Zunächst wieder die Hauptklasse mit der MAIN (UFO)

package ufo;
	import java.awt.*;
	import java.awt.event.*;
	import javax.swing.*;
 
 
	public class Ufo extends JFrame{
		Container c;           
		Zeichenbrett z;   
 
		public Ufo(){
			super("Ufo");
			 c = getContentPane();      
			 z = new Zeichenbrett(); //Alles was passiert, passiert im Zeichenbrett, 
			    // ist ne saubere Lösung oder?  
			 z.setFocusable(true);
			 c.add(z);
			 setTitle("Ufo");
			 setSize(830,700);
			 setVisible(true);
			 setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 
		}
 
	  public static void main(String[] args) {
		  new Ufo();
	  }
	}

Das eigentliche Programm.

package ufo;
 
import java.awt.Graphics;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import javax.swing.JPanel;
 
 
public class Zeichenbrett extends JPanel {
	static int x = 100;
	static int y = 100;
	static double ax = 0;
	static double ay = 0;
	Zeichenbrett z = this;
 
	Zeichenbrett(){
		super();
		this.addKeyListener(new TastaturEingabe());
		Thread g = new Thread(new Gravity());
		g.start();
 
 
	}
 
	public void paintComponent(Graphics g) { 
		super.paintComponent(g);
		zeichneUfo(g, x, y);
 
	}	
 
	void zeichneUfo(Graphics g, int xUfo, int yUfo)
	{
		g.drawRect(xUfo,yUfo,30,15);
		g.drawLine(xUfo+15,yUfo-10,xUfo+15,yUfo);
		g.drawLine(xUfo+10,yUfo+15,xUfo+5,yUfo+25);
		g.drawLine(xUfo+20,yUfo+15,xUfo+25,yUfo+25);
		g.drawLine(0,625,800,625);
	}
 
	class TastaturEingabe implements KeyListener{
 
		@Override
		public void keyPressed(KeyEvent e) {
			if (e.getKeyCode() == KeyEvent.VK_RIGHT)
			{
				ax=ax+10; 
			}
			if (e.getKeyCode() == KeyEvent.VK_LEFT)
			{
				ax=ax-10;   
			}
			if (e.getKeyCode() == KeyEvent.VK_UP)
			{
				ay=ay-1;   
			}
			if (e.getKeyCode() == KeyEvent.VK_DOWN)
			{
				ay=ay+1;
			}
 
 
		}
 
		@Override
		public void keyReleased(KeyEvent e) {
			if (e.getKeyCode() == KeyEvent.VK_RIGHT)
			{
				ax=0; 
			}
			if (e.getKeyCode() == KeyEvent.VK_LEFT)
			{
				ax=0;   
			}
			if (e.getKeyCode() == KeyEvent.VK_UP)
			{
				ay=0;   
			}
			if (e.getKeyCode() == KeyEvent.VK_DOWN)
			{
				ay=0;
			}
		}
 
		@Override
		public void keyTyped(KeyEvent e) {
			// TODO Auto-generated method stub
		}
 
	}
 
	class Gravity implements Runnable{
 
		@Override
		public void run() {
 
 
			double yk = (double)y;	
			double xk = (double)x;
			double vy = 0 ;
			double vx = 0 ;
			long t;
			while (y<1000) {
			t = System.currentTimeMillis();
			try {
				Thread.sleep(20);
			} catch (InterruptedException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} 
			t = System.currentTimeMillis()-t;
			double ayk = 3 + ay;
			vy += ayk*0.005*t; 
			yk+=(vy*t*0.005);
			vx += ax*0.005*t; 
			xk+=(vx*t*0.005);
			if (yk>600) {
				vy=-vy/1.5;
				yk=599;
				vx=-vx/2;
			}
			if (xk>800) xk=0;
			if (xk<0)  yk=800;
 
			y = (int)yk;
			x = (int)xk;
			repaint(); 
		}
	   }
	}
 
}