Java Applet ile yapılmış kendi kendine büyüyüp küçülen top

import java.applet.Applet;
import java.awt.Color;
import java.awt.Graphics;

public class balon extends Applet implements Runnable {
    int r,x,y,d;
    public void init() {
        setSize(500, 500);
        Thread t = new Thread(this);
        t.start();
        x=getWidth()/2;
        y=getHeight()/2;d=10;
    }

    public void paint(Graphics g) {

        g.drawOval(x,y ,r, r);
        r+=d;
        y-=d/2;
        x-=d/2;
    }

    public void run() {
        try{
        while(true){
            Thread.sleep(50);
            if (x<0&&y<0) {
                d=d*-1;
            }
            if(x>(getWidth()/2)&&y>getHeight()/2) {
                d=d*-1;
            }
            repaint();
        }}
        catch(Exception e){}
    }

}

Ekran Görüntüsü;