public class MultiThreadServer {

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

        DatagramSocket serverSocket = new DatagramSocket(6666);

        while (true) {
            byte[] receiveData = new byte[1024];
            byte[] sendData = new byte[1024];

            DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
            serverSocket.receive(receivePacket);

            String message = new String(receivePacket.getData());
            InetAddress clientAddress = receivePacket.getAddress();
            int clientPort = receivePacket.getPort();

            // Create a new thread to handle the client connection
            Thread t = new Thread(new ClientHandler(clientAddress, clientPort));
            t.start();
        }
    }
}

public class ClientHandler implements Runnable {
    private static List<Integer> usedPorts = new ArrayList<>();
    private DatagramSocket socket;
    private InetAddress clientAddress;
    private int clientPort;

    public ClientHandler(InetAddress clientAddress, int clientPort) throws Exception {
        this.clientAddress = clientAddress;
        this.clientPort = clientPort;
        this.socket = getSocket();
    }

    public DatagramSocket getSocket() throws SocketException {
        int port = 0;
        do {
            port = 6666 + (int)(Math.random()*2222);
        } while(usedPorts.contains(port));

        return new DatagramSocket(port);
    }

    public void run() {
        try {
            send("HELLO");

            while(true) {
                String text  = new String(receive());
                send(text);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private void send(String msg) throws IOException {
        DatagramPacket packet = new DatagramPacket(msg.getBytes(), msg.getBytes().length, clientAddress, clientPort);
        socket.send(packet);
    }

    private byte[] receive() throws IOException {
        byte[] buffer = new byte[1024];
        DatagramPacket packet = new DatagramPacket(buffer, buffer.length);
        socket.receive(packet);

        return packet.getData();
    }
}


public class Client {
    private int myPort = 4444+(int)(Math.random()*4444); // to be able to run few clients on same OS
    private int remotePort = 6666;
    private DatagramSocket socket;


    public static void main(String[] args) throws IOException {
        new Client().go();
    }

    private void go() throws IOException {
        System.out.println("My port is " + myPort);
        this.socket = new DatagramSocket(myPort);
        remotePort = getConnection(socket);
        System.out.println("Received remote port is " + remotePort);

        Scanner scanner = new Scanner(System.in);
        String line = scanner.nextLine();
        while (!line.equals("q")) {

            send(line);

            String response = new String(receive());
            System.out.println(response);

            line = scanner.nextLine();
        }

        socket.close();
    }

    private int getConnection(DatagramSocket socket) throws IOException {
        // sending first message on server
        String msg = "HELLO";
        InetAddress remoteAddress = InetAddress.getLocalHost();
        DatagramPacket packet = new DatagramPacket(msg.getBytes(), msg.getBytes().length, remoteAddress, remotePort);
        socket.send(packet);

        // getting first message from server and returning the port
        byte[] buffer = new byte[1024];
        DatagramPacket receivePacket = new DatagramPacket(buffer, buffer.length);
        socket.receive(receivePacket);

        return receivePacket.getPort();
    }

    private void send(String msg) throws IOException {
        DatagramPacket packet = new DatagramPacket(msg.getBytes(), msg.getBytes().length, InetAddress.getLocalHost(), remotePort);
        socket.send(packet);
    }

    private byte[] receive() throws IOException {
        byte[] buffer = new byte[1024];
        DatagramPacket packet = new DatagramPacket(buffer, buffer.length);
        socket.receive(packet);

        return packet.getData();
    }
}