Action Event Listener

You could have picked another name besides WindowQuitter for the listener class.

import java.awt.*; 
import java.awt.event.*;
import javax.swing.*; 

public class TwoButtons extends JFrame implements ActionListener
{
  JButton redButton ;
  JButton grnButton ;

  // constructor for TwoButtons
  public TwoButtons()                           
  {
    redButton = new JButton("Red");
    grnButton = new JButton("Green");
    getContentPane().setLayout( new FlowLayout() ); 
    getContentPane().add( redButton );                      
    getContentPane().add( grnButton );
  }

   . . . . more code will go here . . . . 

  public static void main ( String[] args )
  {
    TwoButtons demo  = new TwoButtons() ;
    
    WindowQuitter wquit = new WindowQuitter(); // 2 
    demo.addWindowListener( wquit ); // 3 
    
    demo.setSize( 200, 150 );     
    demo.setVisible( true );      

  }
}

class WindowQuitter extends WindowAdapter  // 1 
{
  public void windowClosing( WindowEvent e )
  {
    System.exit( 0 );  
  }
}

An action listener needs to be registered for the two buttons. There are various ways that this could be done, but let us use one listener that will listen to both of the buttons. The listener object will be the same object as the container, the object of type TwoButtons.