//
// example of use of java.net.* classes:  send/receive messages using
//      UDP (Datagram* classes).
// this program is the client; UDPserver.java is the server.
//
// adapted from example in ch. 11 of _Exploring Java_, Niemeyer & Peck.
//
import java.net.* ;
import java.io.* ;

//
// a UDPclient object can be used to send messages using UDP to a
//      corresponding UDPserver (see UDPserver.java).
//
class UDPclient {

        String theHost ;
        int thePort ;

        UDPclient(String h, int p) {
                theHost = h ;
                thePort = p ;
        }

        // sends a packet containing "message" to the server.
        private void sendMessage(String message) {
                try {
                        byte[] data = message.getBytes() ;
                        // get IP address for hostname.
                        InetAddress addr =
                                InetAddress.getByName(theHost) ;
                        // build packet to send, including
                        //      message text and destination info.
                        DatagramPacket packet =
                                new DatagramPacket(data, data.length,
                                        addr, thePort) ;
                        // create "sender" socket (no host or port,
                        //      since that's in the messages).
                        DatagramSocket sock = new DatagramSocket() ;
                        // send the message.
                        sock.send(packet) ;
                        sock.close() ;
                } catch (IOException e) {
                        System.out.println(e) ;
                }
        }

        //
        // ---- main ---------------------------------------------------
        //
        // command-line arguments are hostname, port for server.
        //      hostname is the name (or IP address) of the machine
        //      on which UDPserver is executing.  port is the port
        //      on which it was started.
        //
        // program sends "hello" and "goodbye" messages to server.
        //      note that since this is UDP, the program executes
        //      successfully even if no server program is running.
        //      compare to programs using TCP.
        //
        public static void main(String[] args) {

                if (args.length < 2) {
                        System.out.println("Arguments are hostname, " +
                                "portnum") ;
                        System.exit(-1) ;
                }

                UDPclient c = new UDPclient(args[0],
                                Integer.parseInt(args[1])) ;
                c.sendMessage("Hello!") ;
                c.sendMessage("Goodbye!") ;
        }
}