return Collections.unmodifiableMap(map);
    }
  }

  static class MapDifferenceImpl<K extends @Nullable Object, V extends @Nullable Object>
      implements MapDifference<K, V> {
    final Map<K, V> onlyOnLeft;
    final Map<K, V> onlyOnRight;
    final Map<K, V> onBoth;
    final Map<K, ValueDifference<V>> differences;

    MapDifferenceImpl(
        Map<K, V> onlyOnLeft,
        Map<K, V> onlyOnRight,