Java adding to lists

Question

I am trying to make a simple TCP chat program, and when the user sends the message "/list", I want to reply from the server with a list of all online users.

    else if (cmd.text.equals("/list")){
    //Sends the client a list of all registered users
        UserList userList = new UserList();

        System.out.println(online_users);

        if (online_users.values()!=null){

            userList.users.addAll(online_users.values());

        }

        c.sendTCP(userList);

    }

However, even though I have tested and know that online_users does contain a value to add to userList.users, the command userList.users.addAll() keeps throwing an error. This is the error:

Exception in thread "Server" java.lang.NullPointerException
at com.andrewlalisofficial.ChatServer.checkCommand(ChatServer.java:128)
at com.andrewlalisofficial.ChatServer$1.received(ChatServer.java:68)
at com.esotericsoftware.kryonet.Server$1.received(Server.java:61)
at com.esotericsoftware.kryonet.Connection.notifyReceived(Connection.java:246)
at com.esotericsoftware.kryonet.Server.update(Server.java:208)
at com.esotericsoftware.kryonet.Server.run(Server.java:356)
at java.lang.Thread.run(Thread.java:745)

where line 128 is userList.users.addAll(online_users.values()); I want to be able to simply add the collection of values from the Map online_users to a new String list and send it off. I don't understand why there is an error.

Use your debugger, put a breakpoint on that line and checks what is null. E.g. you say that `online_users` contains a value, but `userList.users` might just as well be null — Robin, Sep 26 '15 at 12:57
Looks like the null comes from `usersList.users`. How do you initialize users in UserList costructor? — Nir Levy, Sep 26 '15 at 12:57
you should initialize your `users` inside UserList like this. `public UserList() { users = new ArrayList();}` — Andrei Sfat, Sep 26 '15 at 12:58
This is just a simple message class, it is only: UserList{List users;} — Andrew Lalis, Sep 26 '15 at 13:01
yeah, you need to initialize that class users otherwise it won"t work to call addAll. — Andrei Sfat, Sep 26 '15 at 13:14

Java adding to lists

0 Answers0