ChatServer.java

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

// ChatServer: waits for clients (Student, Teacher) to connect and relays messages to all
public class ChatServer {
    // Keep track of all connected clients' output streams
    private static Set<PrintWriter> clientWriters = new HashSet<>();

    public static void main(String[] args) throws IOException {
        // Create a server socket listening on port 1234
        ServerSocket serverSocket = new ServerSocket(1234);
        System.out.println("Server started on port 1234");

        // Accept clients forever
        while (true) {
            Socket socket = serverSocket.accept(); // wait for a client
            System.out.println("Client connected");
            // Handle each client in a separate thread
            new Thread(new ClientHandler(socket)).start();
        }
    }

    // Inner class: handles communication with one client
    private static class ClientHandler implements Runnable {
        private Socket socket;
        private PrintWriter out;
        private BufferedReader in;

        public ClientHandler(Socket socket) throws IOException {
            this.socket = socket;
            // Input stream: read messages from client
            this.in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            // Output stream: send messages to client
            this.out = new PrintWriter(socket.getOutputStream(), true);
            // Add this client's output stream to the set
            synchronized (clientWriters) {
                clientWriters.add(out);
            }
        }

        public void run() {
            try {
                String message;
                // Continuously read messages from this client
                while ((message = in.readLine()) != null) {
                    System.out.println("Received: " + message);
                    // Broadcast message to all connected clients
                    synchronized (clientWriters) {
                        for (PrintWriter writer : clientWriters) {
                            writer.println(message);
                        }
                    }
                }
            } catch (IOException e) {
                e.printStackTrace();
            } finally {
                // Remove client when disconnected
                synchronized (clientWriters) {
                    clientWriters.remove(out);
                }
            }
        }
    }
}