Java by API/java.awt.event/WindowAdapter

extends WindowAdapter

import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class MainClass extends JPanel {
  public MainClass() {
    JButton btn1 = new JButton("Button1");
    add(btn1);
  }
  public static void main(String[] args) {
    JFrame frame = new JFrame();
    frame.getContentPane().add(new MainClass());
    //frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.addWindowListener(new FrameListener());
    
    frame.setSize(200, 200);
    frame.setVisible(true);
  }
}
class FrameListener extends WindowAdapter
{
   public void windowClosing(WindowEvent e)
  {
    System.exit(0);
  }
}