}
      return String.join(separator, toJoin);
    }
    return join(parts.iterator());
  }

  /*
   * TODO: b/381289911 - Make the Iterator overload use StringJoiner (including Android or not)—or
   * some other optimization, given that StringJoiner can over-allocate:
   * https://bugs.openjdk.org/browse/JDK-8305774
   */

  // TODO: b/381289911 - Optimize MapJoiner similarly to Joiner (including Android or not).