关于awt和swing的一点小问题请教一下

import java.awt.*;

import java.awt.event.*;



public class TankClient extends Frame {

	

	

	int x = 50, y = 50;

	

	Image offScreenImage = null;

	

	public void paint(Graphics g) {

		Color c = g.getColor();

		g.setColor(Color.RED);

		g.fillOval(x, y, 30, 30);

		g.setColor(c);

		

		y += 5;

	}

	

	public void update(Graphics g) {

		if(offScreenImage == null) {

			offScreenImage = this.createImage(800, 600);

		}

		Graphics gOffScreen = offScreenImage.getGraphics();

		Color c = gOffScreen.getColor();

		gOffScreen.setColor(Color.GREEN);

		gOffScreen.fillRect(0, 0, 800, 600);

		gOffScreen.setColor(c);

		paint(gOffScreen);

		g.drawImage(offScreenImage, 0, 0, null);

	}



	public void lauchFrame() {

		this.setLocation(400, 300);

		this.setSize(800, 600);

		this.setTitle("TankWar");

		this.addWindowListener(new WindowAdapter() {

			public void windowClosing(WindowEvent e) {

				System.exit(0);

			}

		});

		this.setResizable(false);

		this.setBackground(Color.GREEN);

		setVisible(true);

		

		new Thread(new PaintThread()).start();

	}



	public static void main(String[] args) {

		TankClient tc = new TankClient();

		tc.lauchFrame();

	}

	

	private class PaintThread implements Runnable {



		public void run() {

			while(true) {

				repaint();

				try {

					Thread.sleep(50);

				} catch (InterruptedException e) {

					e.printStackTrace();

				}

			}

		}

	}



}