Java中，下面一段关于小球运动的单线程动画，为什么"暂停、继续"两按钮按了没效果呢？

图片说明

 import java.awt.*;
import java.awt.event.*;
import java.applet.*;

public class Movement extends Applet implements ActionListener
{
    Frame f=new Frame();
    Panel p=new Panel();
    Button b1=new Button("重新开始");
    Button b2=new Button("暂停");
    Button b3=new Button("继续");

    public Movement()
    {
        p.add(b1);p.add(b2);p.add(b3);
        f.add(p,"South");

        b1.addActionListener(this);
        b2.addActionListener(this);
        b3.addActionListener(this);

        f.pack();
        f.setVisible(true);
    }

    Thread runner;
    public void actionPerformed(ActionEvent e)
    {
        if(e.getSource()==b1)
        {
            i=1;
        }
        if(e.getSource()==b2)
        {
            runner.suspend();           
        }
        if(e.getSource()==b3)
        {
            runner.resume();
        }
    }

    int i=0;
    public void paint(Graphics g)
    {
        i+=10;
        if(i<490)
        {
            g.setColor(Color.red);
            g.fillOval(i,50,10,10);     

            try
            {
                runner.sleep(500);
            }
            catch(InterruptedException e){}


        }
        repaint();
    }

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

 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
    <head>
    </head>
    <body bgcolor="000000">
        <center>
            <applet
                code    = "Movement.java"
                width   = "500"
                height  = "300"
                >
            </applet>
        </center>
    </body>
</html>

https://www.zybang.com/question/cbf55de65a3b45f1c5ef9250fbbb5b05.html