checkArgument(!delegate.isEmpty());
  }

  @Override
  UnmodifiableIterator<K> keyIterator() {
    return Iterators.unmodifiableIterator(delegate.keySet().iterator());
  }

  @Override
  Spliterator<K> keySpliterator() {
    return delegate.keySet().spliterator();
  }

  @Override
  public int size() {
    return delegate.size();
  }

  @Override
  public boolean containsKey(@Nullable Object key) {

  }

  @Override
  public UnmodifiableIterator<K> iterator() {
    return map.keyIterator();
  }

  @Override
  @GwtIncompatible // Spliterator
  public Spliterator<K> spliterator() {
    return map.keySpliterator();
  }

  @Override
  public boolean contains(@Nullable Object object) {
    return map.containsKey(object);
  }

  @Override
  K get(int index) {

      }

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

  @GwtIncompatible // Spliterator
  Spliterator<K> keySpliterator() {
    return CollectSpliterators.map(
        entrySet().spliterator(),
        Spliterator.ORDERED | Spliterator.DISTINCT | Spliterator.NONNULL | Spliterator.IMMUTABLE,
        Entry::getKey);
  }