java se

那位高手给小弟指点哈下边这个程序怎么在没有达到预期的功能。
功能：实现多个小球的运动，开始时没有小球，每单击一次鼠标增加一个小球。注：现在只完成了一个小球的运动，执行多次调用内部类就可以实现其它小球
package eleven;import java.applet.Applet;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import javax.swing.JFrame;public class Eleven extends Applet { 
    int dx = 5, dy = 5;
    int rgb = 0;    
    int x = 60;
    int y = 100;
    Color color = new Color((int)(Math.random() * 0xFFFFFF));    public static void main(String[] args) {
  Eleven e =  new Eleven();
    }    public Eleven(){
        setSize(350, 350);
        setBackground(Color.gray);
        this.addMouseListener(new MouseAdapter(){
             @Override
            public void mousePressed(MouseEvent e){
                 new PaintRound();
            }
        });
        this.setLayout(null);
        setVisible(true);          
    }
    class PaintRound implements Runnable{        public PaintRound(){
            Thread t = new Thread(this);
            t.start();
        }
        public void doColor() {
            rgb = (int)(Math.random()*0xFFFFFF);
            color = new Color(rgb);
        }
        public void run() {
            while (true) {
                if (x <= 0) {
                    dx = -dx;
                    doColor();
                    ThreadGroup tg = new ThreadGroup("first");                }else if ((x + 50) >= new Eleven().getWidth()) {
                    dx = -dx;
                    doColor();
                }
                if (y <= 0) {
                    dy = -dy;
                    doColor();
                }else if ((y + 50) >= new Eleven().getHeight()){
                    dy = -dy;
                    doColor();
                }
                x = x + dx;
                y = y + dy;
                try {
                    Thread.sleep(50);
                    repaint();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
        public void paint( Graphics g) {
            Color c = g.getColor();
            g.setColor(Color.white);
            g.fillOval(x, y, 50, 50);
            g.setColor(c);     
        }    
    }
}