getBufferStrategy()空指针异常，求助！

Question

源码：public Graphics2D getGraphics() {
        Window window = device.getFullScreenWindow();
        if (window != null) {
            BufferStrategy strategy = window.getBufferStrategy();
            return (Graphics2D)strategy.getDrawGraphics();
        }
        else {
            return null;
        }
    }
错误信息：Exception in thread "main" java.lang.NullPointerException
at com.snow.meng.graphics.ScreenManager.getGraphics(ScreenManager.java:36)
at com.snow.meng.test.GameCore.gameLoop(GameCore.java:24)
at com.snow.meng.test.GameCore.run(GameCore.java:19)
at com.snow.meng.tilegame.GameManager.main(GameManager.java:9)谢谢各位大虾。

Answer 1

package com.snow.meng.graphics;import java.awt.DisplayMode;
import java.awt.Graphics2D;
import java.awt.GraphicsDevice;
import java.awt.GraphicsEnvironment;
import java.awt.Window;
import java.awt.image.BufferStrategy;import javax.swing.JFrame;public class ScreenManager {
private GraphicsDevice device;

 public ScreenManager() {
        GraphicsEnvironment environment =
            GraphicsEnvironment.getLocalGraphicsEnvironment();
        device = environment.getDefaultScreenDevice();
    } public void setFullScreen(DisplayMode displayMode) {
// TODO Auto-generated method stub
 final JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setUndecorated(true);
        frame.setIgnoreRepaint(true);
        frame.setResizable(false);         device.setFullScreenWindow(frame);
} public Graphics2D getGraphics() {
        Window window = device.getFullScreenWindow();
        if (window != null) {
            BufferStrategy strategy = window.getBufferStrategy();
            return (Graphics2D)strategy.getDrawGraphics();
        }
        else {
            return null;
        }
    }    public int getWidth() {
        Window window = device.getFullScreenWindow();
        if (window != null) {
            return window.getWidth();
        }
        else {
            return 0;
        }
    }    public int getHeight() {
        Window window = device.getFullScreenWindow();
        if (window != null) {
            return window.getHeight();
        }
        else {
            return 0;
        }
    }

}

Answer 2

本机运行正常啊，你的main函数怎么写的
public static void main(String[] args){
ScreenManager sm = new ScreenManager();
sm.setFullScreen(null);
System.out.println(sm.getHeight()+"==="+sm.getWidth());
sm.getGraphics();
}
控制台输出：
800===1280

Answer 3

Exception in thread "main" java.lang.NullPointerException
at com.snow.meng.graphics.ScreenManager.getGraphics(ScreenManager.java:36)
at Test.main(Test.java:9)
768===1024
我的有问题啊。真不明白怎么回事