小编Bl *_*l H的帖子

所以我有一个聊天室,它曾经工作,但我稍微改变了代码,尝试了一些东西,但它们没有用,所以我试图恢复原来的代码,但我不确定我做错了什么因为现在它抛出一个NullPointerException.我的代码有一个PrintWriters的ArrayList和一个方法showAll()就像它说的那样; 向聊天室中的所有人发送消息.所以基本上我想知道的是我怎么得到例外？

    //This is the main code, used to add printwriters to the arraylist and to connect to clients
    //The Communicate thread is used to get input from users and use the showAll() method with that input
    public void listen() {
        listWriters = new ArrayList<PrintWriter>();
        try {
            scanner = new Scanner(System.in);
            portnum = getPortNumber(scanner);
            System.out.println("Listening on " + portnum);
            serverSocket = new ServerSocket(portnum);
            while(true) {
                clientcommunicate = serverSocket.accept();
                System.out.println("Connection accepted: " + clientcommunicate.toString());

                PrintWriter client = new PrintWriter(clientcommunicate.getOutputStream(), true);
                listWriters.add(client); …