while (true) {
        try {
            if (scanner.hasNext()) {
                String message = scanner.nextLine();
                String message2 = Client.decrypt(message, key);
                ChatGUI.display(message2);
                if (message2.equals("ONLINE")) {

                    ArrayList<String> online = (ArrayList<String>) obIn.readObject();
                    Client.addOnlineUsers(online);
                    System.out.println(online);

public static void addOnlineUsers(ArrayList<String> online) {
    Storage.setOnlineUsers(online); //it's a simple setter
    ChatGUI.setOnline();

public static void setOnline() {
    onlineUsers.setListData(Client.getOnlineUsers().toArray());
}

public void sendNewUserOnline() throws Exception{
    String notify = "ONLINE";
    for (ConnectionPair conPair : connection) {
        PrintWriter out = conPair.getWriter();
        SecretKey clientKey = conPair.getClientAesKey();
        String encMessage = encrypt(notify, clientKey);
        out.println(encMessage);
        out.flush();

        ObjectOutputStream obOut = conPair.getObOut();
        obOut.writeObject(online);
        obOut.flush();
    }
}

java.io.StreamCorruptedException: invalid type code: 35
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1374)
at java.io.ObjectInputStream.readObject(ObjectInputStream.java:369)
at test.ClientService.run(ClientService.java:40)
at java.lang.Thread.run(Thread.java:722)