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