declarations, and the class itself, can be preceded by an access specifier. Public classes can be instantiated by any other code.
LESSON 4: Classes in Java
85
public class Box { double height, width;
// Constructors public Box(double h, double w) { height = h; width = w;
}
public Box(double s) { height = width = s;
}
public double getHeight() { return height;
}
public double getWidth() { return width;
}
public double getArea() { return height * width;
}
}
Figure 4 illustrates what the header communicates.
FIGURE 4
Meaning of headers
public class Box
Any other class can use this class.
This class is named Box.
This is a class type.
The class body (enclosed in {...}) defines the members of the class: variables, or data members, methods, or function members. Members can be
LESSON 4
The following is an example of a Java class definition:
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JAVA PROGRAMMING: PART 1
declared public—they are not public by default. A private member can only be accessed by another member of that class. A public member can be accessed directly from outside the object. The principle of encapsulation infers that data members should be private and that at least some of the function members should be public. The public methods define an object’s interface. Declaring Instance Variables
All variables, unless declared static (discussed later), are replicated for every instance of the class and therefore are referred to as instance variables.
For example, in the class definition for Box, the following code defines two instance variables: double height, width;
Every instance of type Box receives its own height and width.
When a class is instantiated, an object of that type is created; the new instance obtains its own copy of every member variable including the methods. For example, consider the following class: class Color { int r, g, b; public setColor(int red, int grn, int blu) { r = red; g = grn; b = blu;
}
}
If background was the name of an instance of Color, then the color could be defined by calling its setColor method: background.setColor(255, 100, 0);
Note that the color could not be set by assigning values to the data members directly because they are private and not public.
LESSON 4: Classes in Java
87
® background.r = 255;
® background.g = 100;
® background.b = 0;
Declaring Instance Methods
Methods defined inside a class are instance methods by default. An instance method must be applied to an instance of the class of which it is a member.
For example, the class Box defines the methods:
® public double getHeight() { ... }
® public double getWidth() { ... }
® public double getArea() { ... }
These methods can be applied to any instance of the class Box.
Each method is defined fully within the class definition: the body of the method is enclosed within curly braces ({...}).
Methods can refer to instance variables of the target object directly: public double getArea() { return height * width;
}
TRIVIA
This conforms to the concept of encapsulation in object-oriented programming in which class data is held privately within the class and outside code uses functions rather than directly accessing class data.
Suppose we have a variable of type Color named background. We might choose to “blacken” the background color by applying the method setColor() to the instance background, by calling: background.setColor(255, 255, 255);
This applies the method setColor() to the instance background by using the member access symbol “.” (dot). Inside the method, the variables are used by name and they are taken as applying to this instance’s copy of each. Methods like this are called instance methods because they can be applied to individual instances.
LESSON 4
The following statements would all generate compilation errors:
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JAVA PROGRAMMING: PART 1
The this Variable
Java provides a keyword called this which allows an object to refer to itself.
Unlike the this pointer in C++, the Java version is simply another name for the object. For example, if an object has a data member width, then the following statements are equivalent when used within one of the object’s function members: width = w;
this.width = w;
WARNING
If a parameter name is the same as a class member name, an unqualified name will refer to the parameter and not to the class member. this must be used to refer to the class member.
It is a good practice to avoid having the same name for parameters and members. The this keyword is not often needed, but there are times when it is useful. Whenever an instance method must refer to the object of which it is a member (or one of its constructors), it can use this to do so. It can also be used to distinguish between parameter variables and data members having the same name. public class Box { double height, width; public Box(double height, double width) {
// Note that the parameter variables have the same
// name as the data members, so we use the this
// keyword when referring to the data members. this.height = height; this.width = width;
}
public Box(double s) {
// Here we use the this keyword to invoke the
// two-parameter constructor with the single
// parameter,saving us the trouble of
// rewriting the assignment statements. this(s, s);
}
// Other methods...
}
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89
Each class is defined entirely within its source file. Unlike C and C++, there are no header files in Java. While compiling a class, the Java compiler examines other compiled classes to check references and types across class boundaries.
There can be at most, one public class per Java source file. A Java source file may contain any number of non-public classes, but those classes are limited to use within the same package and are best used as “helper” classes for the main public class in the same source file. If a Java source file contains a public class, the name of the source file must match the name of the class, i.e., the source file must be named .java, where is the name of the public class defined within.
LESSON 4
Class Definitions and Source Files
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JAVA PROGRAMMING: PART 1
®®®
LESSON SUMMARY
In this lesson, you have learned:
® Java classes are user defined types that facilitate code reuse and modularity.
® A user defined type is used in the same way as a built-in type.
® Class members are accessed using the dot (.) operator.
® The this keyword allows an object to refer to itself.
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91
1. How would you declare an object of type Animal named Lion that takes a weight of 500 and length of 45 as parameters?
2. What code would change the weight to 250 and the length to 35 in the above Lion instance? Assume that weight and length are the names of the Animal data members.
3. How many public classes are allowed in a Java source file?
Answers on page 197
LESSON 4
REVIEW QUESTIONS
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JAVA PROGRAMMING: PART 1
EXERCISE
1. Create a new class named Clock. (You may put your new class in any package you like.) Class Clock has the following attributes
• Hour (integer 1..12)
• Minutes (integer 0..59)
• Seconds (integer 0..59)
•
isAM (boolean):
These attributes are represented by data fields with default access protection. Class Clock supports the following public extractor methods:
• getHours()
• getMinutes()
• getSeconds()
• getIsAM()
Class Clock may be constructed in any of the following ways:
• no parameters (sets clock to midnight)
• hour and isAM specified (sets minutes and seconds to zero)
• all attributes specified
Class Clock also supports a setTime() method that sets all four attributes from argument values. Compile the new class.
w w w
LESSON 5
Classes in Java—II
OVERVIEW
Java classes have some additional features that aid program development. For example, class methods may be called with varying numbers and types of parameters by using overloading. Two keywords, static and final, can be used to further adjust the behavior of class members. Another useful mechanism is the finalize method, which performs clean-up work when a class instance is destroyed.
LESSON TOPICS q Method Overloading q Constructors q Encapsulation q Access Specifiers q Comparing Objects q Class Variables q Class Methods q Finalization
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®®®
OBJECTIVES
By the end of this lesson, you should be able to:
® Apply overloading mechanisms.
® Employ the static and final keywords.
® Use initialization syntax.
® Understand finalization methods.
® Write complete Java classes.
®®®
METHOD OVERLOADING
Overloading means declaring a method with the same name, more than once. To distinguish between the different versions, the compiler uses the supplied parameter list. In other words, a method is identified by its signature, made up of both the name and the types of its parameters. The return type of a method is not a part of its signature. An overloaded method must differ from other methods of the same name in number and/or types of arguments: ® public double dothis (Circle cx) {...}
® public void dothis (double x) {...}
® public void dothis () {...}
A method’s return type does not distinguish it: public double dothis (Circle cx) {...} public void dothis (Circle cx) {...}
//
// ERROR: these methods conflict
//
When a call to an overloaded method is made, the compiler must perform overload resolution to determine which method is being called. The compiler considers all the methods of the same name and number of arguments
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®®®
WARNING
While there may be multiple constructors for a single class, they must differ somehow in the number or types of arguments. There cannot be two constructors for one class with exactly the same number and types of arguments.
CONSTRUCTORS
A constructor is a special method that initializes a newly instantiated object.
Constructors have the same name as the class and no return type, not even a void. They can take any number of arguments. They are called whenever a class is instantiated, and are therefore used to initialize that instance. Constructors can be overloaded, so a class programmer can offer various ways for a class user to instantiate and initialize the class.
The following example gives the Box class two constructors: public class Box { double height, width; public Box(double hw) { height = width = hw;
}
public Box(double h, double w){ height = h; width = w;
}
...
}
WARNING
A different constructor from within one constructor may not be called with the usual new method, since that would create a brand new object instead of calling a different constructor as part of building the current object. If a class does not provide any constructor explicitly, the compiler generates one that uses no parameters and is sometimes called the default constructor. If a class provides any constructor, the compiler does not provide this noparameter version.
A constructor can use the keyword this() as a method to call a different overload of that class’s constructor. This can help avoid redundant coding: public Box(double s) { this(s, s);
}
Instance variables are already initialized by the time the constructor executes.
The constructor need only perform additional initialization.
LESSON 5
as the call, compares the actual types of arguments of the call with the formal argument types of the methods, and chooses the appropriate method.
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JAVA PROGRAMMING: PART 1
A new operation always calls a constructor to fully initialize a new object.
Remember that the choice of constructor to call depends on normal overload resolution rules based on the initialization parameters:
Box b = new Box (1.0, 2.0);
// calls: Box (double h, double w)
®®®
ENCAPSULATION
Encapsulation is combining of behavior and data into the discrete program units called objects. The programmer hides the data behind a specific interface (usually a purely functional one) that controls access to the data. Encapsulation prevents external code from modifying the data directly avoiding the possibility of unwanted side effects.
By hiding the underlying implementation details, the programmer hides the way in which an object is represented and manipulated from the rest of the program. In this respect, encapsulation contributes to program modularity, as it groups code and data components in discrete, self-contained, and selfprotecting objects. By defining an interface made up only of methods, the programmer provides a clean interface to the object and limits the actions that can be performed on it.
A well-defined, fully encapsulated object provides a clear, comprehensive interface, and takes care of its own security. External code cannot access its internals—and has no reason to do so.
®®®
ACCESS SPECIFIERS
Classes can be declared public. A public class is accessible by any other
Java class. A class that is not declared public has package access, which means that only classes within the same package may access it.
Class members can be declared public or private to enforce proper encapsulation. Certain data elements or methods that are necessary for the class’s internal behavior should be protected from access by outside classes.
These should be declared private. Class members labelled public are accessible to all other classes; members labelled private are accessible only to the code of the class itself. There is also a protected access speci-
LESSON 5: Classes in Java—II
97
®®®
COMPARING OBJECTS
Through the mechanism of inheritance (discussed in a later lesson), every
Java object has a method called equals() that takes another object as its argument and returns true if the target object is equal to the argument object. This sort of equality is not necessarily the same as the one implemented by the == operator, which returns true if and only if its operands refer to exactly the same object. Types such as String, in which equality depends on the value of an object and not simply the object’s identity, redefine the equals() method to compare values instead of references.
To compare the values of two objects, use the equals() method: if (r2.equals (r1)) {
...
}
The behavior of equals() depends on the class. The default behavior is to compare object identities just as the == operator does. Where appropriate, it compares values.
®®®
CLASS VARIABLES
Normally, variables declared in a class are instance variables, which appear once per instance of the class. It is sometimes useful to define variables that, like global variables in other programming languages, are unique and persist throughout the lifetime of the program.
A Java class can have data members that appear once per class, rather than once per instance, and persist for the duration of the program. These members are called class variables.
LESSON 5
fier that will be discussed in the lesson on inheritance. A member with no declared access specifier has package access.
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JAVA PROGRAMMING: PART 1
NOTE
This is similar to the use of static for class members in
C++.
To declare a variable of this sort, add the modifier static to a class variable declaration: public class Box { double height, width; static int BoxCount = 0; public Box(double h, double w) { height = h; width = w;
BoxCount += 1;
}
...
}
Within the class, a class variable may be referred to by just its name and no qualifier. Outside the class, the variable name must be qualified by either an instance of the class or the name of the class. Qualifying a class variable by an instance of the class is identical to accessing an instance data member.
Since class variables are associated with a whole class and not any particular instance, accessing a class variable does not require an instance of the class.
A class variable may be qualified by applying the dot operator to the class name, as if the class name were a class instance.
Class Initialization
TIP
Static initialization blocks can be used to assign the results of a complex computation to be the initial value of a certain static variable. You are not limited to using constant values for initialization.
Class variables may be initialized in the class definition. The required syntax is identical to initialization of instance variables. Any variable not explicitly initialized receives the default value for its type.
The initialization takes place when the interpreter first loads the class. Additionally, a class may specify arbitrary initialization code to be run when the class is loaded. This is accomplished by using a static initialization block. The syntax for defining a static initialization block is the keyword static followed by the body of a method. A class may have more than one static initialization block. When a class is loaded, all its static initialization blocks and static variable initializations execute in the order in which they appear in the source file.
LESSON 5: Classes in Java—II
99
NOTE
Java does not have any preprocessor and thus does not support constants in the sense of C’s
#define directive.
public class Box { static int num_of_Boxs = 0; static float peculiar; static // a static initializer
{
peculiar = 2.7f + 15;
}
...
}
final Variables
A class-member variable can be declared with the modifier final meaning that its value cannot be changed in other parts of the code once the variable is initialized. Java has reserved C’s keyword const though it currently has no meaning.
TRIVIA
Java designers reserved the const keyword to avoid potential confusion from C/C++ programmers inadvertently using const in Java.
A class variable declared final is a class-wide constant. Constants are often declared public to make them available program-wide. class Trig { public static final float PI = 3.14159;
... more stuff ...
}
float zz; zz = 2.0f * Trig.PI;
LESSON 5
The following is an example of static initializer blocks:
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JAVA PROGRAMMING: PART 1
Variables declared static and final are true program constants:
WARNING
If a variable is declared final but not static, it could have different values in different instances of the class. This can be confusing if the programmer expects this variable to be a true constant. It seldom makes sense to use final without static. ®®®
WARNING
If a static method tries to access an instance variable, there will be a compiler error.
// create constant suit public class Suit { public final static public final static public final static public final static
}
values int int int int
DIAMONDS = 0;
CLUBS = 1;
HEARTS = 2;
SPADES = 3;
// create a mathematical constant public class Trig { public static final float PI=3.1416f;
}
CLASS METHODS
As discussed in the preceding section, class variables (as opposed to instance variables) exist before any instances are created. Sometimes the class also needs methods that refer to those variables, and those methods need to be callable by outside code before any instances have been created. Such methods can be declared with the static modifier and are called class methods, because they apply to class variables and not to instances of the class. The following code gives an example of a class method: public class Thing { static int value = 3; public static void resetValue (int val) { value = val;
}
...
}
Outside of a class, class methods can be called either by class name or through an instance:
Thing.resetValue(9);
Thing th = new Thing(6); th.resetValue(9); LESSON 5: Classes in Java—II
101
It is a common practice to define classes that contain only static members.
Since such a class has no instance data or methods, instantiating it is useless.
Such a class is nothing but a wrapper for a related set of global methods and/ or data—a wrapper made necessary by the absence of true globals in Java.
®®®
NOTE
Notice this important distinction in how constructors are declared and finalized. Constructors have no return type, but finalize must have void as a return type.
Unlike constructors, finalize has only one option for arguments—no arguments.
FINALIZATION
When class objects are created, the appropriate constructor is called. What happens when objects go away? In C++, classes can implement a destructor that releases memory when the object is destroyed. Unlike C++, Java does not require a destructor, because memory is not directly allocatable. However, class objects may still create conditions that must be manually undone, such as closing files opened for I/O.
In Java, classes may define a method called finalize(). If finalize() exists for a class, it is called just before an instance of that class is destroyed. The compiler generates a default finalize() method if a class does not provide one. Note that the exact time that finalize() is called can never be predicted, simply because the exact time of garbage collection (i.e., actual destruction of objects) cannot be predicted. Under some circumstances, such as when the interpreter exits at the end of a program, it is possible that finalize() might never be called. Thus, the programmer of a class should never rely on finalize() to do critical tasks. finalize() is only useful to free system resources, such as open files and network connections. A finalizer method is declared by naming it finalize()with no parameters and a void return value.
LESSON 5
Class methods can access class variables, but cannot access instance variables.
Because class methods may be called with no instances available, they are not given a this reference like instance methods.
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®®®
LESSON SUMMARY
In this lesson, you have learned:
® Overloading allows a method to have more than one signature.
® Variables declared static apply to the whole class not to specific instances. ® Variables declared final cannot be changed after they are initialized.
® Finalization carries out various tasks when a class instance is no longer needed. LESSON 5: Classes in Java—II
103
1. If a class is not declared public, then who can instantiate the class?
2. What is the keyword that declares a variable to be constant?
3. What is the keyword that declares a method to be a class method rather than an instance method?
Answers on page 197
LESSON 5
REVIEW QUESTIONS
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JAVA PROGRAMMING: PART 1
EXERCISE
1. Expand the Clock c lass to support a tick() method. The tick() method increments the current time by one second. Add an empty main() method to class Clock, so that you can run the class (see
FirstApp.java for an example of a runnable main() method).
Write test code that instantiates and uses class Clock in main().
Test the clock’s transitions from hour to hour, minute to minute, AM to
PM.
Compile and run the test.
2. Create a class called MathTechniques that supports the following constant and methods:
• pi - 3.14159
• areaOfRectangle that is passed a height and a width
• perimeterOfRectangle that is passed a height and a width
• areaOfCircle that is passed a radius
• perimeterOfCircle that is radius
3. Create a separate class to test out the MathTechniques class.
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LESSON 6
Arrays and Strings
OVERVIEW
The array is a useful construct that stores an indexed collection of identical objects. Perhaps the most useful application of arrays in Java is the character string. A string is an object and may be readily assigned, copied, and accessed. A string cannot, however, be modified.
LESSON TOPICS q Java Arrays q Array Constants q Using Arrays q Copying Array Elements q String Objects q String Methods q String Concatenation q Converting Objects to Strings q Converting Strings to Numbers
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®®®
OBJECTIVES
By the end of this lesson, you should be able to:
® Work with Java arrays.
® Work with Java string objects.
®®®
NOTE
Arrays are neither a true class (there is no class called Array), nor a built-in type
(Arrays have members like a class).
Arrays in Java are in between a class and a data type although they are implemented as a class internally by Java.
JAVA ARRAYS
Arrays are ordered collections of identical objects that can be accessed via an index. In C and C++, an array is implemented as a contiguous block of memory accessed via pointers to the elements. In Java, arrays are first-class objects. Java arrays have their own behavior that is encapsulated into their definition when the compiler creates them. Since arrays are objects, array variables behave like class-type variables. It is important to distinguish between the array variable and the array instance to which it refers. Declaring an array only declares the array variable. To instantiate the array, the new operator must be used with the [] operator to enclose the array size. The array size can be any integer expression.
The following code declares an array variable and initializes it to null: char data[] = null;
This can also be written as: char[] data = null;
To declare an array variable and initialize it to refer to a new instance, use the following syntax: int length = 60; char[] data = new char [length];
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TABLE E. Array Initialization
Type
integers and floating point
zero
characters
null
booleans
false
class-type objects
®®®
Initialization
null
ARRAY CONSTANTS
The declaration of an array must include its type, but not its size. Arrays may be initialized with conventional initialization syntax. An array initializer is either an expression (such as a new expression) that evaluates to the correct array type, or a list of initial element values enclosed in { }. This latter notation is borrowed from C/C++. In Java, this is called an array constant.
Any expression can be used for the initial array element values. (C requires initializers be compiler-time calculable; both C++ and Java allow initializers to be run-time calculable.)
The following array constant provides initial values for its elements: int[] int_array = {1, 3, 4, 15, 0};
Each element of an array of class-type objects behaves like a class-type variable:
® Elements may be allocated by new, making them initially null:
Box[] boxes = new Box [100];
® Elements may be initialized in an array constant:
Box b = new Box (2.2, 3.3);
Box[] boxes = { b, new Box (1.1,2.2) };
LESSON 6
Array elements are always initialized according to their type as shown in
Table E.
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®®®
USING ARRAYS
Array elements can be accessed using C notation (an index in square brackets, [n]), where the index must be an integer. Like arrays in C, arrays start with index number 0, not 1. The index can be thought of as an offset from the beginning of the array.
The index may not be less than zero or greater than the declared size. If the array is declared size n, the index must be in the range 0 to n-1. Unlike C, arrays are not implemented as pointers, so programming techniques like negative array indexing are not allowed. Any attempt to index an array with an illegal value will cause an exception to be thrown.
NOTE
The length() in
String is a method while the length in array is a data member.
All arrays have a data field called length that indicates its size. This value is read-only, and contains the number of elements in the array: int ia[] = new int[100]; for (int i = 0; i < ia.length; i++)
{
ia[i] = i * i;
}
Since length is read-only, its value cannot be changed manually: ia.length = 20; // error
®®®
COPYING ARRAY ELEMENTS
The library method System.arraycopy() is useful for copying a number of elements from one array to another. The method can be used on any type of array and is declared as follows: public static void arraycopy (Object src, int src_position,
Object dst, int dst_position, int length)
The method copies elements from the given source array, beginning at the specified position to the destination array at the specified position. It copies
LESSON 6: Arrays and Strings
109
Any type of array may be copied. If range exceeds bounds of either array, a run-time error results.
®®®
STRING OBJECTS
The Java String c lass (java.lang.String) is a class of object that represents a character array of arbitrary length. While this external class can be used to handle string objects, Java integrates internal, built-in strings into the language. This string literal is one way these strings can be used:
“This is a quoted string literal”
This is actually an object of type String having the value of the quoted character array, which is the most common way to instantiate a string object.
However, the String c lass does contain several constructors, listed in
Table F.
TABLE F String Constructors
.
Description
Syntax
construct an empty string
String ()
copy a string
String (String str)
initialize with a char array
String (char[] chars)
initialize with an ASCII byte array
String (byte[] bytes, int hibyte)
Strings can be used like any other type of Java object: void printMsg (String userName)
{
String msg1 = “Hello, “;
String msg2 = userName; if (msg2 == null) msg2 = “anonymous user”;
System.out.println (msg1 + msg2);
}
An important attribute of the String c lass is that once a string object is constructed, its value cannot change (note that it is the value of an object that
LESSON 6
the number of elements specified by the length argument. The destination array must already be allocated.
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JAVA PROGRAMMING: PART 1
cannot change, not that of a string variable, which is just a reference to a string object). All String data members are private, and no string method modifies the string’s value.
®®®
STRING METHODS
Although a string represents an array, standard array syntax cannot be used to inquire into it. Instead, the String class provides equivalent methods that allow access to a string’s value or any part of it. These are detailed in
Table G.
TABLE G. String Methods
Method
Description
Example
int length()
Returns the number of characters in the string
String str = “Another string”; int len = str.length
();
char charAt (int index)
Extracts one character from the string
char first = str.charAt
(0)
char last = str.charAt
(len - 1)
void getChars (int srcBegin, int srcEnd, char[] dest, int destBegin) Extracts a range of characters from the string into a character array
getChars(3, 6, word,
0);
String substring (int begin)
Return the substring of this string delimited by character position begin and the end of the string
String s3 = str.substring (8);
String substring (int begin, int end)
Returns the substring of this string delimited by character position begin
(inclusive) and character position end
(exclusive)
String s1 = str.substring (0, 7);
String Comparison
Since string variables are Java class-type variables, the equality (==) operator compares whether two strings are the same object, not whether they have
LESSON 6: Arrays and Strings
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TABLE H. More String Methods
Method
Description
Example
boolean equalsIgnoreCase (String otherString) Returns true if this string value equals the other, considering letters equivalent regardless of case
if (s1.equalsIgnoreCase
(s2)) { ... }
int compareTo (String otherString)
Returns zero if the string values are equal, a negative value if this string is less than the other, or a positive value if this string is greater than the other
if (s1.compareTo (s2) >
0) {
String temp = s1; s1 = s2; s2 = temp;
}
boolean startsWith (String prefix)
Returns true if this string starts with the other string if (s1.startsWith (s2))
{ ... }
boolean endsWith (String suffix)
Returns true if this string ends with the other string
if (s1.endsWith (s2)) {
... }
boolean regionMatches (int thisBegin,
String otherString, int otherBegin, int length)
General string region comparison if (s1.regionMatches(0, s2, 5, 10)
{...}
boolean regionMatches (boolean ignoreCase, int thisBegin, String otherString, int otherBegin, int length)
General string region comparison, with option of case insensitivity
if (s1.regionMatches(true, 0, s2, 5,
10) {...}
LESSON 6
the same value. To compare string values, use the equals() method common to all objects, or one of the other methods listed in Table H.
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String Searching
The String class also provides methods that search a string for the occurrence of a single character or substring. These return the index of the matching substring or character if found, or -1 if not found.
® int indexOf (char ch)
® int indexOf (char ch, int begin)
® int lastIndexOf (char ch)
® int lastIndexOf (char ch, int fromIndex)
® int indexOf (String str)
® int indexOf (String str, int begin)
® int lastIndexOf (String str)
® int lastIndexOf (String str, int fromIndex)
The following example shows the usage of these functions: if (s1.indexOf (’:’) >= 0) { ... }
String suffix = s1.substring (s1.lastIndexOf (’.’)); int spaceCount = 0; int index = s1.indexOf (’ ’); while (index >= 0)
{
++spaceCount; index = s1.indexOf (’ ’, index + 1);
}
int index = s1.indexOf (“that”);
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113
The String class provides miscellaneous methods for manipulating string data. Each of the methods listed below returns a new string instead of modifying its given string. (string objects are immutable by design).
String replace (char oldChar, char newChar)
// Replaces all occurrences of oldChar with newChar.
// For example: s2 = s1.replace (’ ’, ’_’);
String toLowerCase ()
// Converts all upper case characters to lower case.
// For example: s2 = s1.toLowerCase ();
String toUpperCase ()
// Converts all lower case characters to upper case
// For example: s2 = s1.toUpperCase ();
String trim ()
// Removes leading and trailing white space
// For example: s2 = s1.trim ();
®®®
STRING CONCATENATION
The String class provides a method for concatenating two strings:
NOTE
C++ programers should note that the
+ and += String operators are built into the Java language; and that Java does not allow programmer- defined operators. String concat (String otherString)
The + and += String operators are more commonly used:
String fname = s1 + “.txt”;
String path = s1 + “/” + s2 + “.txt”;
The Java compiler recognizes the + and += operators as String operators.
For each + expression, the compiler generates calls to methods that carry out the concatentation. For each += expression, the compiler generates calls to methods that carry out the concatenation and assignment.
LESSON 6
Other String Methods
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JAVA PROGRAMMING: PART 1
®®®
CONVERTING OBJECTS TO STRINGS
The String + operator accepts a non-string operand, provided the other operand is a string. The action of the + operator on non-string operands is to convert the non-string to a string, then to do the concatenation. Operands of native types are converted to string by formatting their values. Operands of class types are converted to a string by the method toString() that is defined for all classes. Any object or value can be converted to a string by explicitly using one of the static valueOf() methods defined in class
String:
String str = String.valueOf (obj);
If the argument to valueOf() is of class type, then valueOf() calls that object’s toString() method. Any class can define its own toString() method, or just rely on the default. The output produced by toString() is suitable for debugging and diagnostics. It is not meant to be an elaborate text representation of the object, nor is it meant to be parsed.
These conversion rules also apply to the right-hand side of the String += operator. ®®®
CONVERTING STRINGS TO NUMBERS
Methods from the various wrapper classes, such as Integer and Double, can be used to convert numerical strings to numbers. This is often necessary for command line arguments where the parameter list is available to the main () method as a string array.
The wrapper classes contain static methods ( such as parseInt() ) which convert a string to its own internal data type. These can be used with class names rather than creating a separate object. Be aware that these par-
LESSON 6: Arrays and Strings
115
import java.io.PrintWriter; public class ArgTest { public static void main(String[] args) { int i, j;
// Create an stdout object
PrintWriter stdout = new PrintWriter(System.out, true);
// Make sure we have parameters if (args.length == 0) { stdout.println("No arguments");
System.exit(1);
}
// Loop through the args array for (i = 0; i < args.length; i++) { try { j = Integer.parseInt(args[i]); stdout.println(j + " is an integer");
}
catch (NumberFormatException e) { stdout.println(args[i] + " is not an integer");
}
}
}
}
LESSON 6
ticular conversion methods throw a NumberFormatException and must be used in the context of a try and catch block combination.
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®®®
LESSON SUMMARY
In this lesson, you have learned:
® Arrays are fixed-size, indexed collections of data elements, all of the same type.
® The Java run-time system catches out-of-bounds array indexes.
® Array variables behave like class-type variables.
® If an array contains class-type elements, then each element behaves like a class-type variable.
® Java strings are objects of class String.
® Java strings are immutable.
® Java recognizes + and += concatenation operators.
LESSON 6: Arrays and Strings
117
1. What is wrong with the following code: int arr[] = new int[50]; arr.length = 25; for (int i = 0; i < arr.length; i++)
{
arr[i] = 0;
}
2. Using two different methods, add the text “ball” onto the end of a
String sport already containing the text “base.” Store the result in
String new_sport.
Answers on page 197
LESSON 6
REVIEW QUESTIONS
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JAVA PROGRAMMING: PART 1
EXERCISE
1. Write a class named FloatArray that represents an array of doubleprecision floating point values. Class FloatArray ’s first constructor takes one argument of type integer that specifies the size of the array.
The initial value of the array elements is zero. Do not try to handle invalid size values.
Class FloatArray ’s second constructor takes two arguments: the array size and a double that specifies the initial value of all the array elements.
Write a getValue() method that returns a value from the array, given the index of the element.
Write a setValue() method that sets a value in the array, given the index of the element.
Write a getAverage() method that returns the average of all values in the array.
Write a runnable main() method that tests the class floatarray.
Compile and run the class.
2. Write a static method called removeString. (You may create a new class for your method, or work in one you have already written.) removeString has the following signature: public static String removeString
(String string, String substring)
The function of removeString is to remove all occurrences of the given substring in the given string. For instance, the following call results in the string dated:
String result = removeString (“undaunted”,"un")
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LESSON 7
Inheritance
OVERVIEW
Inheritance is a powerful feature that allows new classes to be derived from existing ones. The derived class inherits the interface of the base class and can add new methods or change existing ones to suit its purpose.
Any number of classes can be added to an inheritance scheme to produce a hierarchy of related types. The Java implementation of inheritance supports polymorphism, which allows objects of different types to be treated as if they were of the same type.
LESSON TOPICS q Introduction to Inheritance q Protected Access q Overriding Methods q Constructor Chaining q Inheritance and Finalization q Abstract Classes q Interfaces q Casting Between Class Types
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®®®
OBJECTIVES
By the end of this lesson, you should be able to:
® Read and understand source code for derived classes.
® Derive a new class.
® Define a set of classes that behave polymorphically.
® Take advantage of Java’s run-time typing.
®®®
INTRODUCTION TO INHERITANCE
Developers working with object-oriented languages visualize classes as being related to each other in hierarchies of “kind of” relationships. Just as programmers generalize the common features of objects in an abstract data type, they also identify and abstract common features of similar types in a superclass. Once the superclass is defined, each subclass is defined as a “kind of” the superclass, so that it can then inherit the common features. The programmer need only specify the behavior that sets the subclass apart from its parent and its siblings. Java programmers say that the subclasses are derived from the superclass, and refer to them as derived types and base types, respectively.
Programmers conceptualize programs both upward and downward, through levels of abstraction during the design and implementation stages of a development cycle. However, it is observed that they usually tend to think predominantly from the bottom up during the design phase, and predominantly top down during the implementation phase. Once the characteristics of a superclass are well described, its interface can be written before the subclasses are developed. Each subclass inherits both its superclass’s interface and its implementation. It modifies and extends the former, or extends only the latter, as needed. The use of inheritance produces a hierarchy of classes.
For example, it is relatively easy to proceed downward from a base class like bank-account to more specialized classes of accounts, provided that the analysis and design phases are complete. By contrast, it is much more difficult proceeding upward, i.e., to try and code many different kinds of bank accounts first, and then coordinating all their differing interfaces into a superclass. LESSON 7: Inheritance
121
An example of inheritance where a programmer might identify a need for two types of text objects would be CompactText and WordProcText.
CompactText emphasizes storage efficiency rather than functionality and
WordProcText is used for word processing that contains font information. While the differences between the text objects are significant enough to warrant implementing them as different classes, the similarities that exist between them suggest the presence of certain shared properties and the possibility of their belonging to one superclass.
In this case, rather than creating two largely similar class definitions, a superclass called TextObject can be first defined. This establishes a common interface for the two. From this base class, the classes CompactText and WordProcText are derived. Each derived type inherits its interface and implementation from the base type. For example, all of the classes have a length() method. The base class might or might not provide a default implementation for this method. If the base class does provide a default implementation, the derived classes can inherit the implementation as their own. A derived class can override or extend the implementation of an inherited method as appropriate. A derived class may not change the interface of a method. CompactText might use the implementation of the length() method defined in TextObject, while WordProcText might override this implementation with its own.
Perhaps it is only later that an EncodedText type is needed. If an inheritance relationship exists, then implementing the new type becomes much simpler. Whether or not derived classes are created at the same time as their base class, each of the derived types extends the interface or implementation of the base class.
Establishing a sensible hierarchy of related types is one of the most challenging aspects of object-oriented program design. It is not easy to generalize a base type in a manner that not only abstracts the common behavior of the types already known, but also provides a good basis for the later derivation of new types. An important reward for utilizing inheritance is a dramatic increase in code reuse, which can save a programmer time and valuable resources. LESSON 7
Example of Inheritance
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Derivation Syntax
The keyword extends in the subclass definition below declares one class as a subclass of another. A Java class may derive from at most one base class. public class TextObject
{
public int length () { ... }
}
public class CompactText extends TextObject
{
} public class WordProcText extends TextObject
{
}
Any user-defined class that does not explicitly derive from another class derives from the library class java.lang.Object. The Object c lass is therefore the ultimate base class of all other Java classes. The Object c lass provides the equals() method as well as other features common to all
Java objects.
Effects of Inheritance
In Java, private methods of the base class are not inherited (the derived class cannot call them anyway), but everything else is inherited.
A derived-type object can be said to have a dual identity. It is a type of both its new class and its parent type. For example, if class Bass is derived from
Fish, an object of type Bass is both a Bass and a Fish. However, the reverse is not necessarily true. The programmer, in this case, must force a conversion from base class type to derived class type with an explicit cast:
BaseClass Obj1; /*...*/
DerivedClass Obj2=(DerivedClass) Obj1;
LESSON 7: Inheritance
NOTE
Since Java has the concept of a package,
(a grouping of classes), Java offers a few additional combinations of access options as compared with C++.
PROTECTED ACCESS
In addition to public, private and default (package-friendly) member access, there are two other access specifiers that apply only to derived classes.
Any member that is declared protected is accessible only by a class that is derived from the current class or is in the same package as the containing class. Any member that is declared private protected is accessible only by a class that is derived from the containing class.
Consider the following example: public class A
{
protected int x; private protected int y;
}
Member x is accessible to any class that derives from class A and to any class in the same package as class A. Member y is only accessible to any class that derives from class A.
Figure 5 below shows the relationship between classes with different access specifiers. LESSON 7
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123
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JAVA PROGRAMMING: PART 1
FIGURE 5
Accessibiliy in an inheritance relationship Package Things
Package Stuff
Class A
Class E
Class C
(derived)
Class B
Class D
Table I details access to class A’s members.
TABLE I. Access to class A’s members
Access specifier
Accessible by classes:
public
A, B, C, D, E
protected
A, B, C
private protected
A, B, D
private
®®®
A, B, C, D
default
A
OVERRIDING METHODS
The process of a derived class redefining a method contained in the parent class (with the same parameter types) is called overriding the method.
Whenever a method is called for a derived-type object, the compiler calls the overriding version instead of the inherited version. The method is thus
LESSON 7: Inheritance
125
public class TextObject
{
public int length ()
{
// TextObject’s implementation
}
} public class CompactText extends TextObject
{
public int length ()
{
// CompactText’s implementation
}
}
This allows a programmer to use a parent type variable to point to a derived type object, call a method for that object, and have the compiler call the specialized version. This is how polymorphism is implemented. C++ programmers will recognize this as a virtual function. In Java, all methods are virtual unless specified otherwise.
Dynamic Method Dispatching
Consider the following example: class Fish { void eat(), other items... } class Bass extends Fish { void eat()...} class Shark extends Fish { void eat()...}
Fish redfish, bluefish; redfish = new Bass(); bluefish = new Shark(); redfish.eat(); bluefish.eat();
The compiler cannot tell at compile-time which method to call here. The solution entails waiting until run-time to determine which function is the
LESSON 7
implemented in a way that is unique to the derived class. The following is a syntax example:
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JAVA PROGRAMMING: PART 1
correct one. In other words, the run-time code must check the actual type rather than the apparent type of an object based on the name of the variable used in the code. This is known as dynamic method dispatching.
Polymorphism
Dynamic method dispatching is a major component of polymorphism. Polymorphism is a very powerful feature of object-oriented languages. It allows a program to treat objects of different types as if they were all of the same type. TRIVIA
The key difference here, compared to a traditional non-polymorphic approach, is that differences in behavior are internalized in different derived classes. This differs from an external body of code selecting a certain behavior from a list of options. The creator of a group of related data types designs them to respond to the same message in ways that are nominally identical, no matter how different they may be in detail. The code which defines and uses objects of these types can then send the same message to all of them and get an appropriate response from each.
A user program can create heterogeneous lists of objects of related types and invoke a virtual function for each one of them. At run time, the program will branch to the version of the function that is appropriate to that object.
This is accomplished by the following manner. When a class declares a member function, the compiler includes a type code in every object of that class. Wherever the function is invoked, the Java run-time system checks the type of each object and invokes the function appropriate to that object’s specific class. By automating type checking, polymorphism eliminates the need for many of the large switch statements that tend to infest C programs.
Coding these is notoriously tedious, mechanical, and error-prone. Such statements also require maintenance whenever a new class of data is introduced, even if that class is closely related to others.
In contrast, the addition of a new data type typically has much less effect on programs that use polymorphism. Since code that requests a string’s length pays no attention to the particular string type, the compiler neither knows nor cares that some of the string types were introduced after the code was written. Such code therefore requires less initial development effort as well as less maintenance.
LESSON 7: Inheritance
127
TextObject[] tarr =
{
new EncodedText (), new WordProcText (), new CompactText (), new TextObject ()
};
int totalLen = 0; for (int i = 0; i < tarr.length; ++i)
{
totalLen += tarr[i].length ();
}
The code that invokes the length() method ignores type differences.
New types of TextObjects can be introduced without forcing changes in calling code.
The super Keyword
It is often desirable for an overriding implementation of a method to call the inherited implementation of the method. Often, the inherited code need not be completely replaced, only augmented to add some new actions. Any method in a derived class that overrides a method in the base class can call the corresponding base class method by using the super keyword as shown below: public class WordProcText extends TextObject
{
public int length ()
{
int len = super.length ();
...
return len;
}
}
LESSON 7
The following polymorphism example invokes the length() method of each element of an array of TextObjects:
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JAVA PROGRAMMING: PART 1
Note that there is no way to access a method overridden more than once.
The following code is illegal: super.super.f() Final Methods and Final Classes
NOTE
final in Java is the counterpart of virtual in C++. In
C++ you need to specify virtual to get dynamic binding.
In Java you need to specify final to get static binding.
®®®
Dynamic lookup takes time. Therefore, there are some situations where it should be disabled. Also, there are some methods for which the compiler must be able to resolve a required action, or for which an overriding version would be too complicated to implement.
Methods labeled final, private, or static are not subject to dynamic lookup because they may not be overridden. private methods are simply not inherited because they would never be callable anyway. static methods apply to a particular class’s static data and thus make no sense in a derivation. final methods are those designated as not-overrideable for reasons of complexity or safety.
CONSTRUCTOR CHAINING
When a derived-type object is instantiated, it is sometimes desirable to have its constructor call a particular overload of its parent class. This is done with what looks like a call to the method super(). Often this is used to pass a particular variable up into a parent class’s constructor. This must be the first statement in a constructor.
If the programmer does not include a super() call, the compiler will provide one. The only exception to this occurs when a constructor calls another constructor via the form this(). In this case, the compiler knows that the first constructor will call super().
Constructors do not use dynamic lookup simply because each class has its own constructor.
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public class A
{
A(int ix){...}
A(float fx){...}// overloaded
}
public class B extends A
{
B(int ix){...}
}
The keyword super can be used to specify which of A’s constructors to call from B’s constructor: class B extends A {
{
B(int ix)
{
super(ix);// calls A(int ix)
}
}
This use of super() is legal only as the first statement of a constructor. If a constructor calls neither super() nor this(), the compiler implicitly calls the base class’s no-parameter constructor as seen in Figure 6.
LESSON 7
Consider the following two classes:
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JAVA PROGRAMMING: PART 1
FIGURE 6
Constructor Chaining
class A
{
A(){...}
A(float xx){...}
}
C om piler calls
class B extends A
{
A() h ere
B(){...}
B(float fx)
B(int ix)
{
this();
...
}
{super(fx);...}
b u t n o t h e re
}
One common mistake is to define a particular constructor in a parent class, and then derive from it without calling super(). In this case, the compiler would call the parent class’s no-parameter constructor, but would not have defined one. This can produce some mystifying error messages.
The appropriate overload must exist in the parent class. Recall that if any constructor is written, the compiler will not provide the default no-parameter version.
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class A { int value;
A(int input) { value = input; }
}
class B extends A { int myvalue;
}
®®®
NOTE
in C++, a parent class’s destructor is called automatically.
This does not happen in Java.
®®®
INHERITANCE AND FINALIZATION
If a parent class contains a finalize() method, it must be called explicitly by the derived class’s finalize() method. Unlike C++, Java destructors are not called automatically. If it is needed, the derived class’s finalize() method must call it manually with: super.finalize(); ABSTRACT CLASSES
Methods declared with the keyword abstract define a skeleton for a method but do not implement it. This requires a derived class to provide the code for this class. A class with one or more abstract methods must itself be declared abstract and cannot be instantiated. It can only be used for derivation.
C++ programmers will recognize this as a pure virtual function that makes an abstract class. A method can be declared abstract with no brackets to enclose any code.
LESSON 7
The following will produce a compiler error:
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FIGURE 7
The relationship between an abstract class and its derived classes abstract float area(); class Shape
(derived)
Circle
Square
Triangle
A generic shape cannot exist. Each subtype implements its own area() method. ®®®
TIP
The rule of thumb in choosing between an abstract class and an interface is as follows: if you want any code in the base class to be reused by derived classes, use an abstract class.
Otherwise use an interface. INTERFACES
Some languages support multiple inheritance in which a child class may derive some of its features from one parent class and other features from another (or others). There is a continuing controversy over whether the benefits promised by this feature justify the extra complexity that comes with it.
Java does not support multiple inheritance. However, it does provide a mechanism called an interface whereby the properties of one class can be used by another without formal inheritance. This enables Java to avoid most of the syntactical complications that multiple inheritance requires. When a parent class is to be defined as abstract, instead of defining one or more methods as abstract, the class can be defined with the keyword interface instead of class. This defines the minimum set of methods a class must contain. An interface type is named for derivation with the keyword implements instead of extends. All of its methods are implicitly public and abstract. If desired, they can be declared that way for clarity. Also, any data defined in an interface must be constant (i.e., final static).
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A derived class cannot have two parent classes, as multiple inheritance leads to problems of how to decide which method to use for inherited methods.
Yet multiple inheritance is desirable because it lets an object be assigned into, and thus used as, variables of different types. For example, a programmer might instantiate a Circle as a class derived from Shape, yet wish to pass it into a method requiring a Graphic object parameter.
The implements declaration in Figure 7, seen as a way of deriving from an interface, also means that the new class can be treated as being of that new type. Though a class can have only one parent class, it can implement any number of classes, and thus behave as more than one type of object. For example: interface Shape{...} class Circle extends Drawable implements Shape{...} class Square extends Drawable implements Shape{...}
Shape sp1 = new Circle(), sp2 = new Square(); sp1.area(); sp2.area():
The classes Circle and Square are derived from class Drawable, but by implementing Shape, they also inherit Shape’s interface and are enabled to act as polymorphic instances of Shape.
With interface and implements, Java supports the polymorphic features of multiple inheritance, while yet avoiding the complications of methods implemented in multiple ways.
®®®
CASTING BETWEEN CLASS TYPES
The Java run-time system keeps track of the type of each object. While this implies some memory space overhead for each object, the programmer can achieve a type-safe run-time environment. The Java language makes some of
LESSON 7
The implements Declaration
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JAVA PROGRAMMING: PART 1
its run-time typing power available to the programmer through the instanceof operator. The form of an instanceof expression is as follows: object-expression instanceof class-name
The left-hand term is any expression that evaluates to a class type. The right-hand term is the name of a class or interface. An instanceof expression returns true if the actual type of the object on the left-hand side is of the type named by the right-hand side, and false otherwise. The object is of the named type if it is of exactly the named type or is of a type that directly or indirectly extends the identified type.
The instanceof operator can be use for safe casting between class types: if (tobj instanceof CompactText)
{
CompactText ct = (CompactText) tobj;
}
LESSON 7: Inheritance
LESSON SUMMARY
In this lesson, you have learned:
® Inheritance involves defining a class as a specific type of another class.
® Inheritance increases code reuse and allows for polymorphism.
® Java supports dynamic method dispatch at run-time.
® Methods and classes may be declared final.
® Abstract methods have no implementation.
® Interface types support limited multiple inheritance.
® Java’s run-time typing allows safe casting between class types.
LESSON 7
®®®
135
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JAVA PROGRAMMING: PART 1
REVIEW QUESTIONS
1. Write the class declarations for the following relationship, assuming that all classes are public: a Bulldog is a kind of Dog, and a Dog is a kind of
Animal.
2. What is the difference between the protected and private protected access methods?
3. What is purpose of the super keyword?
Answers on page 198
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1. Create an interface called SecondObserver that defines a single method, tick(). Go back to your Clock c lass and make class Clock implement the SecondObserver interface.
Create a new class called PrintClock that extends your Clock c lass.
PrintClock provides a method called toString() that produces a string representation of the current time. Test your class.
2. Create a new class called TickingPrintClock that extends your
PrintClock class. TickingPrintClock extends the tick() method to print the new time on each tick, in addition to the normal function of the method.
Test your new class. You can insert the following code block to cause your program to sleep for one second: try {
Thread.sleep (1000);
}
catch (InterruptedException ie)
{
}
LESSON 7
EXERCISE
w w w
LESSON 8
Writing Java Applets
OVERVIEW
Java applets are graphical mini-programs that run in the context of another program. Thanks to the growth of the Internet, applets have become popular additions to World Wide Web pages. This lesson introduces basic applet methods and the means to embed applets in
HTML documents.
LESSON TOPICS q What Is an Applet? q The Applet Class q The Delegation Event Model—Action Events q The paint() Method q The Graphics Class q Java Fonts q Drawing Lines and Shapes q Drawing with Color q The Color Class
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®®®
OBJECTIVES
By the end of this lesson, you should be able to:
® Create a Java applet.
® Embed an applet in an HTML document.
® Use graphics library methods to draw in applets.
® Use the event model to capture click type events.
®®®
TRIVIA
The applets make static documents come alive. This feature of Java was a major reason for its quick popularity as an internet language.
WHAT IS AN APPLET?
An applet is not a standalone application. It is a mini program invoked within a larger program, such as an applet viewer or a Java-enabled World
Wide Web (WWW) browser. An applet depends on the viewer or browser to provide a context in which to run. This means that the viewer or browser implements common applet functions and provides those functions to the applet by way of a Java Application Program Interface (Java API). The viewer or browser also controls the execution of the applet.
An applet is graphical by nature. Each applet occupies a rectangular panel in its viewer's window. Applets are meant to be embedded within a graphical document, such as an HTML document.
A "Hello, World" Applet
The following is a simple applet named HelloWorldApplet.java:
NOTE
Without the import statement, use java.applet.App let instead of
Applet; and java.awt.Graphics instead of
Graphics.
import java.applet.*; import java.awt.*; public class HelloWorldApplet extends Applet
{
public void paint (Graphics g)
{
g.drawString (“Hello World”, 25, 50);
}
}
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® java.applet.Applet
® java.awt.Graphics
Without those import statements, the Java compiler would not recognize the classes Applet and Graphics, which the applet class refers to.
®®®
THE Applet CLASS
Every applet is an extension of the java.applet.Applet c lass. The base Applet c lass provides methods that a derived Applet class may call to obtain information and services from the browser context. These include methods that do the following:
® get applet parameters
® get the network location of the HTML file that contains the applet
® get the network location of the applet class directory
® print a status message in the browser
® fetch an image
® fetch an audio clip
® play an audio clip
® resize the applet
LESSON 8
These import statements bring the classes into the scope of our applet class:
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Additionally, the Applet c lass provides an interface by which the viewer or browser obtains information about the applet and controls the applet's execution. The viewer may:
® request information about the author, version and copyright of the applet ® request a description of the parameters the applet recognizes
® initialize the applet
® destroy the applet
® start the applet's execution
® stop the applet's execution
The Applet class provides default implementations of each of these methods. Those implementations may be overridden as necessary.
The “Hello, World” applet is complete as it stands. The only method overridden is the paint method.
Invoking an Applet
An applet may be invoked by embedding directives in an HTML file and viewing the file through an applet viewer or Java-enabled browser. The tag is the basis for embedding an applet in an HTML file.
Below is an example that invokes the “Hello, World” applet:
The Hello, World Applet
If your browser was Java-enabled, a “Hello, World” message would appear here.
The code attribute of the tag is required. It specifies the
Applet class to run. Width and height are also required to specify the initial size of the panel in which an applet runs. Note that the size of the applet
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If an applet takes parameters, values may be passed for the parameters by adding tags between and . The browser ignores text and other tags between the applet tags. Non-Java-enabled browsers do not process and . Therefore, anything that appears between the tags, not related to the applet, is visible in nonJava-enabled browsers. The programmer can take advantage of this feature to provide filler content for users with older browsers.
The viewer or browser looks for the compiled Java code at the location of the document. To specify otherwise, use the codebase attribute of the tag as shown:
Whether a code base is specified or the default is used, it is important to keep in mind that the code base is the root directory of the package tree. If the code is organized into packages, the Java class loader expects to find the class files in like-named subdirectories of the code base directory. The code base directory itself is the home of the default package.
If an applet resides in a package other than the default, the holding package must be specified in the code attribute using the period character (.) to separate package/class components. For example:
Getting Applet Parameters
The following example demonstrates how to make an applet respond to setup parameters specified in the document. This applet displays a checkerboard pattern of black and a second color. The second color and the size of each square may be specified as parameters to the applet within the document.
LESSON 8
panel is determined in the document, not from within the applet. An applet may call its resize() method, but the browser is not guaranteed to update the display. The applet directive must be closed with a tag.
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CheckerApplet gets its parameters in the init() method. It may also get its parameters in the paint() method. However, getting the values and saving the settings once at the start of the applet, instead of at every refresh, is convenient and efficient. The applet viewer or browser calls the init() method of each applet it runs. The viewer calls init() once, immediately after loading the applet. (Applet.init() is implemented to do nothing.) Override the default implementation to insert custom initialization code.
The Applet.getParameter() method fetches a parameter given the parameter's name (the value of a parameter is always a string). If the value is numeric or other non-character data, the string must be parsed.
The following is a skeleton of CheckerApplet.java: import java.applet.*; import java.awt.*; public class CheckerApplet extends Applet
{
int squareSize = 50;// initialized to default size public void init () {} private void parseSquareSize (String param) {} private Color parseColor (String param) {} public void paint (Graphics g) {}
}
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public void init ()
{
String squareSizeParam = getParameter (“squareSize”); parseSquareSize (squareSizeParam);
String colorParam = getParameter (“color”);
Color fg = parseColor (colorParam); setBackground (Color.black); setForeground (fg);
}
private void parseSquareSize (String param)
{
if (param == null) return; try { squareSize = Integer.parseInt (param);
}
catch (Exception e) {
// Let default value remain
}
}
The applet calls parseSquareSize() to parse the squareSize parameter. parseSquareSize() calls the library method Integer.parseInt(), which parses a string and returns an integer. Integer.parseInt() throws an exception whenever its argument is invalid.
Therefore, parseSquareSize() catches exceptions, rather than allowing the applet to fail on bad input.
The applet calls parseColor() to parse the color parameter into a Color value. parseColor() does a series of string comparisons to match the parameter value to the name of a predefined color. For more information concerning parseColor(), refer to the sample library.
LESSON 8
Here are CheckerApplet ’s init() and private parseSquareSize() methods:
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JAVA PROGRAMMING: PART 1
Specifying Applet Parameters
The following is an example of an HTML file with a CheckerApplet embedded in it. The HTML file specifies both parameters to the applet by means of the tag.
Checkerboard Applet
Parameter names are not case sensitive.
®®®
THE DELEGATION EVENT MODEL—
ACTION EVENTS
Events, such as the clicking of a button or the movement of a scroll bar, can be intercepted and processed by an applet. The technique used by Java to do this is called the Delegation Event Model. This model uses the concept that an object (e.g., a command button) is the source of events; and another object
(e.g., an applet) listens to a source for a specific type of event.
A listener informs the source that it is interested in listening to that source.
The source adds that listener to its list of objects. When an event occurs, the source informs all interested listeners, and passes information regarding the event to the listener.
If a listener is interested in an event it must implement a set of methods that the source can call when the event occurs. This set of methods is called an interface. The concept of an interface will be discussed in greater depth later in the course. Having implemented the interface, the source is then able to call the listener. When a method of the interface is called, a single argument or an event object is passed that conveys information about the event.
The essential steps required for handling events include:
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2. Adding the listener to the source's event list.
3. Implementing the appropriate methods.
4. Using the event object to process the event.
Step 1: Implement the listener:
...
import java.awt.event.*; public class MyApplet extends Applet implements
ActionListener
{
The implements keyword is used in the class definition to inform the java compiler that this class supports a specific type of interface. There are several types of event interfaces supported by Java. The ActionListener and
AdjustmentListener interfaces are described in this section. The
ActionListener interface deals with high level events such as the pressing of a button. The AdjustmentListener is used with controls such as a scroll bar. Other high and low level interfaces exist.
Step 2: Add the listener to the source's event list: public void init ()
{
Button b = new Button (“Help”);
b.addActionListener(this);
add (b);
}
The addActionListener method is used to add the source (the current applet) to the list of listeners interested in listening to the button. The this keyword identifies the current applet as the listener. The method is applied to the button thus making the button the source object.
Step 3: Implement the appropriate method:a public void actionPerformed(ActionEvent evt)
{
LESSON 8
1. Use of the implements keyword by the listener.
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JAVA PROGRAMMING: PART 1
The ActionListener interface has only a single method, actionPerformed. When an interface is implemented by a class, all of its methods must be overridden by that class. The actionPerformed method is passed as a single argument, ActionEvent. This argument contains information about that event.
Step 4: Process the event: if (evt.getSource() instanceof Button)
{
Button source = (Button)evt.getSource(); if (source.getLabel() == “Help”)
/ * ... handle “Help” button ... */
The actionPerformed method may be invoked by more than one source if the listener has added itself to other sources. One of the primary purposes of the ActionEvent object is to differentiate between different sources. The getSource method returns a reference to the object that caused the event. It is used with such methods as getLabel, that returns the button label, to determine the source of the event.
The ActionEvent Class
The ActionEvent c lass contains the event type ACTION_PERFORMED.
An ActionEvent can be fired by:
® clicking a button
® double clicking a list
® choosing a menu item
® pressing when in a text field
The getSource() method is used to determine the object in which the event originated.
An action command is associated with each ActionEvent. This command is a simple String that is assigned to a source object. It is assigned when the object is created using the setActionCommand method. Later, the getActionCommand method can be used within such methods as actionPerformed, to uniquely identify the source of the event. This
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The user may hold down a modifier key such as or when an event is fired. The getModifiers() method can be used to determine which key was held down. The following masks are provided with the
ActionEvent c lass for comparison with the modifier constants:
® ALT_MASK
® CTRL_MASK
® META_MASK
® SHIFT_MASK
Adjustment Events
Adjustment events are fired when the user manipulates an Adjustable item, such as a scrollbar or slider control. They are handled similarly to
Action events.
LESSON 8
technique avoids having to use a label, or some other attribute of the source, that may change with the use of the object.
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JAVA PROGRAMMING: PART 1
To respond to a scrollbar manipulation, implement the adjustmentValueChanged method of the AdjustmentListener interface, and query the AdjustmentEvent object:
...
import java.awt.event.*; public class MyApplet extends Applet implements
AdjustmentListener
{ public void init ()
{
Scrollbar sb = new Scrollbar ( ... ); sb.addAdjustmentListener(this); add (sb);
}
public void adjustmentValueChanged (AdjustmentEvent evt)
{
switch (evt.getAdjustmentType ())
{
case AdjustmentEvent.UNIT_INCREMENT: case AdjustmentEvent.UNIT_DECREMENT: case AdjustmentEvent.BLOCK_INCREMENT: case AdjustmentEvent.BLOCK_DECREMENT: case AdjustmentEvent.TRACK:
Scrollbar sb = (Scrollbar)evt.getAdjustable(); int orientation = sb.getOrientation (); int value = sb.getValue ();
/* ...update display to match value */
}
}
}
The AdjustmentEvent Class
The AdjustmentEvent c lass contains the event type
ADJUSTMENT_VALUE_CHANGED. This event can be fired by:
® manipulating a scrollbar
® manipulating a custom component (such as a slider)
The getAdjustable() method returns a reference to the component that fired the event. If that type of adjustment is needed, the getAdjust-
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® BLOCK_DECREMENT
® BLOCK_INCREMENT
® TRACK
® UNIT_DECREMENT
® UNIT_INCREMENT
®®®
THE paint() METHOD
The Applet c lass derives a class within Java's Abstract Windowing Toolkit
(AWT) library, java.awt.Panel. The Java AWT forms a layer between the viewer or browser and the applet running within it. Every graphical component in the AWT library, including Panel, derives from java.awt.Component. The Component c lass supports a set of methods for drawing, positioning, sizing, etc. The paint() method is one of those common methods.
The Java AWT calls the paint() method to completely render the component. For example, when the window containing the component is resized, it may be necessary to repaint certain window components. The default implementation of paint() does nothing. To draw custom graphical content in your applet, you must override the paint() method. Its signature is: public void paint (Graphics g);
®®®
THE Graphics CLASS
The paint() method takes one parameter, an object of class Graphics.
The object holds a context in which drawing operations may be performed.
The Graphics class provides the methods for drawing; including drawLine() (the method that draws a line), and drawString() (the method that draws text). The paint() method in the “Hello, World” applet calls
LESSON 8
ableType() method will return one of the following values which are defined in the AdjustableEvent c lass:
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Graphics.drawString() to draw the text string “Hello, World” at position (25, 50) within the applet's panel:
FIGURE 8
String placement in Graphics
Class
25
50
Hello, World
The Graphics object clips all graphics operations to within the current draw region.
As shown in the diagram above, the x coordinate of an AWT graphics point represents the number of pixels horizontally from the left of the panel. The y coordinate represents the number of pixels vertically from the top.
®®®
JAVA FONTS
A Java font is represented by an object of class java.awt.Font. A Font object has three attributes:
® family
® style
® size
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® “Helvetica”
® “TimesRoman”
® “Courier”
® “Dialog”
® “DialogInput”
There is also a default font family to be used in the event that a font is missing.
Java provides plain, bold, and italic font styles. Bold and italic may be combined, resulting in four styles, listed in Table J.
TABLE J. Java Styles and Their Symbols
Style
Symbol
Plain
Font.PLAIN
Bold
Font.BOLD
Italic
Font.ITALIC
BoldItalic
Font.BOLD | Font.ITALIC
The third attribute of a font is its point size.
Selecting a Font
To select a font, the programmer must:
® construct a Font object
® install a Font object into the Graphics context
The Font constructor takes three arguments:
® the family (a string)
® style
® point size
LESSON 8
Java provides five font families, each of which maps to a true font on the host platform. A font family is identified by a string:
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For example:
Font myFont = new Font ("Helvetica", Font.PLAIN, 18);
To install a font into a Graphics context, use the Graphics.setFont() method: public class NewHelloWorldApplet extends Applet
{
static Font myFont = new Font ("Courier", Font.PLAIN, 18); public void paint (Graphics g)
{
g.setFont (myFont);
g.drawString ("Hello, World", 25, 50);
}
}
®®®
DRAWING LINES AND SHAPES
Below is an example of an applet (called ShapesApplet.java) that demonstrates some of the drawing functions available through class
Graphics.
import java.applet.*; import java.awt.*; public class ShapesApplet extends Applet
{
public void paint (Graphics g)
{
int left = 0; int top = 0; int right = 319; int bottom = 119;
g.drawRect
g.drawLine
g.drawLine
g.drawOval
}
}
(left, top, right - left, bottom - top);
(left, top, right, bottom);
(right, top, left, bottom);
(left, top, right - left, bottom - top);
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The arguments to drawRect() are: the x coordinate of the left edge of the rectangle, the y coordinate of the top edge of the rectangle, the width of the rectangle in pixels, and the height of the rectangle in pixels drawOval() draws an oval within the rectangle specified by its parameters, which are similar in meaning to those of drawRect().
®®®
DRAWING WITH COLOR
The following example is another applet that demonstrates the use of color for graphics.
ColorsApplet.java:
import java.applet.*; import java.awt.*; public class ColorsApplet extends Applet
{
public void paint (Graphics g)
{
int left = 10; int top = 50; int width = 50; int height = 50;
g.setColor (Color.red);
g.fillRect (left, top, width, height); left += 100;
g.setColor (Color.yellow);
g.fillRect (left, top, width, height); left += 100;
g.setColor (Color.blue);
g.fillRect (left, top, width, height);
}
}
LESSON 8
The arguments to drawLine() are the (x,y) coordinates of the first endpoint and the (x,y) coordinates of the second endpoint. drawLine() draws a line one pixel wide from the first endpoint to the second endpoint, inclusive. 156
JAVA PROGRAMMING: PART 1
This applet displays three square tiles of various colors. The method
Graphics.setColor() is used to set the current color.
®®®
THE Color CLASS
The method Graphics.setColor() installs a color into a graphics context. The color setting remains until another call to setColor(). At the start of the paint() method, the color installed in the graphics context is the foreground color of the applet, usually black. color.red, color.blue, etc., are constant objects of type color, defined as static data members in class Color. The entire list of predefined colors appears in Table K.
TABLE K. RGB Color Values
Color
Red Value
Green Value
Blue Value
white
255
255
255
lightGray
192
192
192
gray
128
128
128
darkGray
64
64
64
black
0
0
0
red
255
0
0
pink
255
175
175
orange
255
200
0
yellow
255
255
0
green
0
255
0
magenta
255
0
255
cyan
0
255
255
blue
0
0
255
If the desired color is not one of the 13 predefined color objects, a new color object can be constructed. The color constructor constructs a new color object from RGB values in the range (0, 255). For example,
Color c = new Color (255, 128, 0);
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c = Color.getHSBColor (0.5, 0.7, 0.2);
HSB values are floating point numbers in the range (0.0, 1.0).
Although these methods allow you to construct millions of different color values, the number of different colors displayable, at one time, is far fewer.
The Java library internals map each color to the closest available color in the current model of the display device.
Foreground and Background Colors
Because an applet is an Abstract Windowing Tookit (AWT) component, it can store two colors: foreground and background. By default, the foreground and background colors of the applet are inherited from the document. The colors are usually black on white or black on gray.
These colors may be accessed through the methods getForeground() and getBackground(), for example:
Color c = getBackground ();
The colors may be set through the methods setForeground() and setBackground(), for example: setForeground (new Color (64, 64, 64));
Before calling the applet's paint() method, the Java AWT fills the panel with the applet's background color. (The viewer or browser calls the init() method before the first call to paint().) At the start of the paint() method, the color installed in the graphics context becomes the applet's foreground color.
CheckerApplet takes advantage of this behavior in the paint() method. Because the Java AWT has already filled the panel with the background color (black), the paint() method does not need to draw the black squares. Because the foreground color is already installed in the graphics
LESSON 8
It is sometimes more convenient to express a color in terms of HSB (hue, saturation, brightness) values. A static method Color.getHSBColor() exists for this purpose:
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JAVA PROGRAMMING: PART 1
context at the start of paint(), the paint method does not need to call
Graphics.setColor(). paint() simply iterates over the visible squares, drawing a foreground square whenever the counter is even: public void paint (Graphics g)
{
Dimension sz = size (); for (int y = 0; y < sz.height; y += squareSize) { int count = y / squareSize; for (int x = 0; x < sz.width; x += squareSize) { if ((count % 2) == 0) {
g.fillRect (x, y, squareSize, squareSize);
}
++count;
}
}
}
The method size() returns an object of type Dimension, giving the current width and height of the applet's pane.
LESSON 8: Writing Java Applets
LESSON SUMMARY
In this lesson, you have learned:
® An applet is a graphical mini-program that runs in the context of another program, such as an applet viewer or a Java-enabled WWW browser. ® The directive is used to embed an applet in HTML documents.
® The Java applet API allow programmers to:
• draw text and graphics into an applet
• work with fonts and colors
LESSON 8
®®®
159
160
JAVA PROGRAMMING: PART 1
REVIEW QUESTIONS
1. What is the fully qualified name of the base class of all applets?
2. What is the name of the method that fetches the value of an applet parameter? 3. Write the full HTML applet tag that starts an applet named
Shape.class at a width of 200 and a height of 100. It also takes a color parameter equal to red and a size parameter equal to 10.
Answers on page 198
LESSON 8: Writing Java Applets
161
1. Modify the “Hello, World” applet to take four parameters named text, font, style and size. These specify what text to display, and what family, style and size of the font to use. Valid values for style include: plain, bold, italic, bolditalic. size is an integer, and must be parsed (see
CheckerApplet for an example). If any of the parameters is unspecified, use a default value.
2. Implement an applet that contains a single button. When the user clicks on that button, display your name at location (100, 100) on the applet.
The code used to illustrate the event handler earlier in this chapter can be used as the basis for this application. Add a string identifier to hold your name and initialize it to an empty string in the init method. In the actionPerformed method, assign your name to the string. The repaint method will force redrawing of the applet. Place this method right after your name has been assigned. Display the string in the paint method. Ensure that the following import statements are included at the beginning of the program: import java.awt.event.*; import java.applet.Applet; import java.awt.*;
LESSON 8
EXERCISE
w w w
APPENDIX A
Hypertext Markup
Language (HTML)
OVERVIEW
This appendix contains a brief overview of some common HTML tags, as well as standard Netscape extensions. Many of the Netscape extensions have been submitted for inclusion in the HTML standard.
164
JAVA PROGRAMMING: PART 1
®®®
HTML HISTORY AND SGML
HTML is an application of the ISO certified Standard Generalized Markup
Language (SGML) standard. SGML was first published in 1988 as a standard for electronic document exchange, archival and processing. There are other de facto standards that have the same objectives, such as Adobe’s
Acrobat, or Microsoft’s RTF (Rich Text Format).
As a subset of SGML, HTML is much simplified. SGML documents are generally more complex and programming-like than HTML documents.
You can use SGML to define HTML, which allows for greater standardization and interchangeability. HTML’s advantage comes from its combination of SGML tags and contructs and standard English text markup notation.
HTML is therefore easy to interpret, yet powerful enough for its purpose.
The HTML standard is maintained by the Internet Engineering Task Force
(IETF).
The next sections list some common HTML tags, and their usage.
APPENDIX A: Hypertext Markup Language (HTML)
Tag
Description
Comments
Document type
Should surround whole document
Document title
Should be in header
Document header
Body
Comment
(not displayed by the browser)
®®®HEAD ELEMENTS
Tag
Description
Comments
Searchable Document
Adds search prompt, only works if document is setup for search
Document’s base URL
xmax) xmax = x; float y = v[i + 1]; if (y < ymin)
APPENDIX B: Java Sample
} this.xmax this.xmin this.ymax this.ymin this.zmax this.zmin
}
=
=
=
=
=
=
xmax; xmin; ymax; ymin; zmax; zmin; }
/** An applet to put a Cehmical model into a page */ public class XYZApp extends Applet implements Runnable {
XYZChemModel md; boolean painted = true; float xfac; int prevx, prevy; float xtheta, ytheta; float scalefudge = 1;
Matrix3D amat = new Matrix3D(), tmat = new Matrix3D();
String mdname = null;
String message = null;
Image backBuffer;
Graphics backGC;
Dimension backSize; private synchronized void newBackBuffer() { backBuffer = createImage(size().width, size().height); backGC = backBuffer.getGraphics(); backSize = size();
}
public void init() { mdname = getParameter(“model”); try { scalefudge = Float.valueOf(getParameter(“scale”)).floatValue();
} catch(Exception e) {
};
amat.yrot(20); amat.xrot(20); if (mdname == null) mdname = “model.obj”; resize(size().width zmax) zmax = z;
179
180
JAVA PROGRAMMING: PART 1
public void run() {
InputStream is = null; try {
Thread.currentThread().setPriority(Thread.MIN_PRIORITY);
is = new URL(getDocumentBase(), mdname).openStream();
XYZChemModel m = new XYZChemModel (is);
Atom.setApplet(this);
md = m;
m.findBB();
float xw = m.xmax - m.xmin; float yw = m.ymax - m.ymin; float zw = m.zmax - m.zmin; if (yw > xw) xw = yw; if (zw > xw) xw = zw; float f1 = size().width / xw; float f2 = size().height / xw; xfac = 0.7f * (f1 < f2 ? f1 : f2) * scalefudge;
} catch(Exception e) {
e.printStackTrace();
md = null; message = e.toString();
}
try { if (is != null) is.close(); } catch(Exception e) {
}
repaint();
}
public void start() { if (md == null && message == null) new Thread(this).start();
}
public void stop() {
}
public boolean mouseDown(Event e, int x, int y) { prevx = x; prevy = y; return true;
}
public boolean mouseDrag(Event e, int x, int y) { tmat.unit(); float xtheta = (prevy - y) * (360.0f / size().width); float ytheta = (x - prevx) * (360.0f / size().height); tmat.xrot(xtheta); tmat.yrot(ytheta); amat.mult(tmat); if (painted) {
APPENDIX B: Java Sample
} prevx = x; prevy = y; return true;
}
public void update(Graphics g) { if (backBuffer == null)
g.clearRect(0, 0, size().width, size().height); paint(g); } public void paint(Graphics g) { if (md != null) { md.mat.unit(); md.mat.translate(-(md.xmin + md.xmax) / 2,
-(md.ymin + md.ymax) / 2,
-(md.zmin + md.zmax) / 2); md.mat.mult(amat); // md.mat.scale(xfac, -xfac, 8 * xfac / size().width); md.mat.scale(xfac, -xfac, 16 * xfac / size().width); md.mat.translate(size().width / 2, size().height / 2, 8); md.transformed = false; if (backBuffer != null) { if (!backSize.equals(size())) newBackBuffer(); backGC.setColor(getBackground()); backGC.fillRect(0,0,size().width,size().height); md.paint(backGC);
g.drawImage(backBuffer, 0, 0, this);
}
else md.paint(g); setPainted();
} else if (message != null) {
g.drawString(“Error in model:”, 3, 20);
g.drawString(message, 10, 40);
}
} private synchronized void setPainted() { painted = true; notifyAll(); } private synchronized void waitPainted()
{
while (!painted)
{
try
{
wait();
APPENDIX B
painted = false; repaint(); 181
182
JAVA PROGRAMMING: PART 1
} catch (InterruptedException e) {}
}
painted = false;
}
}
// end class XYZApp class Atom { private static Applet applet; private static byte[] data; private final static int R = 40; private final static int hx = 15; private final static int hy = 15; private final static int bgGrey = 192; private final static int nBalls = 16; private static int maxr; private int Rl; private int Gl; private int Bl; private Image balls[]; static { data = new byte[R * 2 * R * 2]; int mr = 0; for (int Y = 2 * R; --Y >= 0;) { int x0 = (int) (Math.sqrt(R * R - (Y - R) * (Y - R)) + 0.5); int p = Y * (R * 2) + R - x0; for (int X = -x0; X < x0; X++) { int x = X + hx; int y = Y - R + hy; int r = (int) (Math.sqrt(x * x + y * y) + 0.5); if (r > mr) mr = r; data[p++] = r = 1; --i) { float d = (float) i / maxr; red[i] = (byte) blend(blend(Rl, 255, d), bgGrey, b); green[i] = (byte) blend(blend(Gl, 255, d), bgGrey, b); blue[i] = (byte) blend(blend(Bl, 255, d), bgGrey, b);
}
IndexColorModel model = new IndexColorModel(8, maxr + 1, red, green, blue, 0); balls[r] = applet.createImage( new MemoryImageSource(R*2, R*2, model, data, 0, R*2));
}
} void paint(Graphics gc, int x, int y, int r) {
Image ba[] = balls; if (ba == null) {
Setup();
ba = balls;
}
Image i = ba[r]; int size = 10 + r; gc.drawImage(i, x - (size >> 1), y - (size >> 1), size, size, applet);
}
183
w w w
APPENDIX C
Java Class Hierarchy
OVERVIEW
The Java Class Hierarchy is shown in the next series of pages. Generally, pages are divided by packages, although some packages are too large to fit on a single page. The boxes that are not part of the class library with the centered text serve as keys to the packages represented on that particular page. 186
JAVA PROGRAMMING: PART 1
®®®
JAVA CLASS HIERARCHY
FIGURE 9
KEY
Class
Interface
Extends
Implements
* - Duplicated
APPENDIX C: Java Class Hierarchy
java.lang
Boolean
Character
Class
ClassLoader
Compiler
Math
Number
Double
Float
Integer
Long
Process
Runtime
SecurityManager
String
StringBuffer
System
Thread
ThreadGroup
Throwable
Runnable
APPENDIX C
Object
187
188
JAVA PROGRAMMING: PART 1
O bject (java.lang)
java.io j ava.lang
File
FileDescriptor
InputStream
ByteArraryInputStream
FileInputStream
FilterInputStream
BufferedInputStream
DataInputStream
DataInput*
LineNumberInputStream
PushbackInputStream
PipedInputStream
E xception (java.lang)
SequenceInputStream
IOException
StringBufferInputStream
EOFException
OutputStream
ByteArrayOutputStream
FileNotFoundException
FileOutputStream
InterruptedIOException
FilterOutputStream
BufferedOutputStream
DataOutputStream
DataOutput*
PrintStream
PipedOutputStream
DataInput*
RandomAccessFile
DataInput*
StreamTokenizer
FileNameFilter
APPENDIX C: Java Class Hierarchy
j ava.lang
StreamTokenizer
j ava.io
ContentHandler
E xception (java.lang)
DatagramPacket
DatagramSocket
InetAddress
ServerSocket
Socket
SocketImpl
URL
I OException (java.io)
MalformedURLException
ProtocolException
SocketException
UnknownHostException
UnknowenServiceException
URLConnection
SocketImpFactory
URLEncoder
URLStreamHandlerFactory
URLStreamHandler
ContentHandlerFactory
APPENDIX C
java.net
189
190
JAVA PROGRAMMING: PART 1
java.lang
T hrowable
j ava.awt
Error
A WTError
LinkageError
ClassCircularityError
ClassFormatError
IncompatableClassChangeError
AbstractMethodError
IllegalAccessError
InstantiationError
NoSuchFieldError
NoSuchMethodError
NoClassDefFoundError
UnsatisfiedLinkError
VerifyError
ThreadDeath
VirtualMachineError
InternalError
OutOfMemoryError
StackOverflowError
UnknownError
APPENDIX C: Java Class Hierarchy
j ava.lang
ColorModel
DirectColorModel
IndexColorModel
ImageConsumer*
ImageFilter
C loneable*
CropImageFilter
RGBImageFilter
FilteredImageSource
ImageProducer*
MemoryImageSource
ImageProducer*
PixelGrabber
ImageConsumer*
APPENDIX C
java.awt.image
O bject (java.lang)
191
192
JAVA PROGRAMMING: PART 1
O bject (java.lang)
BorderLayout
LayoutManager*
CardLayout
LayoutManager*
CheckBoxGroup
Color
Component
I mageObserver*
Button
Canvas
java.awt
Checkbox
j ava.awt.image
Choice
j ava.applet
Container
Panel
A pplet
Window
Dialog
FileDialog
Frame
MenuContainer*
Label
List
A ppletContext
Scrollbar
TextComponent
TextArea
TextField
A ppletStub
A udioClip
APPENDIX C: Java Class Hierarchy
j ava.lang
Event
FlowLayout
LayoutManager*
Font
FontMetrics
Graphics
GridBagConstraints
C loneable*
GridBagLayout
LayoutManager*
GridLayout
LayoutManager*
Image
Insets
C loneable*
MediaTracker
MenuComponent
MenuBar
MenuContainer*
MenuItem
Menu
CheckBoxMenuItem
Point
Polygon
Rectangle
Toolkit
MenuContainer*
APPENDIX C
java.awt
Dimension
193
194
JAVA PROGRAMMING: PART 1
java.util
O bject (java.lang)
j ava.lang
B itSet
C loneable*
Date
Dictionary
HashTable
C loneable*
Properties
Observable
Random
StringTokenizer
Enumeration
Vector
C loneable*
Stack
Observer
w w w
ANSWERS
To Review Questions
196
JAVA PROGRAMMING: PART 1
®®®
LESSON 2
Review questions, page 46
1. javac
2. Browsers usually run Java applets, though several programs (including appletviewer and HotJava) can also run Java applets.
3.
4. False. Java applications require a main method, but Java applets do not.
5. False. import merely allows programmers to type shorter names when accessing classes or methods.
®®®
LESSON 3
Review questions, page 74
1. a. int
b.int
c. long
d. double
e. double
2. a. The & operator always evaluates both its left and right operands. The
&& operator always evaluates its left operand, then evaluates its right operand only if the result of the left operand is true.
b. The >> operator fills the high bits of the result with the sign bit of the operand, retaining the original sign in the result. (This is an arithmetic right shift, because it is equivalent to division by a power of two.) The
>>> operator fills the high bits of the result with zero. (This is a logical right shift.)
3. The variable named half has gone out of scope at the close of the compound statement within the do..while loop, but it appears in a relational expression afterward.
ANSWERS: To Review Questions
LESSON 4
Review questions, page 91
1. The declaration is as follows:
Animal Lion = new Animal(500, 45);
2. The following two lines are needed:
Lion.weight = 250;
Lion.length = 35;
3. There may be only one public class in any particular source file.
®®®
LESSON 5
Review questions, page 103
1. A class with default protection can be accessed (and instantiated) only by other classes in the same package.
2. final
3. static
®®®
LESSON 6
Review questions, page 117
1. The length data member is read-only, and as a result can not be explicitly modified.
2. The two methods are: new_sport = sport.concat(“ball”);
// or new_sport = sport + “ball”;
ANSWERS
®®®
197
198
JAVA PROGRAMMING: PART 1
3. StringBuffers are needed to modify character arrays, since regular
Strings are immutable.
®®®
LESSON 7
Review questions, page 136
1. The declarations are as follows: public class Animal
{
} public class Dog extends Animal
{
} public class Bulldog extends Dog
{
}
2. Both protected and private protected members are accessible to derived classes. However, a protected member is accessible to any class in the same package, while a private protected member is not.
3. The super keyword is used in a derived class constructor to call the base class’s constructor.
®®®
LESSON 8
Review questions, page 160
1. java.applet.Applet
2. getParameter
ANSWERS: To Review Questions
ANSWERS
3. The HTML code is:
199
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