帮忙看看代码哪错了.该怎么解决

帮忙看看代码哪错了.
import   java.awt.BorderLayout;
 
import   javax.swing.JButton;
import   javax.swing.JPanel;
 
public   class   ddd   extends   JPanel{

private   JPanel   ctrlPanel   =   new   JPanel();

public   ddd()
{
this.add(ctrlPanel,BorderLayout.SOUTH);
}
public   void   setCtrlPanel(JPanel   ctrlPanel)   {
this.ctrlPanel   =   ctrlPanel;
}

public   JPanel   getCtrlPanel()
{
return   ctrlPanel;
}

}
 
class   CommonPanel   extends   JPanel
{
private   JButton   addBtn   =   new   JButton( "添加 ");
private   JButton   updateBtn   =   new   JButton( "修改 ");
private   JButton   delBtn   =   new   JButton( "删除 ");
private   JButton   cancelBtn   =   new   JButton( "取消 ");

public   CommonPanel()
{
add(addBtn);
add(updateBtn);
add(delBtn);
add(cancelBtn);
}
}
 
import   javax.swing.JButton;
import   javax.swing.JFrame;
import   javax.swing.JPanel;
 
public   class   ee   extends   ddd{
private   JButton   addBtn   =   new   JButton( "添加 ");
private   JButton   updateBtn   =   new   JButton( "修改 ");
private   JButton   delBtn   =   new   JButton( "删除 ");
private   JButton   cancelBtn   =   new   JButton( "取消 ");

ee()
{
// JPanel   p3   =   this.getCtrlPanel();
// p3.add(addBtn);
// p3.add(updateBtn);
// p3.add(delBtn);
// p3.add(cancelBtn);

JPanel   p3   =     new   CommonPanel();

this.setCtrlPanel(p3);


}
 
/**
  *   @param   args
  */
public   static   void   main(String[]   args)   {
//   TODO   自动生成方法存根
JFrame     ff   =   new   JFrame();
ff.setSize(500,400);

ee   e1   =   new   ee();
ff.add(e1);
ff.setVisible(true);
}
 
}

 


我用setCtrlPanel方法改变了this.ctrlPanel   的引用，为什么界面不会出现那4   个按钮。  


------解决方案--------------------
class ddd1 extends JPanel {

private JPanel ctrlPanel = new JPanel();

public ddd1() {
this.add(ctrlPanel, BorderLayout.SOUTH);
}

public void setCtrlPanel(JPanel ctrlPanel) {
this.remove(this.ctrlPanel);
this.add(ctrlPanel);
this.ctrlPanel = ctrlPanel;
}

public JPanel getCtrlPanel() {
return ctrlPanel;
}

}
这样改就可以了
------解决方案--------------------
试试吧，调整好了．
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import java.awt.BorderLayout;

/**
* <p> Title: </p>
*
* <p> Description: </p>
*
* <p> Copyright: Copyright (c) 2007 </p>
*
* <p> Company: </p>
*
* @author not attributable
* @version 1.0