Problem with my animation applet

import java.applet.Applet;
import java.awt.*;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

public class test extends Applet implements ActionListener, Runnable
{
    Button start=new Button("Start");
    Button stop=new Button("Stop"); 
    int base;
    int nextSeq;
    int windowSizeValue = 5;
    int messageSizeValue = 12;

    int packetHeight = 15;
    int packetWidth = 15;
    int senderX = 100;
    int senderY = 150;
    int recieverY = 300;
    int interval;
    Packet sender[];
    Thread animThread;
    Dimension offD;
    Image offImage;
    Graphics offG;
    boolean keepDrawing = false;
    String message;
    public void init()
    {

        
        try
        {

        }
        catch (Exception e)
        {
        }
        

        add(start);
        start.addActionListener(this);
        add(stop);
        stop.addActionListener(this);

        base = 0;
        nextSeq = 0;
        Thread animThread = null;
        sender = new Packet[windowSizeValue];
    }
     public void actionPerformed(ActionEvent e)
     {
         if (e.getSource() == start)
         {
                          
             start.setEnabled(false);

             repaint();
             this.startAnim();
            }
         else if (e.getSource() == stop)
         {              
           start.setEnabled(true);
           this.stopAnim();
         }
    }

        public void startAnim()
        {
            if (animThread == null)
            animThread = new Thread(this);
            keepDrawing = true;
            animThread.start();
            for (int i=0; i<messageSizeValue; i++)
            {
                sender[i] = new Packet(0 , 0);
            }
            
            
            
        }
        public void stopAnim() {

            if ( animThread != null && animThread.isAlive())
            keepDrawing = false;
            animThread = null;         
        }


        public void run()
        {

             while (keepDrawing)
              {

                    while (base<messageSizeValue-windowSizeValue) 
                    {base+=1;

                    repaint();
                    try
                    {
                        animThread.sleep(1000); //sleep a little to slow it down
                    }
                    catch (InterruptedException e) {}
                }

         }
    }

        
        
        public void paint(Graphics g)
        {
            update(g);
        }
        
        public void update(Graphics g)
        {
            Dimension d = getSize();
            
            offD = d;
            offImage = createImage(getWidth(), getHeight());
            offG = offImage.getGraphics();
            
            offG.setColor(Color.WHITE);
            offG.fillRect(0,0,getWidth(), getHeight());

            for (int i = 0 ; i < messageSizeValue; i++)
            {


//                 if (sender[i].getStatus() == 0)
//                 {
                    offG.setColor(Color.BLACK);
                    offG.drawRect(senderX+20*i, 150, 15, 15);
                    offG.drawRect(senderX+20*i, 300, 15, 15);
//                 }

            }
            

            offG.setColor(Color.BLACK);

            offG.drawRect(senderX+base*20-3, 146, windowSizeValue*20+1, 23);
            


            g.drawImage(offImage, 0, 0, this);

            
        }
    }

class Packet
{
    int status;
    int position;
    
    Packet()
    {
        this.status = 0;
        this.position = 0;
    }
    
    Packet (int pStatus, int pPosition)
    {
        status = pStatus;
        position = pPosition;
        /*
         * 0 = null
         * 1 = ready
         * 2 = on the way
         * 3 = acknowledge
         * 4 = on the way ack
         * 5 = recieved
         * 6 = unacknowledge
         */
    }
    public int getStatus() {return status;}
    public int getPosition() {return position;}
    public void setStatus(int s)
    {
        this.status = s;
    }
    public void setPosition(int p)
    {
        this.position = p;
    }
}