[简答题] 阅读下列程序，请写出该程序的功能。

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

public class Class1 extends java.applet.Applet implements Runnable {

Thread myThread = null;

public void start() {

setSize(500, 400);

if (myThread == null) {

myThread = new Thread(this); myThread.start();

}

}

public void run() {

while (myThread != null) {

try { myThread.sleep(500);

} catch (InterruptedException e) { }

repaint();

}

}

public void paint(Graphics g) {

int x = (int)(400*Math.random());

int y = (int)(300*Math.random());

g.setColor(Color.red); g.fillOval(x,y, 10, 10);

}

}