### Question:
SecurityUtil { public static String getHostFromPrincipal(String principalName) { return new HadoopKerberosName(principalName).getHostName(); }  static void fetchServiceTicket(URL remoteHost); static String getServerPrincipal(String principalConfig,
      String hostname); static String getServerPrincipal(String principalConfig,
      InetAddress addr); static void login(final Configuration conf,
      final String keytabFileKey, final String userNameKey); 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 final Log LOG; static final String HOSTNAME_PATTERN; }### Answer:
@Test public void testGetHostFromPrincipal() { assertEquals("host", SecurityUtil.getHostFromPrincipal("service/host@realm")); assertEquals(null, SecurityUtil.getHostFromPrincipal("service@realm")); }