code: protected K standardLastKey() {
      Entry<K, V> entry = lastEntry();
      if (entry == null) {
        throw new NoSuchElementException();
      } else {
        return entry.getKey();
      }
    }
docs: A sensible definition of {@link #lastKey} in terms of {@code lastEntry}. If you override {@code
  lastEntry}, you may wish to override {@code lastKey} to forward to this implementation.