Server.java

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import java.util.HashSet;
import java.util.Scanner;
import java.util.Set;
import java.util.concurrent.Executors;
import java.io.IOException;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
 
public class Server {
    private ServerSocket serverSocket;
    private SharedNumber number;
 
    // The set of all the print writers for all the clients, used for broadcast.
    private Set<PrintWriter> writers = new HashSet<>();
 
    public Server(ServerSocket serverSocket) {
        this.serverSocket = serverSocket;
        this.number = new SharedNumber();
    }
 
    public void start() {
        var pool = Executors.newFixedThreadPool(200);
        int clientCount = 0;
        while (true) {
            try {
                Socket socket = serverSocket.accept();
                pool.execute(new Handler(socket));
                System.out.println("Connected to client " + clientCount++);
            } catch (IOException e) {
                e.printStackTrace();
            }
 
        }
    }
 
    public class Handler implements Runnable {
        private Socket socket;
        private Scanner input;
        private PrintWriter output;
 
        public Handler(Socket socket) {
            this.socket = socket;
        }
 
        @Override
        public void run() {
            System.out.println("Connected: " + socket);
            try {
                input = new Scanner(socket.getInputStream());
                output = new PrintWriter(socket.getOutputStream(), true);
 
                // add this client to the broadcast list
                writers.add(output);
 
                while (input.hasNextLine()) {
                    var command = input.nextLine();
 
                    System.out.println("Server Received: " + command);
 
                    if (command.startsWith("UP")) {
                        number.up();
                    } else if (command.startsWith("DOWN")) {
                        number.down();
                    }
 
                    // broadcast updated number to all clients
                    for (PrintWriter writer : writers) {
                        writer.println(number.getNumber());
                    }
 
                    System.out.println("Server Broadcasted: " + number.getNumber());
 
                }
            } catch (Exception e) {
                System.out.println(e.getMessage());
            } finally {
                // client disconnected
                if (output != null) {
                    writers.remove(output);
                }
            }
        }
    }
}