Sockets.java

package stackoverflow;

import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.*;
import java.util.Scanner;

public class Sockets {

    public static void main(String... args) throws Exception {
        ServerSocket server = new ServerSocket();
        server.bind(new InetSocketAddress("localhost", 0));

        Thread serverThread = new Thread(() -> {
            while (!Thread.currentThread().isInterrupted()) {
                try {
                    Socket socket = server.accept();
                    ObjectInputStream in = new ObjectInputStream(socket.getInputStream());
                    String msg = (String) in.readObject();
                    System.out.println("Server here: read message > " + msg);

                    String ack = "ok";
                    ObjectOutputStream out = new ObjectOutputStream(socket.getOutputStream());
                    out.writeObject(ack);
                    System.out.println("Server here: written acknowledge > " + ack);

                    socket.close();
                } catch (Exception e) {
                    System.err.println("Error while processing request");
                    e.printStackTrace();
                }
            }
        });

        serverThread.setName("server-accepting-thread");
        serverThread.setDaemon(true);
        serverThread.start();

        Scanner shell = new Scanner(System.in);

        while (true) {
            System.out.println("Write your message and press 'Enter'. Type 'quit' to terminate the program");
            try {
                String request = shell.nextLine();

                if (request.equals("quit")) {
                    break;
                }

                Socket socket = new Socket("localhost", server.getLocalPort());
                ObjectOutputStream out = new ObjectOutputStream(socket.getOutputStream());
                System.out.println("Client here: sending request > " + request);
                out.writeObject(request);

                ObjectInputStream in = new ObjectInputStream(socket.getInputStream());
                String response = (String) in.readObject();
                System.out.println("Client here: server responded > " + response);

                socket.close();
            } catch (Exception e) {
                System.err.println("Error while sending request");
                e.printStackTrace();
            }
        }
    }

}

Thank you!