import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class WindowDemo extends JFrame implements ActionListener,WindowListener
{  JButton b1,b2,b3;
   JPanel p1,p2;
   JTextArea jta;
   Container cp=null;
   public WindowDemo()
     { p1=new JPanel();
              jta=new JTextArea(5,5);
       p1.setLayout(new BorderLayout());
       p1.add(jta,BorderLayout.CENTER);
       jta.setLineWrap(true);
       p2=new JPanel();
       p2.setLayout(new FlowLayout());
       p2.add(b1=new JButton("OK"));
       p2.add(b2=new JButton("Cancel"));
       p2.add(b3=new JButton("Exit"));
       cp=getContentPane();
       cp.setLayout(new BorderLayout());
       cp.add(p1,BorderLayout.CENTER);
       cp.add(p2,BorderLayout.SOUTH);
       b1.addActionListener(this);
       b2.addActionListener(this);
       b3.addWindowListener(new CloseWindow());  /这里有误/
       pack();
       setSize(300,300);
       setVisible(true);
       setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
     }
  
public void actionPerformed(ActionEvent e)
    { String command=e.getActionCommand();
                      if(command.equals("OK"))
              jta.setText("OK");
         else if(command.equals("Cancel"))
              jta.setText("Cancel");
    }
class CloseWindow extends WindowAdapter
{ public void windowClosing(WindowEvent e)
     { System.exit(0);}
}

public static void main(String args[])
     { new WindowDemo();}
}


上面的程序提示我  找不着变量b3  但我不明白怎么为何  请明白人帮忙看看  谢谢