@ParametricNullness
      K transform(Entry<K, V> entry) {
        return entry.getKey();
      }
    };
  }

  static <K extends @Nullable Object, V extends @Nullable Object> Iterator<V> valueIterator(
      Iterator<Entry<K, V>> entryIterator) {
    return new TransformedIterator<Entry<K, V>, V>(entryIterator) {
      @Override
      @ParametricNullness
      V transform(Entry<K, V> entry) {

      @ParametricNullness
      K transform(Entry<K, V> entry) {
        return entry.getKey();
      }
    };
  }

  static <K extends @Nullable Object, V extends @Nullable Object> Iterator<V> valueIterator(
      Iterator<Entry<K, V>> entryIterator) {
    return new TransformedIterator<Entry<K, V>, V>(entryIterator) {
      @Override
      @ParametricNullness
      V transform(Entry<K, V> entry) {

    }
  }

  final class KeyIterator extends HashIterator<K> {

    @Override
    public K next() {
      return nextEntry().getKey();
    }
  }

  final class ValueIterator extends HashIterator<V> {

    @Override
    public V next() {
      return nextEntry().getValue();
    }
  }

  /**