### Question:
MRApps extends Apps { @VisibleForTesting static String[] getSystemClasses(Configuration conf) { return conf.getTrimmedStrings( MRJobConfig.MAPREDUCE_JOB_CLASSLOADER_SYSTEM_CLASSES); }  static String toString(JobId jid); static JobId toJobID(String jid); static String toString(TaskId tid); static TaskId toTaskID(String tid); static String toString(TaskAttemptId taid); static TaskAttemptId toTaskAttemptID(String taid); static String taskSymbol(TaskType type); static TaskType taskType(String symbol); static TaskAttemptStateUI taskAttemptState(String attemptStateStr); static TaskStateUI taskState(String taskStateStr); @SuppressWarnings("deprecation") static void setClasspath(Map<String, String> environment,
      Configuration conf); static void setJobClassLoader(Configuration conf); static Path getStagingAreaDir(Configuration conf, String user); static String getJobFile(Configuration conf, String user, 
      org.apache.hadoop.mapreduce.JobID jobId); static Path getEndJobCommitSuccessFile(Configuration conf, String user,
      JobId jobId); static Path getEndJobCommitFailureFile(Configuration conf, String user,
      JobId jobId); static Path getStartJobCommitFile(Configuration conf, String user,
      JobId jobId); static void setupDistributedCache( 
      Configuration conf, 
      Map<String, LocalResource> localResources); static void addLog4jSystemProperties(
      String logLevel, long logSize, int numBackups, List<String> vargs); static String getSystemPropertiesToLog(Configuration conf); static void setEnvFromInputString(Map<String, String> env,
      String envString, Configuration conf); @Public @Unstable static void addToEnvironment(Map<String, String> environment,
      String variable, String value, Configuration conf); static String crossPlatformifyMREnv(Configuration conf, Environment env); static final Log LOG; }### Answer:
@Test public void testSystemClasses() { final List<String> systemClasses = Arrays.asList(MRApps.getSystemClasses(new Configuration())); for (String defaultXml : DEFAULT_XMLS) { assertTrue(defaultXml + " must be system resource", ApplicationClassLoader.isSystemClass(defaultXml, systemClasses)); } for (String klass : SYS_CLASSES) { assertTrue(klass + " must be system class", ApplicationClassLoader.isSystemClass(klass, systemClasses)); } assertFalse("/fake/Klass must not be a system class", ApplicationClassLoader.isSystemClass("/fake/Klass", systemClasses)); }