It's time to write your first application! These detailed instructions are for users of Solaris OS and Linux. Instructions for other platforms are in "Hello World!" for Microsoft Windows and "Hello World!" for Mac OS X.

• A Checklist
• Creating Your First Application
  • Create a Source File
  • Compile the Source File into a .class File
  • Run the Program

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A Checklist 

To write your first program, you'll need:

1. The Java SE Development Kit 6 (JDK 6)

   You can download the Solaris OS or Linux version now. (Make sure you download the JDK, not the JRE.) Consult the installation instructions .

2. A text editor

   In this example, we'll use Pico, an editor available for many UNIX-based platforms. You can easily adapt these instructions if you use a different text editor, such as vi or emacs.

These two items are all you'll need to write your first application.

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Creating Your First Application

Your first application, HelloWorldApp, will simply display the greeting "Hello world!". To create this program, you will: 

• Create a source file

  A source file contains code, written in the Java programming language, that you and other programmers can understand. You can use any text editor to create and edit source files.



• Compile the source file into a .class file

  The Java programming language compiler (javac) takes your source file and translates its text into instructions that the Java virtual machine can understand. The instructions contained within this .class file are known as bytecodes.



• Run the program

  The Java application launcher tool (java) uses the Java virtual machine to run your application.

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Create a Source File

To create a source file, you have two options:

• You can save the file HelloWorldApp.java on your computer and avoid a lot of typing. Then, you can go straight to Compile the Source File.
  
  
• Or, you can use the following (longer) instructions.

First, open a shell, or "terminal," window.

A new terminal window.

When you first bring up the prompt, your current directory will usually be your home directory. You can change your current directory to your home directory at any time by typing cd at the prompt and then pressing Return.

The source files you create should be kept in a separate directory. You can create a directory by using the command mkdir. For example, to create the directory java in your home directory, use the following commands:

            
cd
mkdir java

To change your current directory to this new directory, you then enter:

cd java

Now you can start creating your source file.

Start the Pico editor by typing pico at the prompt and pressing Return. If the system responds with the message pico: command not found, then Pico is most likely unavailable. Consult your system administrator for more information, or use another editor. When you start Pico, it'll display a new, blank buffer. This is the area in which you will type your code.

pico? vi? emacs? Pico is probably the easiest of the three editors to use. If you're curious about how to use the other editors, however, check out these handy reference cards for Vi and Emacs.

Type the following code into the new buffer:

/**
 * The HelloWorldApp class implements an application that
 * simply prints "Hello World!" to standard output.
 */
class HelloWorldApp {
    public static void main(String[] args) {
        System.out.println("Hello World!"); // Display the string.
    }
}

Be Careful When You Type Type all code, commands, and file names exactly as shown. Both the compiler (javac) and launcher tool (java) are case-sensitive, so you must capitalize consistently. HelloWorldApp helloworldapp

Save the code in a file with the name HelloWorldApp.java. In the Pico editor, you do this by typing Ctrl-O and then, at the bottom where you see the prompt File Name to write:, entering the directory in which you wish to create the file, followed by HelloWorldApp.java. For example, if you wish to save HelloWorldApp.java in the directory /home/jdoe/java, then you type /home/jdoe/java/HelloWorldApp.java and press Return.

You can type Ctrl-X to exit Pico.

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Compile the Source File into a .class File

Bring up another shell window. To compile your source file, change your current directory to the directory where your file is located. For example, if your source directory is /home/jdoe/java, type the following command at the prompt and press Return:

cd /home/jdoe/java

If you enter pwd at the prompt, you should see the current directory, which in this example has been changed to /home/jdoe/java.

If you enter ls at the prompt, you should see your file.

Results of the ls command, showing the .java source file.

Now are ready to compile the source file. At the prompt, type the following command and press Return.

javac HelloWorldApp.java

The compiler has generated a bytecode file, HelloWorldApp.class. At the prompt, type ls to see the new file that was generated: the following figure.

Results of the ls command, showing the generated .class file.

Now that you have a .class file, you can run your program.

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Run the Program

In the same directory, enter at the prompt:

java HelloWorldApp

The next figure shows what you should now see.

The output prints "Hello World!" to the screen.

Congratulations! Your program works!

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