How do I solve the problem of "java.lang​.NullPoint​erExceptio​n" in MATLAB?

I am using MATLAB 2017a. Here is my Java Code

 public class HelloWorld {
  public static void main( String args[] )
  {
    System.out.println( "Hello World:" );
    if (args.length >0){
      System.out.println( args[0] );
    }
  }
 public int f1 (String args[]){
      if (args.length >0){
        System.out.println( args[0] );
      }
      return 121;
    }
  }

First I create an object in MATLAB

 o=HelloWorld

Then I run the bellow method for seeing Output.

 javaMethod('main',o,'')

This code runs but gives an error.

 Hello World:
Java exception occurred:
java.lang.NullPointerException
at HelloWorld.main(HelloWorld.java:5)

How do I solve this problem?