public void clear(){
  if (mData.size() > 0) {
    int size=mData.size();
    mData.clear();
    mGroups.clear();
    if (mNotifyOnChange)     notifyItemRangeRemoved(0,size);
  }
}

public void clear ( ) { if ( mData . size ( ) > 0 ) { int size = mData . size ( ) ; mData . clear ( ) ; mGroups . clear ( ) ; if ( mNotifyOnChange ) notifyItemRangeRemoved ( 0 , size ) ; } }
clear all items and groups .
