🕒 Timer Observer Example

Observer in action: Imagine your escape room game has a countdown timer ticking in the background. Instead of hard-coding every reaction, the timer simply notifies observers — and each one decides how to respond.

Why it matters: This keeps the timer focused only on time, while the UI, sound system, and game logic handle their own behaviors. Flexible. Decoupled. Easy to extend.

Scenario

In your escape room:

• A Countdown Timer runs in the background.

• Every second, the timer notifies observers.

• Observers could be:

  • UI Clock → shows time left on screen.

  • Sound System → plays warning beeps in the last 10 seconds.

  • Game Logic → locks the room if time runs out.

---

Timer (Subject)

import java.util.ArrayList;
import java.util.List;
import java.util.Timer;
import java.util.TimerTask;

public class CountdownTimer implements Subject {
    private List<Observer> observers = new ArrayList<>();
    private int secondsRemaining;
    private Timer timer;

    public CountdownTimer(int seconds) {
        this.secondsRemaining = seconds;
    }

    @Override
    public void addObserver(Observer observer) {
        observers.add(observer);
    }

    @Override
    public void removeObserver(Observer observer) {
        observers.remove(observer);
    }

    @Override
    public void notifyObservers(String event) {
        for (Observer observer : observers) {
            observer.update(event);
        }
    }

    public void start() {
        timer = new Timer();
        timer.scheduleAtFixedRate(new TimerTask() {
            @Override
            public void run() {
                if (secondsRemaining > 0) {
                    notifyObservers("Time left: " + secondsRemaining + "s");
                    secondsRemaining--;
                } else {
                    notifyObservers("Time’s up!");
                    timer.cancel();
                }
            }
        }, 0, 1000); // every 1 second
    }
}

---

Timer Observers

// Observer 1: UI
public class GameUI implements Observer {
    @Override
    public void update(String event) {
        if (event.startsWith("Time left")) {
            System.out.println("[UI] " + event);
        } else if (event.equals("Time’s up!")) {
            System.out.println("[UI] GAME OVER!");
        }
    }
}

// Observer 2: Sound system
public class SoundSystem implements Observer {
    @Override
    public void update(String event) {
        if (event.startsWith("Time left: 10s")) {
            System.out.println("[Sound] Beep beep! Hurry up!");
        } else if (event.equals("Time’s up!")) {
            System.out.println("[Sound] Alarm sound!!!");
        }
    }
}

---

Using the Timer in the Game

public class EscapeRoomGame {
    public static void main(String[] args) {
        CountdownTimer timer = new CountdownTimer(12); // 12-second game

        GameUI ui = new GameUI();
        SoundSystem sound = new SoundSystem();

        timer.addObserver(ui);
        timer.addObserver(sound);

        timer.start();
    }
}

---

Sample Output

[UI] Time left: 12s
[UI] Time left: 11s
[UI] Time left: 10s
[Sound] Beep beep! Hurry up!
[UI] Time left: 9s
...
[UI] Time’s up!
[Sound] Alarm sound!!!

---

✅ Why this works so well with Observer Pattern:

• The Timer just keeps track of time — it doesn’t care who’s watching.

• Observers (UI, sound, game logic) react independently.

• You can add/remove observers dynamically (e.g., multiple UIs for co-op players).