在做马士兵的坦克大战，做到第十一个版本了。坦克不发炮弹，空指针异常，望指导
我看了一个钟头也没看出哪里空指针异常。各位帮我瞅瞅。
[code=Java][/code]
TankClient类：
import java.awt.Color;
import java.awt.Frame;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
class TankClient extends Frame
{
public static final int GAME_WIDTH=800;
public static final int GAME_HIGHT=600;
Tank tank=new Tank(50, 50,this);
Misssile misssile=null;
Image offscreenImage =null;
public void paint(Graphics g) {
tank.draw(g);
if(misssile!=null)
misssile.draw(g);
}
public void update(Graphics g) {
if(offscreenImage==null)
{
offscreenImage=this.createImage(GAME_WIDTH,GAME_HIGHT);
}
Graphics gofferScreen=offscreenImage.getGraphics();
Color c=gofferScreen.getColor();
gofferScreen.setColor(Color.GREEN);
gofferScreen.fillRect(0, 0, GAME_WIDTH, GAME_HIGHT);
paint(gofferScreen);
g.drawImage(offscreenImage, 0, 0, null);
}
TankClient ()
{
setBounds(10,10,GAME_WIDTH,GAME_HIGHT);
setVisible(true);
setTitle( "TankWar ");
setResizable(false);
setBackground(Color.GREEN);
this.addKeyListener(new KeyMonitor());
this.addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
new Thread(new paintThread()).start();
}
public static void main(String[] args)
{
TankClient frame=new TankClient();
}
private class paintThread implements Runnable
{
public void run() {
while(true)
{
repaint();
System.out.println( "现车不停重画 ");
try {
Thread.sleep(10);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
private class KeyMonitor extends KeyAdapter
{
public void keyReleased(KeyEvent e) {
tank.keyReleased(e);
}
public void keyPressed(KeyEvent e) {
tank.keyPressed(e);
}
}
}
Tank类：
import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.KeyEvent;
public class Tank {
public static final int XSPEED=5;
public static final int YSPEED=5;
TankClient client=null;
private int x,y;
private boolean bL=false,bU=false,bR =false,bD=false;
enum Direction {L,LU,U,RU,R,RD,D,LD,STOP};
private Direction dir=Direction.STOP;
public Tank(int x, int y)
{
this.x = x;
this.y = y;
}
public Tank(int x, int y,TankClient tc)
{
this.x = x;
this.y = y;
this.client=client;
}
public void draw(Graphics g)
{
Color c=g.getColor();
g.setColor(Color.RED);
g.fillOval(x, y, 30, 30);