大家帮找找这个空直引用的错误！！谢谢！

[hpym365]

错误是在第84行！ 说
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;

public class MoveBall
{
public static void main(String [] args)
{
BallFrame b=new BallFrame();
b.setVisible(true);
}
}

class BallFrame extends JFrame
{
JPanel jp=new JPanel();
public BallFrame()
{
this.setBounds(100,150,400,300);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JPanel con=(JPanel)this.getContentPane();

jp.setBackground(Color.black);
con.add(jp);
new Ball(jp).start();
}
}



class Ball extends Thread
{
int r=0;
int moveX=2;
int moveY=2;
int ballX=50;
int ballY=50;
int WIDTH=10;
JPanel jp=new JPanel();
public Ball(JPanel jp)
{
this.jp=jp;
System.out.println("jpjp");
jp.addKeyListener(new KeyAdapter()
{
public void keyPressed(KeyEvent e)
{
switch(e.getKeyCode())
{
case KeyEvent.VK_LEFT:r=0;System.out.println("VK_LEFT");break;
case KeyEvent.VK_RIGHT:r=1;System.out.println("VK_RIGHT");break;
case KeyEvent.VK_UP:r=2;System.out.println("VK_UP");break;
case KeyEvent.VK_DOWN:r=3;System.out.println("VK_DOWN");break;
default:System.out.println("others");
}
}
});
jp.setFocusable(true);

}

public void move()
{
Graphics g=jp.getGraphics();
g.setXORMode(jp.getBackground());//错误在这里！！！

g.fillOval(ballX,ballY,WIDTH,WIDTH);

switch(r)
{
case 0:ballX+=-moveX;
case 1:ballY+=moveX;
case 2:ballY+=-moveX;
case 3:ballY+=moveX;
}
g.fillOval(ballX,ballY,WIDTH,WIDTH);
g.dispose();
}

public void run()
{
Graphics g=jp.getGraphics();
if(jp==null)
System.out.println("null");
g.setXORMode(jp.getBackground());
g.dispose();
while(true)
{
this.move();
try
{
Thread.sleep(10);
}
catch(Exception ex){}
}
}
}

[hpym365]

主要是得不到图形对象 大家帮忙看看

[hpym365]

自己解决了。

[图门的世界]

修改一下你的BallFrame定义

class BallFrame extends JFrame
{
JPanel jp=new JPanel();
public BallFrame()
{
this.setBounds(100,150,400,300);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JPanel con=(JPanel)this.getContentPane();

jp.setBackground(Color.black);
con.add(jp);
new Ball(jp).start();
}
}

改为:

class BallFrame extends JFrame {
JPanel jp = new JPanel();

public BallFrame() {
this.setBounds(100, 150, 400, 300);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JPanel con = (JPanel) this.getContentPane();

jp.setBackground(Color.black);
con.add(jp);
this.setVisible(true);//增加这行代码,显示之后才能取的到graphics
new Ball(jp).start();
}
}

[ys_wangtao]

跟在后面接分了

[interpb]

jf

[treeroot]

jf]

[WiseDragon]

jf

[iambic]

接两分。

[leekooqi]

jf

[yuzl32]

gx

[如果这都不算爱]

jf

[jinancf]

JF

[cleansunshing]

jf

[sundeveloper]

up

[Mark_Chen]

接分！！