public void addBottomSheetCallback(BottomSheetCallback callback){
  if (mCallback == null)   mCallback=new Vector<>();
  mCallback.add(callback);
}

public void addBottomSheetCallback ( BottomSheetCallback callback ) { if ( mCallback == null ) mCallback = new Vector < > ( ) ; mCallback . add ( callback ) ; }
adds a callback to be notified of bottom sheet events .
