👀Observer Pattern

Define a one-to-many dependency — when one object changes state, dependents update automatically.

📌 Key IdeaSubject maintains observer list; notify pushes changes (push model) or observers pull fresh state. Foundation of event-driven UI, reactive streams, domain events.

🖼

Visual

👀 Publish / subscribe

  Subject.notifyObservers(event)
       │        │
       ├────────┼──▶ ObserverA.update()
       └────────└──▶ ObserverB.update()

Decouples publishers from subscribers — add listeners without editing subject.

✅

When to Use

Model changes must refresh many views / caches / projections.
Event broadcasting inside a process (PropertyChangeSupport).
Bridge from domain layer to messaging (*Spring ApplicationEvent*).

🚫

When NOT to Use

Updates cascade unpredictably — debugging notification storms.
Cycles — observer triggers subject again without guards.

🚨

Anti-Pattern (What NOT to Do)

Subject hard-codes every listener typeBadjava

class Order {
    void onShip() {
        new EmailService().send(this);
        new SmsService().text(this);
        // add Slack? edit Order again
    }
}

You must reopen the core class for every new side effect.

✅

The Right Way

interface OrderListener { void shipped(Order o); }

class Order {
    private final List<OrderListener> listeners = new CopyOnWriteArrayList<>();
    void add(OrderListener l) { listeners.add(l); }
    void ship() {
        // ...
        listeners.forEach(l -> l.shipped(this));
    }
}

Same idea as PropertyChangeSupport, Swing listeners, or Spring ApplicationEventPublisher.

⚖️

Pros & Cons

👍 Pros

Open/Closed — add observers without changing subject.
Supports MVC-style separation.

👎 Cons

Unexpected order of notifications — don't rely on ordering unless documented.
Memory leaks if observers forget to unregister.

🔀

Common Confusions Cleared Up

Observer vs Pub/Sub bus

Classic Observer is in-process object graph. Message broker pub/sub is distributed async — same idea at architectural scale.

🎤

Interview Q&A

🔮

Likely Follow-up Questions

Cold vs hot observables (Rx) vs classic Observer?

🧾

Quick Cheat Sheet

IntentNotify many dependents of change.

JDKPropertyChangeSupport, Swing listeners.

PitfallLeak if observer not removed.

🔗