How to determine when a JFrame opened or closed

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COMPLETE SOURCE CODE FOR : DetermineWhenAJframeIsOpenOrClose.java
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import javax.swing.JFrame;
import java.awt.Frame;
import javax.swing.JButton;
import javax.swing.JOptionPane;

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowListener;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;


public class DetermineWhenAJframeIsOpenOrClose
{
  JFrame frame;

  JFrame a=new JFrame("OPEN A JFRAME WITH TITLE'S MONSTER");
  JButton b=new JButton("CLICK HERE TO OPEN");
  
  public DetermineWhenAJframeIsOpenOrClose()
  {
   
  b.addActionListener(new ActionListener()
  {
   public void actionPerformed(ActionEvent evt)
   {
    frame=new JFrame("MONSTER");
    frame.setSize(300,500);
    frame.setVisible(true);
    
    frame.addWindowListener(new WindowAdapter()
    {
     //windowOpened METHOD WILL BE CALLED WHEN A JFRAME IS OPENED
     public void windowOpened(WindowEvent evt)
     {
      JOptionPane.showMessageDialog(null,"MONSTER IS OPEN");
     }
   
     //windowClosing METHOD WILL BE CALLED WHEN A JFRAME IS CLOSING
     public void windowClosing(WindowEvent evt)
     {
      Frame temp=(Frame)evt.getSource();
      temp.dispose();
      JOptionPane.showMessageDialog(null,"MONSTER IS CLOSE");
     }
   
    });
   
   }
  });
  
  a.add(b);
  
  a.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  a.setSize(500,100);
  a.setVisible(true);
  
  }
 
 public static void main(String[]args)
 {
  DetermineWhenAJframeIsOpenOrClose mainObject=new DetermineWhenAJframeIsOpenOrClose();
 }
}

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JUST COMPILE AND EXECUTE IT
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