21天精通Java：从入门到实战指南

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"《原版21天学通JAVA》是一本面向有一定编程背景读者的Java学习指南。该书旨在全面教授Java语言，并指导读者如何利用Java创建小程序和应用程序。作者假设读者至少具备变量、循环和函数等基础编程概念，无论是拥有多年编程经验的专业人士，还是仅对编程有所了解的新手，都能从中获益。书中内容覆盖广泛，适合那些在HTML方面表现出色，熟悉CGI编程（如Perl、AppleScript、Visual Basic或流行CGI语言）并且寻求进一步提升网页设计技能的人。此外，如果你在学校时期接触过Basic或Pascal，对编程有一定基本理解，但听说Java易于学习且功能强大，这本书也是个理想选择。通过本书，读者将系统地学习Java语法、数据类型、控制结构、类和对象、异常处理、输入/输出以及网络编程等核心概念。同时，书中还包含了大量的实例和练习，帮助读者边学边实践，巩固理论知识。无论是初学者希望通过21天快速入门Java，还是有一定基础想要深入探索的开发者，这本书都能提供一个高效的学习路径，让你在Java的世界里游刃有余。"

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Acknowledgments

From Laura Lemay:

To Sun’s Java team, for all their hard work on Java the language and on the browser, and

particularly to Jim Graham, who demonstrated Java and HotJava to me on very short notice in

May and planted the idea for this book.

To everyone who bought my previous books, and liked them. Buy this one too.

From Charles L. Perkins:

To Patrick Naughton, who first showed me the power and the promise of OAK (Java) in early

1993.

To Mark Taber, who shepherded this lost sheep through his first book.

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Introduction

The World Wide Web, for much of its existence, has been a method for distributing passive

information to a widely distributed number of people. The Web has, indeed, been exceptionally

good for that purpose. With the addition of forms and image maps, Web pages began to become

interactive—but the interaction was often simply a new way to get at the same information. The

limitations of Web distribution were all too apparent once designers began to try to stretch the

boundaries of what the Web can do. Even other innovations, such as Netscape’s server push to

create dynamic animations, were merely clever tricks layered on top of a framework that wasn’t

built to support much other than static documents with images and text.

Enter Java, and the capability for Web pages of containing Java applets. Applets are small

programs that create animations, multimedia presentations, real-time (video) games, multi-user

networked games, and real interactivity—in fact, most anything a small program can do, Java

applets can. Downloaded over the net and executed inside a Web page by a browser that supports

Java, applets are an enormous step beyond standard Web design.

The disadvantage of Java is that to create Java applets right now, you need to write them in the

Java language. Java is a programming language, and as such, creating Java applets is more

difficult than creating a Web page or a form using HTML. Soon there will be tools and programs

that will make creating Java applets easier—they may be available by the time you read this. For

now, however, the only way to delve into Java is to learn the language and start playing with the

raw Java code. Even when the tools come out, you may want to do more with Java than the tools

can provide, and you’re back to learning the language.

That’s where Teach Yourself Java in 21 Days comes in. This book teaches you all about the Java

language and how to use it to create not only applets, but also applications, which are more

general Java programs that don’t need to run inside a Web browser. By the time you get through

with this book, you’ll know enough about Java to do just about anything, inside an applet or

out.

Who Should Read This Book

Teach Yourself Java in 21 Days is intended for people with at least some basic programming

background—which includes people with years of programming experience and people with

only a small amount of experience. If you understand what variables, loops, and functions are,

you’ll be just fine for this book. The sorts of people who might want to read this book include

you, if one or more of the following is true:

■■ You’re a real whiz at HTML, understand CGI programming (in perl, AppleScript,

Visual Basic, or some other popular CGI language) pretty well, and want to move

onto the next level in Web page design.

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■■ You had some Basic or Pascal in school, you’ve got a basic grasp of what programming

is, but you’ve heard Java is easy to learn, really powerful, and very cool.

■■ You’ve programmed C and C++ for many years, you’ve heard this Java thing is

becoming really popular, and you’re wondering what all the fuss is all about.

■■ You’ve heard that Java is really good for Web-based applets, and you’re curious about

how good it is for creating more general applications.

What if you know programming, but you don’t know object-oriented programming? Fear not.

Teach Yourself Java in 21 Days assumes no background in object-oriented design. If you know

object-oriented programming, the first couple of days will be easy for you.

What if you’re a rank beginner? This book might move a little fast for you. Java is a good language

to start with, though, and if you take it slow and work through all the examples, you may still

be able to pick up Java and start creating your own applets.

How This Book Is Organized

Teach Yourself Java in 21 Days describes Java primarily in its current state—what’s known as the

beta API (Application Programming Interface). This is the version of Java that Netscape and

other browsers, such as Spyglass’s Mosaic, support. A previous version of Java, the alpha API,

was significantly different from the version described in this book, and the two versions are not

compatible with each other. There are other books that describe only the alpha API, and there

may still be programs and browsers out there that can only run using alpha Java programs.

Teach Yourself Java in 21 Days uses primarily Java beta because that is the version that is most

current and is the version that will continue to be used in the future. The alpha API is obsolete

and will eventually die out. If you learn Java using beta API, you’ll be much better prepared for

any future changes (which will be minor) than if you have to worry about both APIs at once.

Java is still in development. “Beta” means that Java is not complete and that things may change

between the time this book is being written and the time you read this. Keep this in mind as you

work with Java and with the software you’ll use to create and compile programs. If things aren’t

behaving the way you expect, check the Web sites mentioned at the end of this introduction for

more information.

Teach Yourself Java in 21 Days covers the Java language and its class libraries in 21 days, organized

as three separate weeks. Each week covers a different broad area of developing Java applets and

applications.

In the first week, you’ll learn about the Java language itself:

■■ Day 1 is the basic introduction: what Java is, why it’s cool, and how to get the

software. You’ll also create your first Java applications and applets.

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■■ On Day 2, you’ll explore basic object-oriented programming concepts as they apply to

Java.

■■ On Day 3, you start getting down to details with the basic Java building blocks: data

types, variables, and expressions such as arithmetic and comparisons.

■■ Day 4 goes into detail about how to deal with objects in Java: how to create them,

how to access their variables and call their methods, and how to compare and copy

them. You’ll also get your first glance at the Java class libraries.

■■ On Day 5, you’ll learn more about Java with arrays, conditional statements. and

loops.

■■ Day 6 is the best one yet. You’ll learn how to create classes, the basic building blocks

of any Java program, as well as how to put together a Java application (an application

being a Java program that can run on its own without a Web browser).

■■ Day 7 builds on what you learned on Day 6. On Day 7, you’ll learn more about how

to create and use methods, including overriding and overloading methods and

creating constructors.

Week 2 is dedicated to applets and the Java class libraries:

■■ Day 8 provides the basics of applets—how they’re different from applications, how to

create them, and the most important parts of an applet’s life cycle. You’ll also learn

how to create HTML pages that contain Java applets.

■■ On Day 9, you’ll learn about the Java classes for drawing shapes and characters to the

screen—in black, white, or any other color.

■■ On Day 10, you’ll start animating those shapes you learned about on Day 9, includ-

ing learning what threads and their uses are.

■■ Day 11 covers more detail about animation, adding bitmap images and audio to the

soup.

■■ Day 12 delves into interactivity—handling mouse and keyboard clicks from the user

in your Java applets.

■■ Day 13 is ambitious; on that day you’ll learn about using Java’s Abstract Windowing

Toolkit to create a user interface in your applet including menus, buttons, checkboxes,

and other elements.

■■ On Day 14, you explore the last of the main Java class libraries for creating applets:

windows and dialogs, networking, and a few other tidbits.

Week 3 finishes up with advanced topics, for when you start doing larger and more complex Java

programs, or when you want to learn more:

■■ On Day 15, you’ll learn more about the Java language’s modifiers—for abstract and

final methods and classes as well as for protecting a class’s private information from

the prying eyes of other classes.

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