checkUnmodifiableMultimap(
        synchronizedSetMultimap(HashMultimap.<@Nullable String, @Nullable Integer>create()), false);
  }

  @J2ktIncompatible
  @GwtIncompatible // SerializableTester
  public void testSerializingUnmodifiableSynchronizedHashMultimap() {
    Multimap<String, Integer> unmodifiable =
        prepareUnmodifiableTests(

      SetMultimap<K, V> multimap, @CheckForNull Object mutex) {
    if (multimap instanceof SynchronizedSetMultimap || multimap instanceof BaseImmutableMultimap) {
      return multimap;
    }
    return new SynchronizedSetMultimap<>(multimap, mutex);
  }

  static class SynchronizedSetMultimap<K extends @Nullable Object, V extends @Nullable Object>
      extends SynchronizedMultimap<K, V> implements SetMultimap<K, V> {

      SetMultimap<K, V> multimap, @CheckForNull Object mutex) {
    if (multimap instanceof SynchronizedSetMultimap || multimap instanceof BaseImmutableMultimap) {
      return multimap;
    }
    return new SynchronizedSetMultimap<>(multimap, mutex);
  }

  static class SynchronizedSetMultimap<K extends @Nullable Object, V extends @Nullable Object>
      extends SynchronizedMultimap<K, V> implements SetMultimap<K, V> {