### Question:
NetUtils { public static String getHostNameOfIP(String ipPort) { if (null == ipPort || !ipPortPattern.matcher(ipPort).matches()) { return null; } try { int colonIdx = ipPort.indexOf(':'); String ip = (-1 == colonIdx) ? ipPort : ipPort.substring(0, ipPort.indexOf(':')); return InetAddress.getByName(ip).getHostName(); } catch (UnknownHostException e) { return null; } }  static SocketFactory getSocketFactory(Configuration conf,
      Class<?> clazz); static SocketFactory getDefaultSocketFactory(Configuration conf); static SocketFactory getSocketFactoryFromProperty(
      Configuration conf, String propValue); static InetSocketAddress createSocketAddr(String target); static InetSocketAddress createSocketAddr(String target,
                                                   int defaultPort); static InetSocketAddress createSocketAddr(String target,
                                                   int defaultPort,
                                                   String configName); static InetSocketAddress createSocketAddrForHost(String host, int port); static URI getCanonicalUri(URI uri, int defaultPort); static void addStaticResolution(String host, String resolvedName); static String getStaticResolution(String host); static List <String[]> getAllStaticResolutions(); static InetSocketAddress getConnectAddress(Server server); static InetSocketAddress getConnectAddress(InetSocketAddress addr); static SocketInputWrapper getInputStream(Socket socket); static SocketInputWrapper getInputStream(Socket socket, long timeout); static OutputStream getOutputStream(Socket socket); static OutputStream getOutputStream(Socket socket, long timeout); static void connect(Socket socket,
      SocketAddress address,
      int timeout); static void connect(Socket socket, 
                             SocketAddress endpoint,
                             SocketAddress localAddr,
                             int timeout); static String normalizeHostName(String name); static List<String> normalizeHostNames(Collection<String> names); static void verifyHostnames(String[] names); static String getHostNameOfIP(String ipPort); static String getHostname(); static String getHostPortString(InetSocketAddress addr); static InetAddress getLocalInetAddress(String host); static boolean isLocalAddress(InetAddress addr); static IOException wrapException(final String destHost,
                                          final int destPort,
                                          final String localHost,
                                          final int localPort,
                                          final IOException exception); static boolean isValidSubnet(String subnet); static List<InetAddress> getIPs(String subnet,
      boolean returnSubinterfaces); static final String UNKNOWN_HOST; static final String HADOOP_WIKI; }### Answer:
@Test public void testGetHostNameOfIP() { assertNull(NetUtils.getHostNameOfIP(null)); assertNull(NetUtils.getHostNameOfIP("")); assertNull(NetUtils.getHostNameOfIP("crazytown")); assertNull(NetUtils.getHostNameOfIP("127.0.0.1:")); assertNull(NetUtils.getHostNameOfIP("127.0.0.1:-1")); assertNull(NetUtils.getHostNameOfIP("127.0.0.1:A")); assertNotNull(NetUtils.getHostNameOfIP("127.0.0.1")); assertNotNull(NetUtils.getHostNameOfIP("127.0.0.1:1")); }