if (impl == null) throw new SocketException ( "MulticastSocket: Cant access socket implementation");
if (isClosed()) throw new SocketException("socket is closed");
  public NetworkInterface getNetworkInterface()    throws SocketException  {    if (impl == null)      throw new SocketException (		      "MulticastSocket: Cant access socket implementation");    InetAddress address =	    (InetAddress) impl.getOption (SocketOptions.IP_MULTICAST_IF);    NetworkInterface netIf = NetworkInterface.getByInetAddress (address);    return netIf;  }