### Question:
ConverterUtils { public static String toString(ApplicationId appId) { return appId.toString(); }  static Path getPathFromYarnURL(URL url); static Map<String, String> convertToString(
      Map<CharSequence, CharSequence> env); static URL getYarnUrlFromPath(Path path); static URL getYarnUrlFromURI(URI uri); static String toString(ApplicationId appId); static ApplicationId toApplicationId(RecordFactory recordFactory,
      String appIdStr); static String toString(ContainerId cId); static NodeId toNodeId(String nodeIdStr); static ContainerId toContainerId(String containerIdStr); static ApplicationAttemptId toApplicationAttemptId(
      String applicationAttmeptIdStr); static ApplicationId toApplicationId(
      String appIdStr); static final String APPLICATION_PREFIX; static final String CONTAINER_PREFIX; static final String APPLICATION_ATTEMPT_PREFIX; }### Answer:
@Test public void testContainerIdNull() throws URISyntaxException { assertNull(ConverterUtils.toString((ContainerId)null)); }