一个随机移动的小球怎么让它从上到下移动
完整程序如下：

自己写的随机移动的小球
import javax.microedition.midlet.MIDlet;
import javax.microedition.lcdui.*;
import java.util.*;
import javax.microedition.midlet.MIDletStateChangeException;


public class BallTest5 extends MIDlet implements CommandListener {
private Display display;
private Command cmdExit;
private Ball ball;
public BallTest5() {
// TODO 自动生成构造函数存根
display=Display.getDisplay(this);
ball=new Ball(display);
cmdExit=new Command( "退出 ",Command.SCREEN,1);
ball.addCommand(cmdExit);
ball.setCommandListener(this);
}

protected void destroyApp(boolean arg0) throws MIDletStateChangeException {
// TODO 自动生成方法存根

}

protected void pauseApp() {
// TODO 自动生成方法存根

}

protected void startApp() throws MIDletStateChangeException {
// TODO 自动生成方法存根
ball.start();

}
public void commandAction(Command c,Displayable d)
{
if(c==cmdExit)
{
notifyDestroyed();
}
}


public class Ball extends Canvas implements Runnable{
Random random=new Random();
int posX=20;
int posY=20;
int ballsize=60;
Display display;

public Ball(Display display)
{
super();
this.display=display;
}


public void run()
{
while(true)
{
this.posX=(random.nextInt()> > > 1)%(this.getWidth()-20)+10;
this.posY=(random.nextInt()> > > 1)%(this.getHeight()-20)+10;
try{
Thread.sleep(2000);
}
catch(Exception e)
{
e.printStackTrace();
}
repaint();
}
}

public void start()
{
display.setCurrent(this);
Thread t=new Thread(this);
t.start();
repaint();
}

protected void paint(Graphics g)
{
int x=g.getClipX();
int y=g.getClipY();
int width=g.getClipWidth();
int height=g.getClipHeight();
g.setColor(230);
g.fillRect(x, y, width, height);
g.setColor(10);
g.fillArc(posX, posY, ballsize, ballsize, 0, 360);
}
}

}


[解决办法]
public void run()
{
while(true)
{
this.posY -= 20;
if(this.posY > getHeight() - 10) [
posY = 10;
}
try{
Thread.sleep(2000);
} catch (Exception e) {
e.printStackTrace();
}
repaint();
}
}