关于模态窗体中子窗体的退出问题，急、急......

哪位帮忙给看一下下面的程序，我想在模态和非模态下实现相同的功能，既点子窗体的“退出”按钮后，主窗体仍然可用。我现在的问题是当为模态时，点子窗体退出按钮后，主窗体仍然不可用，请给实现一下：////主文件 Test.java  
    
  import   javax.swing.*;   
  import   java.awt.*;   
  import   java.awt.event.*;   
    
  public   class   Test   {   
          private   JFrame   frame   =   null;   
          private   JButton   button   =   null;   
          private   JCheckBox   b   =   null;   
    
          public   Test()   {   
                  frame   =   new   JFrame("测试窗体");   
                  JPanel   pane1   =   new   JPanel(); 
     JPanel   pane2   =   new   JPanel();
                  b   =   new   JCheckBox("模态窗口");   
                  pane1.add(b);   
                  button   =   new   JButton("弹出窗体");
  pane2.add(button);
                  button.addActionListener(new   ActionListener()   {   
                  public   void   actionPerformed(ActionEvent   actionEvent)   {   
                                 My_Frame   my_f   =   new   My_Frame(frame,   b.isSelected(),   String.valueOf(b.isSelected()));   
                                  my_f.setSize(320,200);   
                                  my_f.setLocationRelativeTo(null);   
                                  my_f.setVisible(true);   
                          }   
                  });   
                  frame.getContentPane().add(pane1,   BorderLayout.NORTH);   
                  frame.getContentPane().add(pane2,   BorderLayout.CENTER);   
                  frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);   
                  frame.setSize(600,   450);   
                  frame.setLocationRelativeTo(null);   
                  frame.setVisible(true);   
          }   
    
          public   static   void   main(String[]   args)   {   
                  new   Test();   
          }   
  }   //////子文件 My_Frame.java  
    
  import   javax.swing.*; 
  import   java.awt.*;   
  import   java.awt.event.*; 
  import javax.swing.event.*;   
  import   java.awt.event.WindowListener;   
  import   java.awt.event.WindowEvent;   
    
  public   class   My_Frame extends   JFrame   implements   WindowListener{   
          private   JFrame   frame   =   null;   
          private   boolean   modal   =   false;   
          private   String   title   =   null;   
          public   My_Frame()   {   
                  this(null,   false);   
          }   
          public   My_Frame(JFrame   frame){   
                  this(frame,   false);   
          }   
          public   My_Frame(JFrame   frame,   boolean   modal)   {   
                  this(frame,   modal,   "");   
          }   
          public   My_Frame(JFrame   frame,   boolean   modal,   String   title)   {   
                  super(title);   
                  this.frame   =   frame;   
                  this.modal   =   modal;   
                  this.title   =   title;   
                  this.init();  setLayout(new FlowLayout(1)); 
JButton btb=new JButton("退出"); 
btb.addActionListener(new ActionListener(){ 
public void actionPerformed(ActionEvent e){ 
My_Frame.this.dispose(); //退出子窗体
} 
}); 
   add(btb); 
   setVisible(true);  
          }   
          private   void   init(){   
                  if(modal)   
                          frame.setEnabled(false);   
                  this.addWindowListener(this);   
          }   
          public   void   windowOpened(WindowEvent   windowEvent)   {}   
          public   void   windowClosing(WindowEvent   windowEvent)   {   
                  if(modal)   
                          frame.setEnabled(true);   
          }   
          public   void   windowClosed(WindowEvent   windowEvent)   {}   
          public   void   windowIconified(WindowEvent   windowEvent)   {}   
          public   void   windowDeiconified(WindowEvent   windowEvent)   {}   
          public   void   windowActivated(WindowEvent   windowEvent)   {}   
          public   void   windowDeactivated(WindowEvent   windowEvent)   {   
                  if(modal)   
                          this.requestFocus();   
          }   
  }