试图画到画布时得到一个nullpointerexception错误

我试图通过使用2d数组绘制基本上棋盘图案到屏幕上，并在当前坐标上绘制10x10像素块（基于从阵列位置读取什么字符）。我想这是所有相关的问题代码：试图画到画布时得到一个nullpointerexception错误

public void paint(Graphics g) { 
     super.paint(g); 
     Graphics2D g2d = (Graphics2D) g; 
     g.fillRect(0, 0, this.getWidth(),this.getHeight()); 
     for(int x = 0;x<=3;x++){ 
      for(int y = 0;y<=3;y++){ 
           // NPE occurs on this line: 
       if (globalmap[x][y] == '1'){g2d.fillRect(10*y, 10*x, 10,10);} 
      } 
     } 
}

这是地图数组：

堆栈跟踪：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException 
     at com.side.side.GameEngine.paint(GameEngine.java:64) 
     at javax.swing.JComponent.paintChildren(Unknown Source) 
     at javax.swing.JComponent.paint(Unknown Source) 
     at javax.swing.JComponent.paintChildren(Unknown Source) 
     at javax.swing.JComponent.paint(Unknown Source) 
     at javax.swing.JLayeredPane.paint(Unknown Source) 
     at javax.swing.JComponent.paintChildren(Unknown Source) 
     at javax.swing.JComponent.paintToOffscreen(Unknown Source) 
     at javax.swing.RepaintManager$PaintManager.paintDoubleBuffered(Unknown Source) 
     at javax.swing.RepaintManager$PaintManager.paint(Unknown Source) 
     at javax.swing.RepaintManager.paint(Unknown Source) 
     at javax.swing.JComponent.paint(Unknown Source) 
     at java.awt.GraphicsCallback$PaintCallback.run(Unknown Source) 
     at sun.awt.SunGraphicsCallback.runOneComponent(Unknown Source) 
     at sun.awt.SunGraphicsCallback.runComponents(Unknown Source) 
     at java.awt.Container.paint(Unknown Source) 
     at java.awt.Window.paint(Unknown Source) 
     at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source) 
     at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source) 
     at javax.swing.RepaintManager.seqPaintDirtyRegions(Unknown Source) 
     at javax.swing.SystemEventQueueUtilities$ComponentWorkRequest.run(Unknown Source) 
     at java.awt.event.InvocationEvent.dispatch(Unknown Source) 
     at java.awt.EventQueue.dispatchEvent(Unknown Source) 
     at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source) 
     at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source) 
     at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source) 
     at java.awt.EventDispatchThread.pumpEvents(Unknown Source) 
     at java.awt.EventDispatchThread.pumpEvents(Unknown Source) 
     at java.awt.EventDispatchThread.run(Unknown Source)

64号线：

if (globalmap[x][y] == '1') { g2d.fillRect(10*y, 10*x, 10,10); }

来源

2011-12-18 purpleladydragons

......哪条线是线64？ – 2011-12-18 15:33:47

你确定globalmap不是null吗？ – 2011-12-18 15:37:07

说真的，第64行是什么？你知道我们看不到你的显示器，对吧？ – 2011-12-18 15:51:07

可能是你的全局映射数组元素是空的，但很难根据你发布的内容来判断。请注意，此行是可怕的：

if (globalmap[x][y] == '1'){g2d.fillRect(10*y, 10*x, 10,10);}

您需要如果只用于调试目的传播这一点上几行：

if (globalmap[x][y] == '1') { 
    g2d.fillRect(10*y, 10*x, 10,10); 
}

来源

2011-12-18 15:57:41

发现将值分配给永不执行的全局映射的函数。非常感谢你指出这一点，除了帮助我的坏习惯外。 – purpleladydragons 2011-12-18 16:08:59

@purpleladydons：aha，很高兴你有它的工作。对于这种类型的问题，我有时在抛出NPE的行上添加println语句来查看错误，像'System.out.println（“globalmap is null：”+（globalmap == null））;'' System.out.println（“globalmap [x] [y]为null：”+（globalmap [x] [y] == null））;'等等......这就是为什么println有时被称为“穷人男人的调试器“。您也可以使用大多数IDE的调试功能。 – 2011-12-18 16:11:10

试图画到画布时得到一个nullpointerexception错误

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