Server.java

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.ArrayList;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class Server implements Runnable {

    private ArrayList<ConnectionHandler> connections ;
    private ServerSocket server ;
    private boolean done ;
    private ExecutorService pool;
    public Server(){
        connections = new ArrayList<>();
        done = false ;
    }

    @Override
    public void run(){
        try {
            server = new ServerSocket(9999);
            pool = Executors.newCachedThreadPool();
            while (!done){
                Socket client = server.accept();
                ConnectionHandler handler = new ConnectionHandler(client);
                connections.add(handler);
                pool.execute(handler);
            }

        } catch (IOException e) {
            shutdown();

        }
    }

    public void broadcast(String message){
        for (ConnectionHandler ch : connections){
            if (ch!=null){
                ch.sendMessage(message);
            }
        }
    }

    public void shutdown(){
        try {
            done = true;
            pool.shutdown();
            if (!server.isClosed()) {
                server.close();
            }
            for (ConnectionHandler ch : connections) {
                ch.shutdown();
            }
        } catch (IOException e){
            //ignore
        }
    }
    class ConnectionHandler implements Runnable {

        private Socket client;
        private BufferedReader in;
        private PrintWriter out;

        public ConnectionHandler(Socket client) {
            this.client = client;
        }

        @Override
        public void run() {
            try {
                out = new PrintWriter(client.getOutputStream(), true);
                in = new BufferedReader(new InputStreamReader(client.getInputStream()));
                out.println("Please enter a nickname : ");
                String nickname;
                nickname = in.readLine();
                System.out.println(nickname + " connected !");
                broadcast(nickname + " joined the chat!");
                String message;
                while ((message = in.readLine()) != null) {
                    if (message.startsWith("/nick")) {
                        String[] messageSplit = message.split("", 2);
                        if (messageSplit.length == 2) {
                            broadcast(nickname + " renamed themselves to " + messageSplit[1]);
                            System.out.println(nickname + " renamed themselves to " + messageSplit[1]);
                            nickname = messageSplit[1];
                            out.println("Successfully changed your nickname to" + nickname);
                        } else {
                            out.println(" No nickname provided !");
                        }
                    } else if (message.startsWith("/q")) {
                        broadcast(nickname + " left the chat");
                        shutdown();
                    } else {
                        broadcast(nickname + " :" + message);
                    }
                }
            } catch (IOException e) {
                shutdown();
            }

        }

        public void sendMessage(String message) {
            out.println(message);
        }

        public void shutdown() {
            try {
                in.close();
                out.close();
                if (!client.isClosed()) {
                    client.close();
                }
            } catch (Exception e) {
                //ignore
            }
        }


    }
    public static void main(String[] args) {
        Server server = new Server();
        server.run();
    }
}