JAVA 模拟小球自由落体和平抛运动

这个绝对可以的，我都试过了。请采纳，谢谢。
import java.awt.*;
import java.awt.event.*;
import java.applet.*;
public class BallThread extends Applet implements Runnable{
Thread red, blue;
Graphics redPen, bluePen;
int t=0;

public void init(){
red = new Thread(this);
blue = new Thread(this);
redPen = getGraphics();
bluePen = getGraphics();
redPen.setColor(Color.red);
bluePen.setColor(Color.blue);
}
public void start(){
red.start();
blue.start();
}

public void run(){
while(true){
t=t+1;
if(Thread.currentThread()==red){
if(t>100)t=0;
redPen.clearRect(0,0,110,400);
redPen.fillOval(50,(int)(1.0/2*t*9.8),15,15);
try{
red.sleep(40);
}catch(InterruptedException e){}
}else if(Thread.currentThread()==blue){
bluePen.clearRect(120,0,900,500);
bluePen.fillOval(120+7*t,(int)(1.0/2*t*9.8),15,15);
try{
blue.sleep(40);
}catch(Interr