A Simple Java Program

In this tutorial I explain

• How to write an elementary Java program
• How to save
• How to compile & run
• And also a few points

Here's a simple Java source code -

	/* this is a simple Java program */
	class Example {
		// your program begins with a call to main()
		public static void main(String args[])
		{
			System.out.println("This is a simple Java program. Hello Java!");
		}
	}

Here's what we do to save the above piece of code as a valid Java file.

• Open up notepad & write the above piece of code in it .
• Make sure you don't miss out on any of the characters .

Saving the source -

Now that you've written the code in notepad , this is how you'll save it

• Select File/Save As from the notepad menu .
• In the File name field type "Example.java" including the double quotes
• In the Save as type field select All Files ( *.* ) .
• Click enter & the file is saved .

Compiling & running the source -

To compile the source code open up your DOS prompt & move over to the folder that contains the saved java file .

Now execute the compiler , javac , specifying the name of the source file on the command line as shown below .

C:\>javac Example.java

The javac compiler creates a file called Example.class that contains the bytecode version of the program . The output of javac is not a code that can be directly executed but is an intermediate representation of the program that contains instructions that the Java interpreter will execute .

 To run the program , use the Java interpreter , called java & pass the class name Example as the command line argument as shown here .

C:\>java Example

The output of the program is 

  This is a simple Java program

                 When Java source code is compiled , each individual class is put into it's own output file named after the class & using the .class extension . This is why it is a good idea to give the Java source files the same name as the class they contain - the name of the source file will match the name of the .class file .

Why save as Example.java ?

For most compute languages , the name of the file that holds the source code to a program is arbitrary .This is not the case in Java . For the above example , the name of the source file should be Example.java .

In Java ,a source file is officially called a compilation unit . It is a text file that contains one or more class definitions . The Java compiler requires that a source file use the .java filename extension . As the file extension is four characters long DOS & Windows 3.1 are not capable of supporting Java .

In Java , all code must reside inside a class . By convention , the name of that class should match the name of the file that holds theprogram & that is why we named the example as Example.java .This convention makes it easier to maintain & organize programs .

Another important point to note here is that Java is case-sensitive .

A closer look at Example.java !

The program begins with the following lines :

/*

  This is a simple Java program .

  Call this file "Example.java" .

*/

This is a comment & is ignored by the compiler . java supports three types of comments & the one used here is a multiline comment .

The next line of code in the program is :

class Example {

This line uses the keyword class to declare that a new class is being defined . Example is an identifier that is the name of the class . The entire class definition , including all of it's members , will be between the opening curly brace ( { ) & the closing curly brace ( } ) & the use of the curly braces in Java is identical to the way they're used in C/C++ .

An another type of comment is used next :

//your program begins with a call to main

This type of comment is called a single line comment .

 The next line of code is as follows :

  public static void main ( String args [ ] ) {

This line begins the main method . This is the line @ which the program will begin executing . All java applications begin execution by calling main ( ) .

The public keyword is an access specifier , which allows the programmer to control the visibility of class members . When a class member is preceded by or declared as public ,then that member may be accessed by code outside the class in which it is declared . main ( ) is declared public because it must be called by code outside of it's class when the program is started .

The keyword static allows main ( ) to be called without having to instantiate a particular instance of the class . This is necessary since main ( ) is called by the Java interpreter before any objects are made

      The keyword void simply tells the compiler that main ( ) does not return any value .

Note -

The Java compiler will compile classes that do not contain a main ( ) method , but the Java interpreter has no way to run these classes .

The next line of code is :

 System.out.println ( " This is a simple Java program " );

This line outputs the string  " This is a simple Java Program "

      System is a predefined class that provides access to the system , & out is the output stream connected to the console .

Statements end in a semi-colon ( ; ) .The reason why lines other than println ( ) in the example don't end with a " ; " is  'cause they are technically not statements .