Unknown error: java.lang.NullPointerException.

Unknown error: java.lang.NullPointerException.

編譯工程的時候遇到RT錯誤，詭異的是之前都是好的，我就改了改makefile和lib工程的文件

最後找到了方法

I was getting the error when I created a new Android project from source in eclipse and it stumped me for a bit until I took a look at the .project file.  For some reason even after I updated the library links, the old links were not removed.  This
is what was causing the Unknown error: java.lang.NullPointerException.

This should cause the workspace to be re-built and the .project file will get corrected.To fix it close Eclipse and edit your .project file and remove the <linkResources> section and ALL of the links inside. Then go back into eclipse re-setup the links and

do a clean (Project->Clean in the menu) on the affected project.

Happy Developing!

把 .project 下面的 <linkResource>標籤刪掉，clean再build就對了

以上是轉別人的 我的具體解決方法爲：

方法一：更換工作空間File---->Switch WorkSpace

方法二：關閉其他工程，只留當前工程和appcompat_v7工程。