stream.defaultReadObject();
      factory = (Supplier<? extends Collection<V>>) requireNonNull(stream.readObject());
      Map<K, Collection<V>> map = (Map<K, Collection<V>>) requireNonNull(stream.readObject());
      setMap(map);
    }

    @GwtIncompatible // java serialization not supported
    @J2ktIncompatible
    private static final long serialVersionUID = 0;
  }

  /**