Create your first Java program

Create Java file

Let's start by compiling and running the following short sample program.

/* /*w w  w  .  ja v a2 s . c om*/
 This is a simple Java program. Call this file "Main.java". 
 */

public class Main {
  // Your program begins with a call to main().
  public static void main(String args[]) {
    System.out.println("Java.");
  }
}

In Java, a source file is 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.

In Java, all code must reside inside a class. By convention, the name of the public class should match the its file name. And Java is case-sensitive.

The code above generates the following result.

Compiling the Program

To compile the program, execute the compiler, javac, specifying the name of the source file on the command line:

C:\>javac Main.java 

The javac compiler creates a file called Main.class. Main.class contains the byte code version of the program.

To run the program, use the Java interpreter, called java. Pass the class name Main as a command-line argument, as shown here:

C:\>java Main 

When the program is run, the following output is displayed:

When Java source code is compiled, each individual class is put into its own file named classname.class.

A Closer Look at the Main.java

The first part is a comment.

/* 
   This is a simple Java program. Call this file "Main.java". 
*/ 

Comment is a remark for a program. The contents of a comment are ignored by the compiler. The next line of code in the program is shown here:

public class Main { 

The keyword class declares that a new class is being defined. Main is the name of the class. The entire class definition is between the opening curly brace ({) and the closing curly brace (}). The next line in the program is the single-line comment, shown here:

// Your program begins with a call to main(). 

A single-line comment begins with a // and ends at the end of the line. The next line of code is shown here:

public static void main(String args[]) { 

Java applications begin execution by calling main(String args[]). Java is case-sensitive. Thus, Main is different from main.