return new MapEntry(index);
  }

  class MapEntry extends AbstractEntry<K> {
    @ParametricNullness final K key;

    int lastKnownIndex;

    @SuppressWarnings("unchecked") // keys only contains Ks
    MapEntry(int index) {
      this.key = (K) keys[index];
      this.lastKnownIndex = index;
    }

    @Override
    @ParametricNullness
    public K getElement() {
      return key;
    }