import java.net.*;

public class EchoServer {

    public static void main(String[] args) {
	if (args.length!=1) {
	    System.out.println("java EchoServer <port>");
	    System.exit(0);
	}
	int port=0;
	try {
	    port=Integer.parseInt(args[0]);
	} catch (Exception ex) {
	    System.out.println("java EchoServer <port>");
	    System.exit(0);
	}
	EchoServer server = new EchoServer(port);

    }

    public EchoServer(int port) {

	try {

	    // Server Socket auf bestimmtem Port oeffnen: 
	    DatagramSocket sock = new DatagramSocket(port); 
	    byte[] data = new byte[1];

	    // Datenstruktur fuer dem Empfang eines UDP Paketes anlegen
	    DatagramPacket pack = new DatagramPacket(data, data.length); 
	    System.out.println("Echo Server ready at port: "+port);

	    while (true) {
		// warten auf UDP Pakete von clients
		sock.receive(pack);

		// Antwort aus dem request zusammenbauen
		DatagramPacket reply = new DatagramPacket(pack.getData(),
							  pack.getLength(),
							  pack.getAddress(),
							  pack.getPort());

		// und weg damit ...
		sock.send(reply);
	    }
	} catch (Exception ex) {
	    System.out.println("Encountered unexpected exception: "+ex);
	}
    }
}