帮帮忙 看哪里出错了 swing 控件编写一个摇奖程序

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class aa extends JFrame implements ActionListener
{
        JButton jbr = new JButton("摇奖");
        JPanel jp = new JPanel();
        CardLayout cl = new CardLayout();
        public aa()
        {
                this.setTitle("使用卡片布局管理器");
                this.setLayout(null);
                jbr.setBounds(120,40,100,20);
                this.add(jbr);
                jbr.addActionListener(this);
                jp.setBounds(300,260,300,200);
                jp.setLayout(cl);
                for(int i=1;i<6;i++)
                {
                    JButton jb = new JButton(i + "等奖");//创建表示奖级别的多个按钮
                    jp.add(jb,""+i);//将新创建的按钮添加到面板中
                }
                    this.add(jp);
                    this.setBounds(200,200,300,200);
                    this.setSize(800,600);
                    this.setVisible(true);
        }
        public void actionPerformed(ActionEvent e)
        {
                int i =(int)Math.floor(Math.random()*6);
                //随即产生一个从1到5之间的随机数
                cl.show(jp,""+i);
                // 显示随即控件
        }
        public static void main(String args[])
        {
                aa s= new aa();
        }
}