Teh MVC... kind of

App.java

public class App {
    public static void main(String[] args) {
        NumberModel model = new NumberModel();
        NumberController controller = new NumberController(model);
    }
}

NumberController.java

public class NumberController {

    private final NumberModel model;

    public NumberController(NumberModel model) {
        this.model = model;
        NumberView view = new NumberView(this, model);
    }

    public void onUserInput(String input) {
        try {
            int inputNumber = Integer.parseInt(input);
            model.setNumber(inputNumber);
        }
        catch (NumberFormatException e) {
            // do nothing
        }
    }
}

NumberModel.java

public class NumberModel {

    private int number;
    private List<NumberObserver> observers = new ArrayList<NumberObserver>();

    public NumberModel() {
        number = 0;
    }

    public int getNumber() {
        return number;
    }

    public void setNumber(int number) {
        this.number = number;
        notifyObservers();
    }

    public void addObserver(NumberObserver observer) {
        observers.add(observer);
    }

    public void removeObserver(NumberObserver observer) {
        observers.remove(observer);
    }

    private void notifyObservers() {
        for (NumberObserver observer : observers) {
            observer.onNumberChanged(number);
        }
    }

    public interface NumberObserver {
        void onNumberChanged(int newValue);
    }
}

NumberView.java

public class NumberView implements NumberModel.NumberObserver {

    public NumberView(NumberController controller, NumberModel model) {
        model.addObserver(this);
        displayNumber(model.getNumber());

        Scanner sc = new Scanner(System.in);
        while (sc.hasNextLine()) {
            controller.onUserInput(sc.nextLine());
        }
    }

    @Override
    public void onNumberChanged(int newValue) {
        displayNumber(newValue);
    }

    private void displayNumber(int number) {
        System.out.println("" + number);
    }
}