### Question:
FifoScheduler extends
    AbstractYarnScheduler<FiCaSchedulerApp, FiCaSchedulerNode> implements
    Configurable { @Override public synchronized List<ApplicationAttemptId> getAppsInQueue(String queueName) { if (queueName.equals(DEFAULT_QUEUE.getQueueName())) { List<ApplicationAttemptId> attempts = new ArrayList<ApplicationAttemptId>(applications.size()); for (SchedulerApplication<FiCaSchedulerApp> app : applications.values()) { attempts.add(app.getCurrentAppAttempt().getApplicationAttemptId()); } return attempts; } else { return null; } } FifoScheduler(); @Override void serviceInit(Configuration conf); @Override void serviceStart(); @Override void serviceStop(); @Override synchronized void setConf(Configuration conf); @Override synchronized Configuration getConf(); @Override int getNumClusterNodes(); @Override synchronized void setRMContext(RMContext rmContext); @Override synchronized void reinitialize(Configuration conf, RMContext rmContext); @Override Allocation allocate(
      ApplicationAttemptId applicationAttemptId, List<ResourceRequest> ask,
      List<ContainerId> release, List<String> blacklistAdditions, List<String> blacklistRemovals); @VisibleForTesting synchronized void addApplication(ApplicationId applicationId,
      String queue, String user, boolean isAppRecovering); @VisibleForTesting synchronized void addApplicationAttempt(ApplicationAttemptId appAttemptId,
          boolean transferStateFromPreviousAttempt,
          boolean isAttemptRecovering); @Override void handle(SchedulerEvent event); @Override QueueInfo getQueueInfo(String queueName,
      boolean includeChildQueues, boolean recursive); @Override List<QueueUserACLInfo> getQueueUserAclInfo(); @Override ResourceCalculator getResourceCalculator(); @Override void recover(RMState state); @Override RMContainer getRMContainer(ContainerId containerId); @Override QueueMetrics getRootQueueMetrics(); @Override synchronized boolean checkAccess(UserGroupInformation callerUGI,
      QueueACL acl, String queueName); @Override synchronized List<ApplicationAttemptId> getAppsInQueue(String queueName); Resource getUsedResource();  }### Answer:
@Test public void testGetAppsInQueue() throws Exception { Application application_0 = new Application("user_0", resourceManager); application_0.submit(); Application application_1 = new Application("user_0", resourceManager); application_1.submit(); ResourceScheduler scheduler = resourceManager.getResourceScheduler(); List<ApplicationAttemptId> appsInDefault = scheduler.getAppsInQueue("default"); assertTrue(appsInDefault.contains(application_0.getApplicationAttemptId())); assertTrue(appsInDefault.contains(application_1.getApplicationAttemptId())); assertEquals(2, appsInDefault.size()); Assert.assertNull(scheduler.getAppsInQueue("someotherqueue")); }