code: @Override
  	public Collection<Session> getSessionsForTopic(String topic) {
  		Collection<Session> result;
  		if (map.containsKey(topic)) {
  			return Collections.unmodifiableCollection(map.get(topic));
  		} else {
  			result = Collections.EMPTY_LIST;
  		}
  		return result;
  	}
docs: Get Sessions for topics
  
  @param topic
  @return