ch1.htm

Java_by_Example,初级经典例子哦,珍藏版本

was already claimed and they changed the name to Java, they had

a powerful, yet simple, language on their hands.

<P>

More importantly, Java was a platform-neutral language, which

meant that programs developed with Java could run on any computer

system with no changes. This platform independence was attained

by using a special format for compiled Java programs. This file

format, called byte-code, could be read and executed by any computer

system that has a Java interpreter. The Java interpreter, of course,

must be written specially for the system on which it will run.

<P>

In 1993, after the World Wide Web had transformed the text-based

Internet into a graphics-rich environment, the Java team realized

that the language they had developed would be perfect for Web

programming. The team came up with the concept of Web applets,

small programs that could be included in Web pages, and even went

so far as to create a complete Web browser (now called HotJava)

that demonstrated the language's power.

<P>

In the second quarter of 1995, Sun Microsystems officially announced

Java. The &quot;new&quot; language was quickly embraced as a powerful

tool for developing Internet applications. Netscape Communications,

the developer of the popular Netscape Navigator Web browser (Figure

1.2), added support for Java to its new Netscape Navigator 2.0.

Other Internet software developers are sure to follow suit, including

Microsoft, whose Internet Explorer 3 (currently in beta) offers

Java support. After more than five years of development, Java

has found its home.

<P>

<A HREF="f1-2.gif"><B> Figure 1.2 : </B><I>The new Netscape Navigator 2.0 Web browser is Java capable.



</I></A><P>

<H2><A NAME="IntroducingJava"><FONT SIZE=5 COLOR=#Ff0000>

Introducing Java</FONT></A></H2>

<P>

By now, you may be curious why Java is considered such a powerful

tool for Internet development projects. You already know that

Java is a simplified version of C++.  Anyone who has struggled

with learning C++ knows that the key word in the previous sentence

is &quot;simplified.&quot; C++ added so much to the C language

that even professional programmers often have difficulty making

the transition.

<P>

According to Sun Microsystems, Java is &quot;simple, object-oriented,

statically typed, compiled, architecture neutral, multi-threaded,

garbage collected, robust, secure, and extensible.&quot; That's

a mouthful, but this description of Java probably doesn't help

you understand the language much. The following list of Java's

attributes, however, should clear out some of the cobwebs:

<UL>

<LI>Simple. Java's developers deliberately left out many of the

unnecessary features of other high-level programming languages.

For example, Java does not support pointer math, implicit type

casting, structures or unions, operator overloading, templates,

header files, or multiple inheritance.

<LI>Object-oriented. Just like C++, Java uses classes to organize

code into logical modules. At runtime, a program creates objects

from the classes. Java classes can inherit from other classes,

but multiple inheritance, wherein a class inherits methods and

fields from more than one class, is not allowed.

<LI>Statically typed. All objects used in a program must be declared

before they are used. This enables the Java compiler to locate

and report type conflicts.

<LI>Compiled. Before you can run a program written in the Java

language, the program must be compiled by the Java compiler. The

compilation results in a &quot;byte-code&quot; file that, while

similar to a machine-code file, can be executed under any operating

system that has a Java interpreter. This interpreter reads in

the byte-code file and translates the byte-code commands into

machine-language commands that can be directly executed by the

machine that's running the Java program. You could say, then,

that Java is both a compiled and interpreted language.

<LI>Multi-threaded. Java programs can contain multiple threads

of execution, which enables programs to handle several tasks concurrently.

For example, a multi-threaded program can render an image on the

screen in one thread while continuing to accept keyboard input

from the user in the main thread. All applications have at least

one thread, which represents the program's main path of execution.

<LI>Garbage collected. Java programs do their own garbage collection,

which means that programs are not required to delete objects that

they allocate in memory. This relieves programmers of virtually

all memory-management problems.

<LI>Robust. Because the Java interpreter checks all system access

performed within a program, Java programs cannot crash the system.

Instead, when a serious error is discovered, Java programs create

an exception. This exception can be captured and managed by the

program without any risk of bringing down the system.

<LI>Secure. The Java system not only verifies all memory access

but also ensures that no viruses are hitching a ride with a running

applet. Because pointers are not supported by the Java language,

programs cannot gain access to areas of the system for which they

have no authorization.

<LI>Extensible. Java programs support native methods, which are

functions written in another language, usually C++. Support for

native methods enables programmers to write functions that may

execute faster than the equivalent functions written in Java.

Native methods are dynamically linked to the Java program; that

is, they are associated with the program at runtime. As the Java

language is further refined for speed, native methods will probably

be unnecessary.

<LI>Well-understood. The Java language is based upon technology

that's been developed over many years. For this reason, Java can

be quickly and easily understood by anyone with experience with

modern programming languages such as C++.

</UL>

<P>

As you can tell from the preceding list of features, a great deal

of thought went into creating a language that would be fairly

easy to use but still provide the most powerful features of a

modern language like C++. Thanks to features such as automatic

garbage collection, programmers can spend more time developing

their programs rather than wasting valuable man-hours hunting

for hard-to-find memory-allocation bugs. However, features such

as Java's object-oriented nature, as well as its ability to handle

multiple threads of execution, ensure that the language is both

up-to-date and powerful.

<H2><A NAME="JavaPrograms"><FONT SIZE=5 COLOR=#Ff0000>

Java Programs</FONT></A></H2>

<P>

As I mentioned previously. Java can be used to create two types

of programs: applets and stand-alone applications. An Applet is

simply a part of a Web page, just as an image or a line of text

can be. Just as a browser takes care of displaying an image referenced

in an HTML document, a Java-enabled browser locates and runs an

Applet . When your Java-capable Web browser loads the HTML document,

the Java applet is also loaded and executed.

<P>

Using applets, you can do everything from adding animated graphics

to your Web pages to creating complete games and utilities that

can be executed over the Internet. Some applets that have already

been created with Java include Bar Chart, which embeds a configurable

bar chart in an HTML document; Crossword Puzzle, which enables

users to solve a crossword puzzle on the Web; and LED Sign, which

presents a scrolling, computerized message to viewers of the Web

page within which the applet is embedded. Figure 1.3 shows a spreadsheet

applet running in Netscape Navigator 2.0.

<P>

<A HREF="f1-3.gif"><B> Figure 1.3 : </B><I>Applets are small programs that are run from within an HTML document.



</I></A><P>

<P>

Although most Java programmers are excited by the ability to create

applets, Java can also be used to create stand-alone applications-that

is, applications that don't need to be embedded in an HTML document.

The most well-known application is the HotJava Web browser itself,

shown in Figure 1.4. This basic browser is completely written

in the Java language, showing how Java handles not only normal

programming tasks such as looping and evaluating mathematical

expressions, but also how it can handle the complexities of telecommunications

programming.

<P>

<A HREF="f1-4.gif"><B> Figure 1.4 : </B><I>The HotJava Web browser is written entirely in the Java programming language.



</I></A><P>

<H2><A NAME="TheJavaDevelopersKit"><FONT SIZE=5 COLOR=#Ff0000>

The Java Developer's Kit</FONT></A></H2>

<P>

Java is actually more than a computer language; it's also a programming

environment that includes a complete set of programming tools.

These tools include a compiler, an interpreter, a debugger, a

disassembler, a profiler, and more. To create a Java program,

you first use a text editor to create the source-code file. You

write the source code, of course, in the Java language. After

completing the source code, which is always saved with a .java

file extension, you compile the program into its byte-code format,

the file for which has the .class file extension. It is the .class

file that the interpreter loads and executes. Because the byte-code

files are fully portable between operating systems, they can be

executed on any system that has a Java interpreter.

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

<TR VALIGN=TOP><TD><B>NOTE</B></TD></TR>

<TR VALIGN=TOP><TD>

<BLOCKQUOTE>

Note that many of Java's tools require long file names, especially the long extensions .java and .class. Both Windows 95 and Windows NT allow these long file names, even under DOS sessions.</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<P>

After compiling and running a Java program, you may discover that

the source code needs modification. The Java debugger can help

you find your errors, whereas the Java profiler provides handy

information about your program. If you run into a compiled Java

program that you'd like to see in source-code form, the Java disassembler

will do the translation for you. Java also includes a program

that creates the files you need to take advantage of native methods

(functions written in another language, such as C++). There's

even a program that can create HTML documents from Java source-code

files. Although all the development tools are DOS applications-that

is, they don't run under Windows-they provide a complete environment

for creating and managing Java projects.

<P>

If you're a little confused about how the many Java programs work

together, don't worry about it. You'll get a chance to learn more

about the Java tools as you work through this book. At this point,

just be aware that Java provides everything you need to create

your own applets and stand-alone applications. In the second part

of this book, you'll start learning the Java language, and in

the third part, you'll start using Java's tools to create your

own applets.

<H2><A NAME="WhereIsJava"><FONT SIZE=5 COLOR=#Ff0000>

Where Is Java?</FONT></A></H2>

<P>

All this talk about Java doesn't do you much good until you get

your own copy of the Java Development Kit (JDK). You'll probably

also want a copy of HotJava and Netscape Navigator 2.0, so that

you can try out the Web pages you create with your Java applets.

Two versions of the JDK are included on this book's CD-ROM.  While