First Program in Java

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 (notepad) 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.

1.      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 into a class File.

·         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(Java is case-sensitive)     A     a

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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 is not the same as helloworldapp.

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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 java on the C drive.

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.

2.      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  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 java on the C drive, 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 java directory on the D drive, you must enter D:, as shown in the following figure.

Changing directory on an alternate drive.

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

Directory listing showing the .java source 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 dir to see the new file that was generated, as shown in the following figure.

Directory listing, showing the generated .class file

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

If you encounter problems with the instructions in this step, consult the Common Problems (and Their Solutions).

3.      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!

Understanding Hello World App Java Program    

Now you are familiar with the Java program. Before start hard programming in Java, its necessary to understand each and every part of the program. Lets understand the meaning of  public, class, main, String[] args, System.out, and so on.

public class HelloWorldApp{   public static void main(String[] args) {   System.out.println("Hello World!");    }    }

Class Declaration:

Class is the building block in Java, each and every methods & variable exists within the class or object. (instance of program is called object ). The public word specifies the accessibility of the class. The visibility of the class or function can be public, private, etc. The following code declares a new class "HelloWorldApp" with the public accessibility:

public class HelloWorldApp{ 

The main Method: 

The main method is the entry point in the Java program and java program can't run without main method. JVM calls the main method of the class. This method is always first thing that is executed in a java program. Here is the main method:

public static void main(String[] args) {......
.....

}

{ and is used to start the beginning of main block and } ends the main block. Every thing in the main block is executed by the JVM.

The code:

System.out.println("Hello World");

prints the "Hello World" on the console. The above line calls the println method of System.out class.

The keyword static:

The keyword static indicates that the method is a class method, which can be called without the requirement to instantiate an object of the class. This is used by the Java interpreter to launch the program by invoking the main method of the class identified in the command to start the program.

/*

        Swap Numbers Without Using Third Variable Java Example

        This Swap Numbers Java Example shows how to

        swap value of two numbers without using third variable using java.

*/

public class SwapElementsWithoutThirdVariableExample {

        public static void main(String[] args) {

               

                int num1 = 10;

                int num2 = 20;

               

                System.out.println("Before Swapping")

                System.out.println("Value of num1 is :" + num1);

                System.out.println("Value of num2 is :" +num2);

               

                //add both the numbers and assign it to first

                num1 = num1 + num2;

                num2 = num1 - num2;

                num1 = num1 - num2;

               

                System.out.println("Before Swapping");

                System.out.println("Value of num1 is :" + num1);

                System.out.println("Value of num2 is :" +num2);

        } 

 }

/*

Output of Swap Numbers Without Using Third Variable example would be

Before Swapping

Value of num1 is :10

Value of num2 is :20

Before Swapping

Value of num1 is :20

Value of num2 is :10

*/