NoClassFoundError in an Android App

HI guys, I have a problem that I am getting an error as NoClassFound Error in an Activity which is already added in a Manifest File. Thing is that I am usin开发者_如何学Gog FBRocket jar for Facebook implementation. For this we have to implements LoginListner interface to your Activity. And when we implement this interface this error occurred. How this problem be resolved?

Did you add the FBRocket jar to your apk ?

Places the jar on buildpath (right click on jar to do this), so that it can be used to compilation as well as while preparing apk.