如何使用JPanel中间容器

package test3;
//这段程序代码主要是展示如何使用JPanel中间容器
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class test3
{
 static final int WIDTH=300;
 static final int HEIGHT=150;
 public static void main(String[] args)
 {
     JFrame jf=new JFrame("测试程序");
     jf.setLayout(new BorderLayout());
     jf.setSize(WIDTH,HEIGHT);
     jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
     jf.setVisible(true);
     JPanel contentPane=new JPanel();//创建六个中间容器，并且将contentPane放到顶层容器内
     JPanel p1=new JPanel();
     JPanel p2=new JPanel();
     JPanel p3=new JPanel();
     JPanel p4=new JPanel();
     JPanel p5=new JPanel();
     jf.setContentPane(contentPane);
     JButton b1=new JButton("小赵");//创建九个普通按钮组件，将p1到p5五个面板设置为流布局。
     JButton b2=new JButton("小李");
     JButton b3=new JButton("小王");
     JButton b4=new JButton("小孙");
     JButton b5=new JButton("小钱");
     JButton b6=new JButton("小周");
     JButton b7=new JButton("小政");
     JButton b8=new JButton("小武");
     JButton b9=new JButton("姓");
     FlowLayout fl=new FlowLayout();
     FlowLayout fl1=new FlowLayout();
     FlowLayout fl2=new FlowLayout();
     FlowLayout fl3=new FlowLayout();
     FlowLayout fl4=new FlowLayout();
     p1.setLayout(fl);
     p2.setLayout(fl1);
     p3.setLayout(fl2);
     p4.setLayout(fl3);
     p5.setLayout(fl4);

     p1.add(b1); //将b1、b2加到p1中，将b3、b4加到p2中，将b5、b6加到p3中，将b7、b8加到p4中，将b9加到p5中
     p1.add(b2);
     p2.add(b3);
     p2.add(b4);
     p3.add(b5);
     p3.add(b6);
     p4.add(b7);
     p4.add(b8);
     p5.add(b9);
     contentPane.add(p1,"North");//将p1到p5五个面板按照BorderLayout布局方式放置到contentPane面板中
     contentPane.add(p2,"South");
     contentPane.add(p3,"East");
     contentPane.add(p4,"West");
     contentPane.add(p5,"Center");
}
}