this.map = map;
  }

  /** Used during deserialization only. */
  final void setMap(Map<K, Collection<V>> map) {
    this.map = map;
    totalSize = 0;
    for (Collection<V> values : map.values()) {
      checkArgument(!values.isEmpty());
      totalSize += values.size();
    }
  }

  /**
   * Creates an unmodifiable, empty collection of values.
   *