ButtonFrame.java

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


public class ButtonFrame extends JFrame implements WindowListener{
    private Container contentPane;
    private ButtonPanel b1, b2;

    public ButtonFrame(){
        setTitle("ButtonTest");
        setSize(300, 200);
        addWindowListener(this);

        b1 = new ButtonPanel();   // Create two button panels
        b2 = new ButtonPanel();

        b1.addListener(b1);       // Make each panel a listener for
        b1.addListener(b2);       //   both sets of buttons
        b2.addListener(b1);
        b2.addListener(b2);

        contentPane = this.getContentPane();

        contentPane.setLayout(new BorderLayout());  // Set layout to
        contentPane.add(b1,"North");                //   ensure that
        contentPane.add(b2,"South");                //   panels don't
                                                    //   overlap
    }

    public void windowClosing(WindowEvent e){
      System.exit(0);
    }
    public void windowActivated(WindowEvent e){}
    public void windowClosed(WindowEvent e){}
    public void windowDeactivated(WindowEvent e){}
    public void windowDeiconified(WindowEvent e){}
    public void windowIconified(WindowEvent e){}
    public void windowOpened(WindowEvent e){}

}

The main program that creates and displays a ButtonFrame is the same as before:

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

public class ButtonTest
{  public static void main(String[] args)
   {  JFrame frame = new ButtonFrame();
      frame.show();  
   }
}