UDPEcho.java

import java.net.DatagramPacket;
import java.net.DatagramSocket;

public class UDPServer {
    public static void main(String[] args) throws Exception {
        DatagramSocket socket = new DatagramSocket(6969);
        System.out.println("Started server");
        while (true) {
            byte[] req = new byte[128];
            DatagramPacket request = new DatagramPacket(req, req.length);
            socket.receive(request);
            StringBuilder data = new StringBuilder();
            for (int i = 0; i < req.length; i+=2) {
                data.append((char) ((req[i] << 8) + req[i+1]));
            }
            data.append("\n").append(request.getAddress().toString()).append(":").append(request.getPort());
            byte[] res = new byte[data.length()*2];
            for (int i = 0; i < res.length; i+=2) {
                res[i] = (byte)(data.charAt(i/2) & 0xFF00);
                res[i+1] = (byte)(data.charAt(i/2) & 0x00FF);
            }

            DatagramPacket response = new DatagramPacket(res, res.length, request.getAddress(), request.getPort());
            socket.send(response);
        }
    }
}