if (impl == null) throw new SocketException ( "MulticastSocket: Cant access socket implementation");
if (isClosed()) throw new SocketException("socket is closed");
  public void setNetworkInterface(NetworkInterface netIf)    throws SocketException  {    if (impl == null)      throw new SocketException (		      "MulticastSocket: Cant access socket implementation");    Enumeration e = netIf.getInetAddresses ();    if (!e.hasMoreElements ())      throw new SocketException ("MulticastSocket: Error");    InetAddress address = (InetAddress) e.nextElement ();    impl.setOption (SocketOptions.IP_MULTICAST_IF, address);  }