JFrame的show（）方法 问题

package Swing2;



import java.awt.*;

import javax.swing.*;

import java.awt.event.*;



public class EventDemo1 implements ActionListener,WindowListener {



public EventDemo1()

{

JFrame f=new JFrame("EventDemo1");

Container contentPane=f.getContentPane();

JButton b=new JButton("L can speak!");

b.addActionListener(this);

contentPane.add(b);

f.pack();

f.show();                                                          //这里有一处警告！！！

f.addWindowListener(this);

}



public void actionPerformed(ActionEvent e)

{

Toolkit.getDefaultToolkit().beep();

}

public void windowIconified(WindowEvent e){}

public void windowActivated(WindowEvent e){}

public void windowClosed(WindowEvent e){}

public void windowOpened(WindowEvent e){}

public void windowDeactivated(WindowEvent e){}

public void windowDeiconified(WindowEvent e){}

public void windowClosing(WindowEvent e){

System.exit(0);

}



public static void main(String arg[])

{

new EventDemo1();

}



}