code: private void incrementModCounts() {
      if (externalMessageList != null) {
        externalMessageList.incrementModCount();
      }
      if (externalBuilderList != null) {
        externalBuilderList.incrementModCount();
      }
      if (externalMessageOrBuilderList != null) {
        externalMessageOrBuilderList.incrementModCount();
      }
    }
docs: Increments the mod counts so that an ConcurrentModificationException can be thrown if calling
  code tries to modify the builder while its iterating the list.