throw new InternalError ("PlainDatagramSocketImpl::joinGroup is not implemented");
if (address == null) throw new NullPointerException(); if (!(address instanceof InetSocketAddress)) throw new SocketException("unknown address type"); impl.joinGroup((InetSocketAddress) address, netIf);
  public void joinGroup(SocketAddress address, NetworkInterface netIf)  {    throw new InternalError      ("PlainDatagramSocketImpl::joinGroup is not implemented");  }