JAVA EE: Java Tutorial : The Main method

Wednesday, 29 July 2015

Java Tutorial : The Main method

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Understanding the Main Method in Java

In Java, every standalone application needs an entry point where execution begins. That entry point is the main method. Without it, the Java Virtual Machine (JVM) does not know where to start running your program.

Breaking Down the Method Signature

The standard signature looks like this:

    public static void main(String[] args)
  

Let's understand what each keyword represents:

public: An access modifier that lets the JVM invoke this method from outside the class.
static: Allows the JVM to call the method directly without needing to create an object of the class first.
void: Indicates that the method doesn't return any value after execution finishes.
main: The exact name that the JVM looks for to begin program execution.
String[] args: An array of strings used to pass command-line arguments to your application at runtime.

Complete Example: Reading Command-Line Arguments

Here is an example demonstrating how your program receives and iterates through command-line inputs:

public class MyApp {
    public static void main(String[] args) {
        System.out.println("Argument count: " + args.length);

        if (args.length == 0) {
            System.out.println("No arguments passed");
        } else {
            for (String arg : args) {
                System.out.println("Argument: " + arg);
            }
        }
    }
}

Running with Arguments via Command Prompt

To compile and run this code with values passed to String[] args, follow these terminal commands:

    javac MyApp.java

    java MyApp Peter John Julie Ram

The console will output the count along with each argument printed on a new line:

    Argument count: 4

    Argument: Peter

    Argument: John

    Argument: Julie

    Argument: Ram

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MyApp.java

/**
 * This program prints arguments passed in the output screen.
 */
public class MyApp
{

    public static void main(String[] args)
    {

        System.out.println("Argument Length : "+args.length);
        
        if (args.length == 0)
        {
            System.out.println("No arguments passed");  
        }
        else
        {
            for (String arg : args)
            {
                System.out.println(arg);
            }
        }

    }
}