code: public Object firstKey() throws NoSuchElementException {
          Entry first = (mReverse) ? mLeastRecent : mMostRecent;
          if (first != null) {
              return first.mKey;
          }
          else if (mRecentMap.size() == 0) {
              throw new NoSuchElementException();
          }
          else {
              return null;
          }
      }
docs: Returns the first key in the map, the most recently used. If reverse
  order, then the least recently used is returned.