### Question:
HelixNotifier implements Notifier<String> { @Override public void unsubscribe(String topic, TopicListener<String> topicListener) { if (topic == null) { throw new IllegalArgumentException("topic cannot be null"); } if (topicListener == null) { throw new IllegalArgumentException("topicListener cannot be null"); } String topicPath = getTopicPath(topic); HelixPropertyListener helixListener = topicListenerToHelixListenerMap.remove(topicListener); helixStore.unsubscribe(topicPath, helixListener); logger.trace("TopicListener={} has been unsubscribed from topic={}", topicListener, topic); } HelixNotifier(HelixPropertyStore<ZNRecord> helixStore);  HelixNotifier(String zkServers, HelixPropertyStoreConfig storeConfig); @Override boolean publish(String topic, String message); @Override void subscribe(String topic, TopicListener<String> topicListener); @Override void unsubscribe(String topic, TopicListener<String> topicListener);  }### Answer:
@Test public void testUnsubscribeNonExistentListeners() { helixNotifier.unsubscribe(refTopics.get(0), listeners.get(0)); }