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

//
// ---- main -----------------------------------------------------------
//
// command-line argument is port number.
//
// program receives and prints messages sent by UDPclient program
//      (called one or more times, possibly from different machines).
//
// instructions for testing:
//
// (1) start this program:
//      java UDPserver nnnn
//              (nnnn is any port number you choose, higher than 1024)
//      suppose mymachine.org is the name of the machine on which 
//              you've done this, and nnnn = 1234.
//
// (2) run UDPclient one or more times:
//      java UDPclient mymachine.org 1234
//
//      you can do this on the same machine (in a different window
//              usually) or on a different machine.  
//
//      if you don't know the name of the machine on which you're
//              running, you can use its IP address instead.  for
//              a Windows machine, you can (at least on the CIRCA
//              machines) get its IP address by typing "winipcfg"
//              at a DOS prompt.
//
// when done, stop the server with control-C.
//
// these instructions have been tested on the CISE Unix machines and
//      the CIRCA Windows machines.  
//
class UDPserver {
        static final int PKSIZE = 1024 ;

        public static void main(String[] args) throws IOException {

                if (args.length < 1) {
                        System.out.println("Argument is portnum") ;
                        System.exit(-1) ;
                }

                // make a "listener" socket (specify port number).
                DatagramSocket s = 
                        new DatagramSocket(Integer.parseInt(args[0])) ;
                while (true) {
                        // prepare packet object to receive message.
                        DatagramPacket packet = 
                                new DatagramPacket(
                                        new byte [PKSIZE], PKSIZE) ;
                        // receive message.
                        s.receive(packet) ;
                        // extract data (message text sent by client).
                        //      ("trim" added to cope with JDK 1.1.3 
                        //      bug (?) that added blanks to packet 
                        //      data.)
                        String msg = 
                                new String(packet.getData()).trim() ;
                        // print message text and sender's hostname.
                        System.out.println("Message from UDPclient at "
                                + packet.getAddress().getHostName() +
                                ":  " + msg) ;
                }
        }
}