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我已经为一个创建类似乒乓球的游戏的课程准备了这个实验室，并且我已经将.java这个实验室的文件相应地放在了以下链接的 pastebin 上：

Ball.java： http: //pastebin.com/VMg75i8U

Block.java： http: //pastebin.com/T7GCTh4z

BallTestTwo.java： http: //pastebin.com/gZgR8d2J

Tester.java： http: //pastebin.com/fG5f5zZR

Locatable.java： http: //pastebin.com/GE77PLJk

我的问题是我的积木工作得很好，因为我可以在Tester.java注释掉球线的情况下运行并获得我想要的结果。但是，当我在未注释球线的情况下运行它时，我得到了错误并且只是一个空白的白色画布。

如果您需要澄清最终结果应该是什么，这里是指向实验室描述的文字文档的链接。https://docs.google.com/file/d/0B_ifaCiEZgtcTVFPX0t2Mzhzczg/edit?usp=sharing https://docs.google.com/file/d/0B_ifaCiEZgtcMFVWYXIxdjZsdDQ/edit?usp=sharing

编辑 我收到的错误将永远重复，直到我强制终止程序：

at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$200(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(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)

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at BallTestTwo.paint(BallTestTwo.java:42)
    at BallTestTwo.update(BallTestTwo.java:37)
    at sun.awt.RepaintArea.updateComponent(Unknown Source)
    at sun.awt.RepaintArea.paint(Unknown Source)
    at sun.awt.windows.WComponentPeer.handleEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$200(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(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)
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