Android Unable to instantiate activity ComponentInfo

Android开发环境，真是脆弱....折腾人。

昨晚敲点书上的代码，今天来公司运行，就各种报错，如下：

10-18 10:03:56.063: W/dalvikvm(14661): Unable to resolve superclass of Lcom/example/geoquizfrist/QuizActivity; (761)
10-18 10:03:56.063: W/dalvikvm(14661): Link of class 'Lcom/example/geoquizfrist/QuizActivity;' failed
10-18 10:03:56.073: D/AndroidRuntime(14661): Shutting down VM
10-18 10:03:56.073: W/dalvikvm(14661): threadid=1: thread exiting with uncaught exception (group=0x40ad0228)
10-18 10:03:56.073: E/AndroidRuntime(14661): FATAL EXCEPTION: main
10-18 10:03:56.073: E/AndroidRuntime(14661): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.geoquizfrist/com.example.geoquizfrist.QuizActivity}: java.lang.ClassNotFoundException: com.example.geoquizfrist.QuizActivity
10-18 10:03:56.073: E/AndroidRuntime(14661):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2110)
10-18 10:03:56.073: E/AndroidRuntime(14661):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2229)
10-18 10:03:56.073: E/AndroidRuntime(14661):     at android.app.ActivityThread.access$600(ActivityThread.java:139)
10-18 10:03:56.073: E/AndroidRuntime(14661):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1261)
10-18 10:03:56.073: E/AndroidRuntime(14661):     at android.os.Handler.dispatchMessage(Handler.java:99)
10-18 10:03:56.073: E/AndroidRuntime(14661):     at android.os.Looper.loop(Looper.java:154)

QuizActivity这个类明明有，怎么都不行....

后来发现在，是build path的问题。如下图：

必须按上面的这样，要把Android Private Libraries勾上才可以。而且Android Private Libraries下面必须要有appcompat_v7下的libs，如果没有，CLEAN代码。