observer-pattern

🧩 Concept

The Observer Pattern defines a one-to-many relationship between objects, so that when one object changes state (Subject), all its dependents (Observers) are notified automatically.

---

🧠 Key Roles

Role	Description
Subject	Holds data/state and maintains a list of observers.
Observer	Gets notified when the subject changes.
ConcreteSubject	Actual object being observed.
ConcreteObserver	Responds to updates from the subject.

---

💻 Simple Java Example

// Observer interface
interface Observer {
    void update(String news);
}

// Subject class
class NewsAgency {
    private List<Observer> observers = new ArrayList<>();
    private String news;

    void addObserver(Observer o) { observers.add(o); }
    void removeObserver(Observer o) { observers.remove(o); }

    void setNews(String news) {
        this.news = news;
        notifyObservers();
    }

    private void notifyObservers() {
        for (Observer o : observers) o.update(news);
    }
}

// Concrete observer
class NewsChannel implements Observer {
    private String news;
    public void update(String news) { this.news = news; }
}

✅ Usage Example:

NewsAgency agency = new NewsAgency();
NewsChannel channel1 = new NewsChannel();
NewsChannel channel2 = new NewsChannel();

agency.addObserver(channel1);
agency.addObserver(channel2);

agency.setNews("Breaking: New Java Release!");
// Both channel1 and channel2 receive update

---

🧱 Flow Summary (Step-by-Step)

Step	Action	Subject (NewsAgency)	Observers	Notes
1️⃣	addObserver(channel1)	[channel1]	—	Register observer
2️⃣	addObserver(channel2)	[channel1, channel2]	—	Register another
3️⃣	setNews("Breaking...")	Notifies both	channel1, channel2 updated	Trigger update
4️⃣	removeObserver(channel2)	[channel1]	channel2 removed	Stop receiving updates

---

🪜 Summary

• Subject → keeps track of observers

• Observers → automatically get updates

• Loose coupling → Subject doesn’t need to know concrete observer types

• Common use cases: UI listeners, event systems, notifications

---

🔔 Real-world Analogy

📰 News Agency & Subscribers — when the agency publishes news, all subscribed channels automatically receive it.