if (impl == null) throw new SocketException ( "MulticastSocket: Cant access socket implementation");
if (isClosed()) throw new SocketException("socket is closed");
  public void setLoopbackMode(boolean disable) throws SocketException  {    if (impl == null)      throw new SocketException (		      "MulticastSocket: Cant access socket implementation");    impl.setOption (SocketOptions.IP_MULTICAST_LOOP, new Boolean (disable));  }