Observer Pattern Design Pattern – Clean Implementation

What Problem It Solves

When one object changes, multiple dependent objects need to be notified. You don't want the subject to know about every observer. Observer decouples the subject (publisher) from observers (subscribers).

Observer vs Pub-Sub

Aspect	Observer	Pub-Sub
Coupling	Subject knows observers (via interface)	Publisher and subscriber don't know each other
Delivery	Direct (in-process)	Via broker (Kafka, Redis, in-memory bus)
Failure	One slow observer blocks others	Async queue decouples
LLD fit	In-process notification, small scale	Cross-service, large scale

Common Interview Traps

Trap	Why	Fix
Forgetting observer removal	Memory leak (observer holds reference to subject)	Provide `removeObserver` weak references
Blocking in observer callback	Slow observer delays all others	Use async notification (event queue)
Notification order dependency	Observers assume specific order	Observers should be order-independent
Observing the same subject twice	Double processing	Check before adding

Observer Variants

Variant	How	When
Pull	Subject notifies "something changed" observers call back to get data	Observers need different data subsets
Push	Subject sends full data with notification	Observers need the same data
Async (Event Bus)	Uses a message queue between subject and observers	Decouple in time, not just space (Kafka/Redis pub/sub)

Where Observer Appears in LLD Problems

Problem	Subject	Observers	Notification Trigger
Notification Service (#18)	NotificationManager	EmailHandler, SMSHandler, PushHandler	New notification created
Live Comments (#73)	CommentStream	WebSocketPush, CacheUpdater, AnalyticsTracker	New comment posted
Stock Brokerage (#29)	StockMarket	MobileApp, EmailAlert, TradingBot	Price change
Pub-Sub (#11)	Topic/PubSubManager	RegisteredSubscriber instances	Message published
Chat (#37)	ChatRoom	UserConnections	New message in room
Metrics (#75)	MetricCollector	DashboardUpdater, AlertEvaluator, LogWriter	Metric recorded

Implementation

// ─── EXAMPLE 1 ──────────────────────────────────────────────────────────────
// WHAT WE ARE IMPLEMENTING:
// An online store inventory monitor alerting waiting buyers immediately when
// an out-of-stock item is replenished.
//
// WHERE THE OBSERVER FITS IN:
// Subject is the Observable (e.g., Product). Observer is the Subscriber
// interface. Customer classes act as Concrete Observers.
// ────────────────────────────────────────────────────────────────────────────
import java.util.concurrent.CopyOnWriteArrayList;

// --- Observer interface ---
interface StockObserver {
    void update(String symbol, double price);
}

// --- Subject ---
class StockMarket {
    private final java.util.List<StockObserver> observers = new CopyOnWriteArrayList<>();
    private final java.util.Map<String, Double> prices = new java.util.HashMap<>();

    public void addObserver(StockObserver obs) { observers.add(obs); }
    public void removeObserver(StockObserver obs) { observers.remove(obs); }

    public void setPrice(String symbol, double price) {
        prices.put(symbol, price);
        notifyObservers(symbol, price);
    }

    private void notifyObservers(String symbol, double price) {
        for (StockObserver obs : observers) {
            obs.update(symbol, price);
        }
    }
}

// --- Concrete observers ---
class MobileApp implements StockObserver {
    private final String name;
    public MobileApp(String name) { this.name = name; }
    public void update(String symbol, double price) {
        System.out.println("  [MobileApp " + name + "] " + symbol + " now $" + price);
    }
}

class EmailAlertService implements StockObserver {
    public void update(String symbol, double price) {
        if (price > 500) {
            System.out.println("  [EMAIL ALERT] " + symbol + " crossed $500 at $" + price);
        }
    }
}

class TradingBot implements StockObserver {
    public void update(String symbol, double price) {
        if (price < 50) {
            System.out.println("  [TradingBot] Buying " + symbol + " at $" + price);
        }
    }
}

public class Main {
    public static void main(String[] args) {
        StockMarket market = new StockMarket();

        market.addObserver(new MobileApp("Alice"));
        market.addObserver(new MobileApp("Bob"));
        market.addObserver(new EmailAlertService());
        market.addObserver(new TradingBot());

        System.out.println("--- Price Update: AAPL ---");
        market.setPrice("AAPL", 150.0);

        System.out.println("\n--- Price Update: TSLA ---");
        market.setPrice("TSLA", 520.0);
    }
}

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Observer Pattern

✦What Problem It Solves

⚖Observer vs Pub-Sub

⚖Common Interview Traps

✦Observer Variants

🧩Where Observer Appears in LLD Problems

💻Implementation

💬Review