code: public RepeatedFieldBuilder<MType, BType, IType> setMessage(
        int index, MType message) {
      if (message == null) {
        throw new NullPointerException();
      }
      ensureMutableMessageList();
      messages.set(index, message);
      if (builders != null) {
        SingleFieldBuilder<MType, BType, IType> entry =
            builders.set(index, null);
        if (entry != null) {
          entry.dispose();
        }
      }
      onChanged();
      incrementModCounts();
      return this;
    }
docs: Sets a  message at the specified index replacing the existing item at
  that index.
  
  @param index the index to set.
  @param message the message to set
  @return the builder