Server.java

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

class Server {
  private ArrayList<Connection> clientConnections;
  private ServerSocket serverSocket;
  private ExecutorService connectionThreadPool;
  private boolean running;
  
  public Server() {
    clientConnections = new ArrayList<>();
    running = true;
  }

  public void listen(int port) throws IOException {
      serverSocket = new ServerSocket(port);
      connectionThreadPool = Executors.newCachedThreadPool();
      while(running) {
        Socket connectionSocket = serverSocket.accept();
        Connection clientConnection = new Connection(this, connectionSocket);
        clientConnections.add(clientConnection);
        connectionThreadPool.execute(clientConnection);
      }
  }

  public void broadcast(String message) {
    for(Connection connection : clientConnections) {
      if(connection != null) {
        connection.sendMessage(message);
      }
    }
  }

  public void close() {
    try {
      running = false;
      if(!serverSocket.isClosed()) {
        serverSocket.close();
      }
      for(Connection connection : clientConnections) {
        connection.close();
      }
    } catch(Exception e) {
      System.out.println("Error occured closing server: " + e);
    }
  }

}