main method in java explained

Exploring main() method

• main() is a special method which is called by JVM.
• main() has the standard signature as follows:
• public static void main(String args[])
• static public void main(String args[])
• main method can be overloaded, but JVM always invokes main() method() with the standard signature.
• main() method works as application launches i.e starting point of the application.

Explanation of main method

• public - It can be accessed from anywhere.
• static - It can be accessed using Class Name directly.
• void - It will not return any value to caller.
• main - Method name (Taken from C/C++).
• String[] - To pass command line values.

Example 1:

main method signature has to be the standard one.

class Lab2
{
    static{
        System.out.println("Iam SIB");
    }

    public static void main(String args) 
    {
        System.out.println("Iam main method!");
    }
}

/*
compilation is successful.

output:
Error: Main method not found in class Lab2, please define the main method as:
   public static void main(String[] args)
or a JavaFX application class must extend javafx.application.Application
*/

Example 2:

var args can be used instead of String[] args

class Lab4
{
    public static void main(String...args) 
    {
        System.out.println("Iam main method!");
    }
}

/*
output:
Iam main method
*/

Example 3:

main method can be made final. It cannot be overriden anyway.

class Lab5
{
    final public static void main(String...args) 
    {
        System.out.println("Iam main method!");
    }
}

/*
output:
Iam main method
*/

Summary

1. Running Java Application
   Ex: java Hello
   Before JAVA 7
   • Loads the class
   • Checks whether main() method with standard signature is available or not.
   • If available then invokes main method.
   • If not available then JVM will throw "java.lang.NoSuchMethodError:main" error
   
   Java 7 onwards
   • Checks whether main() method with standard signature is available or not.
   • If available, then invokes then loads the class first and invokes the main() method.
   • If not available, then JVM will throw Error:
     Main method not found in class Hello, please define the main method as:
     public static void main (String args[])
2. You are allowed to use Var-Args as main method parameter.
3. You can use following modifiers for standard main() method.
   • public
   • static
   • final
   • strictfp
   • synchronized
4. You cannot use following modifiers for standard main() method.
• abstract
• native
• private
• protected
5. You can invoke main() method explicitly.
6. You can overload main method.