private void HandleLogin() throws IOException {
        String line;
        String[] parts;
        while (true) {
            line = in.readLine();
            parts = line.split(" ");
            if (parts[0].equals("USER")) {
                uid = parts[1];
                UserControl.addUserIfNotExist(uid);
                out.println("OK");
                return;
            } else {
                out.println("INV");
            }
        }
    }