}
  }

  /**
   * Any object can be the result of a Future, and not every object has a reasonable toString()
   * implementation. Using a reconstruction of the default Object.toString() prevents OOMs and stack
   * overflows, and helps avoid sensitive data inadvertently ending up in exception messages.
   */
  private void appendResultObject(StringBuilder builder, @CheckForNull Object o) {
    if (o == null) {