使用双缓冲区在applet上绘图,为什么会有空指针异常?
package usedoublebuffering;
// 程序:使用双缓冲区
// 范例文件:UseDoubleBuffering.java
import java.awt.*;
import java.applet.*;
public class UseDoubleBuffering extends Applet implements Runnable
{
int X,Y,moveX,moveY,width,height;
Thread newThread;
Image OffScreen;
Graphics drawOffScreen;
public void init()
{
X = 0;
Y = 0;
moveX = 2;
moveY = 3;
//Dimension d=getSize();
//d.width=400;d.height=300;
//setSize(d);
width = getSize().width;
height = getSize().height;
OffScreen = createImage(width,height); //此处会出现空指针异常
drawOffScreen = OffScreen.getGraphics();
}
public void start()
{
newThread = new Thread(this);
newThread.start();
}
public void stop()
{
newThread = null;
}
public void paint(Graphics g)
{
drawOffScreen.setColor(Color.black);
drawOffScreen.fillRect(0,0,width,height);
drawOffScreen.setColor(Color.white);
drawOffScreen.fillOval(X,Y,30,30);
g.drawImage(OffScreen,0,0,this);
}
public void update(Graphics g)
{
paint(g);
}
public void run()
{
while(newThread != null)
{
repaint();
try
{
Thread.sleep(50);
}
catch(InterruptedException E){ }
X = X + moveX;
Y = Y + moveY;
if(X >= (width - 30))
{
X = width - 30;
moveX = -moveX;
}
if(X <= 0)
{
X = 0;
moveX = -moveX;
}
if(Y >= (height - 30))
{
Y = height - 30;
moveY = -moveY;
}
if(Y <= 0)
{
Y = 0;
moveY = -moveY;
}
}
}
}