### Question:
QueueConfig extends SerializableBase { public boolean addDeadLetterQueue(DeadLetterQueue deadLetterQueue) { if (deadLetterQueues == null) { deadLetterQueues = new LinkedList<>(); } DeadLetterQueue existing = null; for (DeadLetterQueue dlq : deadLetterQueues) { if (dlq.getName().equals(deadLetterQueue.getName())) { if (dlq.isConsumerEnabled() == deadLetterQueue.isConsumerEnabled()) { return false; } existing = dlq; break; } } if (existing != null) { deadLetterQueues.remove(existing); } deadLetterQueues.add(deadLetterQueue); return true; }  void updateTime(); boolean updateRetryCount(int newRetryCount); boolean addDeadLetterQueue(DeadLetterQueue deadLetterQueue); boolean updateVisibilityTimeout(long newTimeOut); boolean updateConcurrency(MinMax<Integer> concurrency); boolean updatePriorityGroup(String priorityGroup); boolean updatePriority(Map<String, Integer> newPriority); @JsonIgnore boolean isDeadLetterQueue(String name); @JsonIgnore boolean hasDeadLetterQueue();  }### Answer:
@Test public void addDeadLetterQueue() { QueueConfig queueConfig = new QueueConfig(); List<DeadLetterQueue> queues = new LinkedList<>(); queues.add(new DeadLetterQueue("test-dlq", false)); assertTrue(queueConfig.addDeadLetterQueue(new DeadLetterQueue("test-dlq", false))); assertEquals(queues, queueConfig.getDeadLetterQueues()); assertFalse(queueConfig.addDeadLetterQueue(new DeadLetterQueue("test-dlq", false))); assertEquals(queues, queueConfig.getDeadLetterQueues()); assertTrue(queueConfig.addDeadLetterQueue(new DeadLetterQueue("test2-dlq", false))); queues.add(new DeadLetterQueue("test2-dlq", false)); assertEquals(queues, queueConfig.getDeadLetterQueues()); }