### Question:
SecurityUtil { static boolean isTGSPrincipal(KerberosPrincipal principal) { if (principal == null) return false; if (principal.getName().equals("krbtgt/" + principal.getRealm() + "@" + principal.getRealm())) { return true; } return false; } private  SecurityUtil(); @InterfaceAudience.Public @InterfaceStability.Evolving static void setConfiguration(Configuration conf); @InterfaceAudience.Private @VisibleForTesting static void setTokenServiceUseIp(boolean flag); @InterfaceAudience.Public @InterfaceStability.Evolving static String getServerPrincipal(String principalConfig,
      String hostname); @InterfaceAudience.Public @InterfaceStability.Evolving static String getServerPrincipal(String principalConfig,
      InetAddress addr); @InterfaceAudience.Public @InterfaceStability.Evolving static void login(final Configuration conf,
      final String keytabFileKey, final String userNameKey); @InterfaceAudience.Public @InterfaceStability.Evolving static void login(final Configuration conf,
      final String keytabFileKey, final String userNameKey, String hostname); static String buildDTServiceName(URI uri, int defPort); static String getHostFromPrincipal(String principalName); @InterfaceAudience.Private static void setSecurityInfoProviders(SecurityInfo... providers); static KerberosInfo getKerberosInfo(Class<?> protocol, Configuration conf); static TokenInfo getTokenInfo(Class<?> protocol, Configuration conf); static InetSocketAddress getTokenServiceAddr(Token<?> token); static void setTokenService(Token<?> token, InetSocketAddress addr); static Text buildTokenService(InetSocketAddress addr); static Text buildTokenService(URI uri); static T doAsLoginUserOrFatal(PrivilegedAction<T> action); static T doAsLoginUser(PrivilegedExceptionAction<T> action); static T doAsCurrentUser(PrivilegedExceptionAction<T> action); @InterfaceAudience.Private static InetAddress getByName(String hostname); static AuthenticationMethod getAuthenticationMethod(Configuration conf); static void setAuthenticationMethod(
      AuthenticationMethod authenticationMethod, Configuration conf); static boolean isPrivilegedPort(final int port); static List<ZKUtil.ZKAuthInfo> getZKAuthInfos(Configuration conf,
      String configKey); static final Logger LOG; static final String HOSTNAME_PATTERN; static final String FAILED_TO_GET_UGI_MSG_HEADER; }### Answer:
@Test public void isOriginalTGTReturnsCorrectValues() { assertTrue(SecurityUtil.isTGSPrincipal (new KerberosPrincipal("krbtgt/foo@foo"))); assertTrue(SecurityUtil.isTGSPrincipal (new KerberosPrincipal("krbtgt/foo.bar.bat@foo.bar.bat"))); assertFalse(SecurityUtil.isTGSPrincipal (null)); assertFalse(SecurityUtil.isTGSPrincipal (new KerberosPrincipal("blah"))); assertFalse(SecurityUtil.isTGSPrincipal (new KerberosPrincipal("krbtgt/hello"))); assertFalse(SecurityUtil.isTGSPrincipal (new KerberosPrincipal("krbtgt/foo@FOO"))); }