private int lastKnownIndex;

    MapEntry(int index) {
      this.key = key(index);
      this.lastKnownIndex = index;
    }

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

    private void updateLastKnownIndex() {
      if (lastKnownIndex == -1
          || lastKnownIndex >= size()