return new CustomListMultimap<>(map, factory);
  }

  private static class CustomListMultimap<K extends @Nullable Object, V extends @Nullable Object>
      extends AbstractListMultimap<K, V> {
    transient Supplier<? extends List<V>> factory;

    CustomListMultimap(Map<K, Collection<V>> map, Supplier<? extends List<V>> factory) {
      super(map);
      this.factory = checkNotNull(factory);