code: private void ensureBuilders() {
      if (this.builders == null) {
        this.builders = new ArrayList<SingleFieldBuilder<MType, BType, IType>>(messages.size());
        for (int i = 0; i < messages.size(); i++) {
          builders.add(null);
        }
      }
    }
docs: Ensures that the list of builders is not null. If it's null, the list is created and
  initialized to be the same size as the messages list with null entries.