public static MultimapBuilderWithKeys<@Nullable Object> hashKeys() {
    return hashKeys(DEFAULT_EXPECTED_KEYS);
  }

  /**
   * Uses a hash table to map keys to value collections, initialized to expect the specified number
   * of keys.
   *
   * @throws IllegalArgumentException if {@code expectedKeys < 0}
   */
  public static MultimapBuilderWithKeys<@Nullable Object> hashKeys(int expectedKeys) {