A Word About the Java Platform
The Java platform consists of the Java application programming interfaces (APIs) and the Java virtual machine (JVM).
http://java.sun.com/developer/online...t/platform.gif Java APIs are libraries of compiled code that you can use in your programs. They let you add ready-made and customizable functionality to save you programming time.
The simple program in this lesson uses a Java API to print a line of text to the console. The console printing capability is provided in the API ready for you to use; you supply the text to be printed.
Java programs are run (or interpreted) by another program called the Java VM. If you are familiar with Visual Basic or another interpreted ********, this concept is probably familiar to you. Rather than running directly on the native operating system, the program is interpreted by the Java VM for the native operating system. This means that any computer system with the Java VM installed can run Java programs regardless of the computer system on which the applications were originally developed.
For example, a Java program developed on a Personal Computer (PC) with the Windows NT operating system should run equally well without modification on a Sun Ultra workstation with the Solaris operating system, and vice versa.
Setting Up Your Computer
Before you can write and run the simple Java program in this lesson, you need to install the Java platform on your computer system.
The Java platform is available free of charge from the
java.sun.com web site. You can choose between the Java® 2 Platform software for Windows 95/98/NT or for Solaris. The download page contains the information you need to install and configure the Java platform for writing and running Java programs.
Writing a Program
The easiest way to write a simple program is with a text editor. So, using the text editor of your choice, create a text file with the following text, and be sure to name the text file ExampleProgram.java. Java programs are case sensitive, so if you type the code in yourself, pay particular attention to the capitalization.
كود:
//A Very Simple Example
class ExampleProgram {
public static void main(String[] args){
System.out.println("I'm a Simple Program");
}
}
Here is the
ExampleProgram.java source code file if you do not want to type the program text in yourself.
Compiling the Program
A program has to be converted to a form the Java VM can understand so any computer with a Java VM can interpret and run the program. Compiling a Java program means taking the programmer-readable text in your program file (also called source code) and converting it to bytecodes, which are platform-independent instructions for the Java VM.
The Java compiler is invoked at the command line on Unix and DOS shell operating systems as follows:
javac ExampleProgram.java
http://java.sun.com/im/a.gif
Note: Part of the configuration process for setting up the Java platform is setting the class path. The class path can be set using either the -classpath option with the javac compiler command and java interpreter command, or by setting the CLASSPATH environment variable. You need to set the class path to point to the directory where the ExampleProgram class is so the compiler and interpreter commands can find it. See
Java 2 SDK Tools for more information.
http://java.sun.com/im/a.gif Interpreting and Running the Program
Once your program successfully compiles into Java bytecodes, you can interpret and run applications on any Java VM, or interpret and run applets in any Web browser with a Java VM built in such as Netscape or Internet Explorer. Interpreting and running a Java program means invoking the Java VM byte code interpreter, which converts the Java byte codes to platform-dependent machine codes so your computer can understand and run the program.
The Java interpreter is invoked at the command line on Unix and DOS shell operating systems as follows:
java ExampleProgram
At the command line, you should see:
I'm a Simple Program
Here is how the entire sequence looks in a terminal window:
http://java.sun.com/developer/online...t/examprog.gif
Common Compiler and Interpreter Problems
If you have trouble compiling or running the simple example in this lesson, refer to the
Common Compiler and Interpreter Problems lesson in
The Java Tutorial for troubleshooting help.
Code Comments
Code comments are placed in source files to describe what is happening in the code to someone who might be reading the file, to comment-out lines of code to isolate the source of a problem for debugging purposes, or to generate API documentation. To these ends, the Java ******** supports three kinds of comments: double slashes, C-style, and doc comments.
Double Slashes
Double slashes (//) are used in the C++ programming ********, and tell the compiler to treat everything from the slashes to the end of the line as text.
كود:
//A Very Simple Example
class ExampleProgram {
public static void main(String[] args){
System.out.println("I'm a Simple Program");
}
}
C-Style Comments
Instead of double slashes, you can use C-style comments (/* */) to enclose one or more lines of code to be treated as text.
كود:
/* These are
C-style comments
*/
class ExampleProgram {
public static void main(String[] args){
System.out.println("I'm a Simple Program");
}
}
Doc Comments
To generate documentation for your program, use the doc comments (/** */) to enclose lines of text for the javadoc tool to find. The javadoc tool locates the doc comments embedded in source files and uses those comments to generate API documentation.
كود:
/** This class displays a text string at
* the console.
*/
class ExampleProgram {
public static void main(String[] args){
System.out.println("I'm a Simple Program");
}
}
With one simple class, there is no reason to generate API documentation. API documentation makes sense when you have an application made up of a number of complex classes that need documentation. The tool generates HTML files (Web pages) that describe the class structures and contain the text enclosed by doc comments. The
javadoc Home Page has more information on the javadoc command and its output.
API Documentation The Java platform installation includes API Documentation, which describes the APIs available for you to use in your programs. The files are stored in a doc directory beneath the directory where you installed the platform. For example, if the platform is installed in /usr/local/java/jdk1.2, the API Documentation is in /usr/local/java/jdk1.2/doc/api.
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