code: public int getAdapterPosition(long identifier) {
          for (int i = 0, size = mOriginalItems.size(); i < size; i++) {
              if (mOriginalItems.get(i).getIdentifier() == identifier) {
                  return i;
              }
          }
          return -1;
      }
docs: Searches for the given identifier and calculates its relative position
  
  @param identifier the identifier of an item which is searched for
  @return the relative position