Java imitates js to implement tree-like folding menu

package com.tuzi;import java.awt.BorderLayout;  

import java.awt.Color;  import java.awt.GridLayout;  
import java.awt.event.ActionEvent;  
import java.awt.event.ActionListener;   

import javax.swing.JButton;  
import javax.swing.JFrame;  
import javax.swing.JPanel;  
import javax.swing.JScrollPane;  

public class B extends JFrame{
      private JButton btn1,btn2,btn3,btn4,btn5;      
      private JPanel pNorth,pSouth,subMenuContainer;      
      private JScrollPane pCenter;      
      private JButton[] btn = null;      
      private static boolean expand=false;      
      
      public B(){
          btn1=new JButton("Grade1 menu1");
          btn1.setBackground(Color.CYAN);
          btn2=new JButton("Grade1 menu2");
          btn2.setBackground(Color.CYAN);
          btn3=new JButton("Grade1 menu3");
          btn3.setBackground(Color.CYAN);
          btn3.addActionListener(new ActionHandler());

          btn4=new JButton("Grade1 menu4");
          btn4.setBackground(Color.CYAN);
          btn5=new JButton("Grade1 menu5");
          btn5.setBackground(Color.CYAN);
          pNorth=new JPanel();
          pNorth.setLayout(new GridLayout(3,1));
          pSouth=new JPanel();
          pSouth.setLayout(new GridLayout(2,1));
          subMenuContainer=new JPanel();
          subMenuContainer.setLayout(new GridLayout(25,1));

          btn=new JButton[25];        
          for(int i=0;i<btn.length;i++){
              btn[i]=new JButton("[菜单"+i+"]");
              btn[i].setBackground(Color.WHITE);
          }          this.setLayout(new BorderLayout());

          pNorth.add(btn1); pNorth.add(btn2); pNorth.add(btn3); 
          for(int i=0;i<btn.length;i++){
              subMenuContainer.add(btn[i]);
          }
          pCenter=new JScrollPane(subMenuContainer);

          pSouth.add(btn4);pSouth.add(btn5);          
          this.add(pNorth,"North");          
          this.add(pCenter,"Center");          
          this.add(pSouth,"South");         
          
          this.setVisible(true);          
          this.setSize(500,600);          
          this.setResizable(false);          
          this.setLocationRelativeTo(null);         
          this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

      } 
      private class ActionHandler implements ActionListener{

          @Override
          public void actionPerformed(ActionEvent e) {         
               if(btn3==e.getSource()){                  
                  if(expand){//折叠
                     pNorth.setLayout(new GridLayout(3,1));
                     pNorth.remove(btn4);pNorth.remove(btn5);
                     pSouth.add(btn4);pSouth.add(btn5);                     
                     for(int i=0;i<btn.length;i++){
                         subMenuContainer.add(btn[i]);
                     }     
                     validate();
                     getContentPane().repaint();
                     expand=false;
                  }else{//展开
                      for(int i=0;i<btn.length;i++){
                          subMenuContainer.remove(btn[i]);
                      }
                      pSouth.removeAll();
                      pNorth.setLayout(new GridLayout(5,1));
                      pNorth.add(btn4);
                      pNorth.add(btn5);
                      pNorth.repaint();
                      pCenter.repaint();
                      pSouth.repaint();
                      validate();
                      getContentPane().repaint();
                      expand=true;
                 }
             }
         }

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

 }