Android Runtime Exception: Unable to instantiate activity componentInfo?

i am running a list-fragment program and at run-time i got the below error.

Error:

**02-09 09:03:40.213: ERROR/AndroidRuntime(572): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{ni.android.Fragment/ni.android.Fragment.Fragment}: java.lang.ClassNotFoundException: ni.android.Fragment.Fragment in loader dalvik.system.PathClassLoader[/data/app/ni.android.Fragment-1.apk]**

I don't know the reason.My program contains three classes- DetailsFragment.java, FragmentsTitles.java and Shakespear.java can any one help me out please....