Builder<E> combine(Builder<E> other) {
      if (hashTable != null) {
        for (int i = 0; i < other.size; ++i) {
          // requireNonNull is safe because the first `size` elements are non-null.
          add((E) requireNonNull(other.contents[i]));
        }
      } else {
        addAll(other.contents, other.size);
      }
      return this;
    }

    /**