Java多线程-两个小球

import java.applet.*;
import java.awt.*;
import javax.swing.*;

public class MyBall {
public static void main(String[] args) {
JFrame w = new JFrame();
w.setSize(300, 400);
MyPanel mp = new MyPanel();
w.add(mp);
Thread b1 = new Thread(mp, "b1");
Thread b2 = new Thread(mp, "b2");
b1.start();
b2.start();
w.show();
}
}

class MyPanel extends JPanel implements Runnable {
int gx = 40, gy = 30, tx = 60, ty = 30;
String str1 = "b1", str2 = "b2";

public void run() {
boolean stop = false;
while (!stop) {
String s = Thread.currentThread().getName().toString();
if (s.equals(str1)) {
gy++;
try {
Thread.sleep(60);
} catch (Exception e) {
}
if (gy >= 200)
stop = true;
} else {
ty++;
try {
Thread.sleep(30);
} catch (Exception e) {
}
if (ty >= 200)
stop = true;
}
repaint();
}
}

public void paint(Graphics g) {
super.paint(g);
g.fillOval(gx, gy, 20, 20);
g.fillOval(tx, ty, 20, 20);
}
}