### Question:
FutureMessageBuffer { public List<Message> retrieveMessagesForHeight(final long height) { chainHeight = height; final List<Message> messages = buffer.getOrDefault(height, Collections.emptyList()); discardPreviousHeightMessages(); return messages; } FutureMessageBuffer(
      final long futureMessagesMaxDistance,
      final long futureMessagesLimit,
      final long chainHeight); void addMessage(final long msgChainHeight, final Message rawMsg); List<Message> retrieveMessagesForHeight(final long height);  }### Answer:
@Test public void retrieveMessagesOnHeightWithNoMessagesReturnsEmptySet() { assertThat(futureMsgBuffer.retrieveMessagesForHeight(3)).isEmpty(); }