class MainWindow {
  //create an object of MainWindow
  private static MainWindow instance = new MainWindow();

  //make the constructor private so that this class cannot be
  //instantiated by other class
  private MainWindow(){}

  //Get the only object available
  public static MainWindow getInstance(){
   return instance;
  }

  public void showMessage(){
   System.out.println("Hello World!");
  }
}

public class Main {
  public static void main(String[] args) {
   //Get the only object available
   MainWindow object = MainWindow.getInstance();

   //show the message
   object.showMessage();
  }
}