if (isClosed()) throw new SocketException("socket is closed");
  public void joinGroup(InetAddress mcastaddr) throws IOException  {    if (! mcastaddr.isMulticastAddress())      throw new IOException("Not a Multicast address");    SecurityManager s = System.getSecurityManager();    if (s != null)      s.checkMulticast(mcastaddr);    impl.join(mcastaddr);  }