AppController.java

class AppController {

    public static void main(String[] args) {
        new AppController(new AppModel(),
                new ToDoView(new PrintWriter(System.out)),
                new Scanner(System.in))
        .start(Arrays.asList(args));
    }

    private final AppModel model;
    private final ToDoView view;
    private final Scanner input;

    private AppController(AppModel model, ToDoView view, Scanner input) {
        this.input = input;
        this.model = model;
        this.view = view;
    }

    private void start(List<String> args) {
        if (args.contains("-register")) {
            execute(new RegistrationController(view, input));
            return;
        }
        if ( args.contains("-u") && args.size()==3 ) {
            model.authenticate(args.get(1), args.get(2));
        }
        do {
            if ( model.isAuthenticated() ) {
                execute(new TasksController(view, input));
            } else {
                execute(new AuthenticationController(view, input));
            }
        } while ( model.getCurrentAction()!=QUIT);
    }

    private void execute(Consumer<AppModel> subsystem) {
        subsystem.accept(model);
    }
}

TasksController.java

class TasksController implements Consumer<AppModel> {
    private enum Menu implements ToDoView.MenuEntry<TasksController> {
        List("List tasks", TasksController::list),
        Add("Add task", TasksController::add),
        Close("Complete task", TasksController::list),
        Quit("Quit", (c, m) -> {/*nothing*/});
        private final BiConsumer<TasksController, AppModel> code;
        private final String label;
        Menu(String label, BiConsumer<TasksController, AppModel> code) {
            this.label = label;
            this.code = code;
        }

        @Override
        public String label() {
            return label;
        }

        @Override
        public void accept(TasksController ctrl, AppModel model) {
            code.accept(ctrl, model);
        }
    }

    private final ToDoView view;
    private final Scanner input;

    public TasksController(ToDoView view, Scanner input) {
        this.input = input;
        this.view = view;
    }

    @Override
    public void accept(AppModel model) {
        Menu selection = null;
        do {
            selection = view.menu(Menu.values(), input);
            selection.accept(this, model);
        } while (Menu.Quit!=selection);

    }

    private void list(AppModel model) {
        model.getTasks().forEach(this::print);
    }

    private void print(Task task) {
        view.text(format(" - %1$s", task.getTaskName()));
    }

    private void add(AppModel model) {
        String name = view.read("Name", input);
        model.add(new Task(name));
    }

    private void close(AppModel model) {
        String name = view.read("Name", input);
        model.findTaskByName(name)
                .map(t -> {
                    model.complete(t);
                    return Void.TYPE;
                })
                .orElseGet(()->{
                    view.error(format("No tasks \"%1$s\" found.", name));
                    return Void.TYPE;
                });
    }
}

ToDoView.java

class ToDoView {

    interface MenuEntry<T> extends BiConsumer<T, AppModel> {
        String label();
    }

    private final PrintWriter output;

    ToDoView(PrintWriter output) {
        this.output = output;
    }

    String read(String label, Scanner in) {
        in.reset();
        output.printf(" | %1$s :", label).flush();
        return in.nextLine();
    }

    void title(String text) {
        output.printf("%n >  %1$s%n", text)
                .flush();
    }

    void text(String text) {
        output.printf("%1$s%n", text).flush();
    }

    void error(String text) {
        output.printf(" /!\\ %1$s%n", text).flush();
    }

    void info(String text) {
        output.printf("[i] %1$s%n", text).flush();
    }

    <E extends MenuEntry> E menu(E[] menu, Scanner in) {
        for (int i=0; i<menu.length; i++) {
            output.printf(" %1$d - %2$s%n", i+1, menu[i].label());
        }
        output.flush();
        int selection;
        do {
            // https://stackoverflow.com/a/13102066/1632752
            selection = Integer.parseInt(in.nextLine())-1;
        } while ( selection<0 || selection>menu.length);
        return menu[selection];
    }
}