My First program in java

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 file are known as bytecodes.
  
• Run the program
  The Java application launcher tool (java) uses the Java virtual machine to run your application.
  

Create a Source File

To create a source file, you have two options:

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

First, start your editor. You can launch the Notepad editor from the Start menu by selecting Programs > Accessories > Notepad. In a new document, type in the following code:

/**
 * 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 

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Note: Type all code, commands, and file names exactly as shown. Both the compiler (javac) and launcher (java) are case-sensitive, so you must capitalize consistently.

HelloWorldApp
Save the code in a file with the name HelloWorldApp.java. To do this in Notepad, first choose the File > Save As menu item. Then, in the Save As dialog box:

1. Using the Save in combo box, specify the folder (directory) where you'll save your file. In this example, the directory is javaon the Cdrive.
2. In the File name text field, type "HelloWorldApp.java", including the quotation marks.
3. From the Save as type combo box, choose Text Documents (*.txt).
4. In the Encoding combo box, leave the encoding as ANSI.

When you're finished, the dialog box should look like this.

The Save As dialog just before you click Save.

Now click Save, and exit Notepad.

Compile the Source File into a .class File

Bring up a shell, or "command," window. You can do this from the Start menu by choosing Command Prompt (Windows XP), or by choosing Run... and then entering cmd. The shell window should look similar to the following figure.

A shell window.

The prompt shows your current directory. When you bring up the prompt, your current directory is usually your home directory for Windows XP (as shown in the preceding figure.
To compile your source file, change your current directory to the directory where your file is located. For example, if your source directory is javaon the Cdrive, type the following command at the prompt and press Enter:

cd C:\java

Now the prompt should change to C:\java>.

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Note: To change to a directory on a different drive, you must type an extra command: the name of the drive. For example, to change to the javadirectory on the Ddrive, you must enter D:, as shown in the following figure.

Changing directory on an alternate drive.

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If you enter dirat the prompt, you should see your source file, as the following figure shows.

Directory listing showing the .javasource file.

Now you are ready to compile. At the prompt, type the following command and press Enter.

javac HelloWorldApp.java

The compiler has generated a bytecode file, HelloWorldApp.class. At the prompt, type dirto see the new file that was generated, as shown in the following figure.

Directory listing, showing the generated .classfile

Now that you have a .classfile, you can run your program.
If you encounter problems with the instructions in this step, consult the Common Problems (and Their Solutions).

Run the Program

In the same directory, enter the following command at the prompt:

java HelloWorldApp

The next figure shows what you should now see:

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

Congratulations! Your program works!