import java.io.*;
import java.net.*;
public class TCPServer {
    public static void main(String[] args) throws IOException {
        ServerSocket serverSocket = new ServerSocket(9999);
        System.out.println("Server started.");
        while (true) {
            Socket clientSocket = serverSocket.accept();
            System.out.println("Connected: " + clientSocket);
            BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
            PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true);
            String inputLine, outputLine;
            while ((inputLine = in.readLine()) != null) {
                System.out.println("Received message: " + inputLine);
                outputLine = "Server: " + inputLine;
                out.println(outputLine);
                if (outputLine.equals("Bye."))
                    break;
            }
            clientSocket.close();
            System.out.println("Client disconnected.");
        }
    }
}

import java.io.*;
import java.net.*;
public class TCPClient {
    public static void main(String[] args) throws IOException {
        String serverHostname = "localhost";
        int serverPort = 9999;
        Socket socket = new Socket(serverHostname, serverPort);
        BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
        BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in));
        String userInput;
        while ((userInput = stdIn.readLine()) != null) {
            out.println(userInput);
            System.out.println("Sent message: " + userInput);
            String receivedMessage = in.readLine();
            System.out.println("Received message: " + receivedMessage);
            if (receivedMessage.equals("Bye."))
                break;
        }
        socket.close();
        System.out.println("Connection closed.");
    }
}

import java.io.*;
import java.net.*;
public class UDPServer {
    public static void main(String[] args) throws IOException {
        DatagramSocket serverSocket = new DatagramSocket(9999);
        System.out.println("Server started.");
        byte[] receiveData = new byte[1024];
        byte[] sendData = new byte[1024];
        while (true) {
            DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
            serverSocket.receive(receivePacket);
            String message = new String(receivePacket.getData(), 0, receivePacket.getLength());
            System.out.println("Received message: " + message);
            InetAddress IPAddress = receivePacket.getAddress();
            int port = receivePacket.getPort();
            String responseMessage = "Server: " + message;
            sendData = responseMessage.getBytes();
            DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, port);
            serverSocket.send(sendPacket);
            if (responseMessage.equals("Server: Bye.")) {
                System.out.println("Client disconnected.");
                break;
            }
        }
        serverSocket.close();
    }
}

import java.io.*;
import java.net.*;
public class UDPClient {
    public static void main(String[] args) throws IOException {
        String serverHostname = "localhost";
        int serverPort = 9999;
        DatagramSocket clientSocket = new DatagramSocket();
        BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in));
        InetAddress IPAddress = InetAddress.getByName(serverHostname);
        byte[] sendData = new byte[1024];
        byte[] receiveData = new byte[1024];
        String userInput;
        while ((userInput = inFromUser.readLine()) != null) {
            sendData = userInput.getBytes();
            DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, serverPort);
            clientSocket.send(sendPacket);
            System.out.println("Sent message: " + userInput);
            DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
            clientSocket.receive(receivePacket);
            String receivedMessage = new String(receivePacket.getData(), 0, receivePacket.getLength());
            System.out.println("Received message: " + receivedMessage);
            if (receivedMessage.equals("Server: Bye.")) {
                System.out.println("Connection closed.");
                break;
            }
        }
        clientSocket.close();
    }
}