code: public static <T> List<Callback<? super T>> createAdd(
        List<Callback<? super T>> list, Callback<? super T> callback) {
      if (list == null)
        list = new ArrayList<Callback<? super T>>();
      list.add(callback);
      return list;
    }
docs: Adds {@code callback} to {@code list}, creating {@code list} if it is null, and returning (the
  possibly newly created) {@code list}.