import java.net.*;

public class UDPServer {
    public static void main(String[] a) throws Exception {
        DatagramSocket s = new DatagramSocket(9876);
        byte[] buf = new byte[1024];
        System.out.println("Server running...");
        while (true) {
            DatagramPacket p = new DatagramPacket(buf, buf.length);
            s.receive(p);
            String msg = new String(p.getData(), 0, p.getLength());
            byte[] out = ("ACK: " + msg.toUpperCase()).getBytes();
            s.send(new DatagramPacket(out, out.length, p.getAddress(), p.getPort()));
        }
    }
}

import java.net.*;
import java.io.*;

public class UDPClient {
    public static void main(String[] a) throws Exception {
        DatagramSocket s = new DatagramSocket();
        InetAddress ip = InetAddress.getByName("localhost");

        System.out.print("Enter msg: ");
        byte[] out = new BufferedReader(new InputStreamReader(System.in)).readLine().getBytes();
        s.send(new DatagramPacket(out, out.length, ip, 9876));

        byte[] buf = new byte[1024];
        DatagramPacket p = new DatagramPacket(buf, buf.length);
        s.receive(p);

        System.out.println("SERVER: " + new String(p.getData(), 0, p.getLength()));
        s.close();
    }
}