Lesson 45: Design Pattern: Observer

Observer lets objects subscribe to events on another object — the foundation of event-driven systems and GUI/reactive frameworks.

Real-time example

A stock-price ticker notifies every subscribed dashboard widget the instant a price changes, without knowing what those widgets are.

interface PriceListener { void onPriceChange(double newPrice); }

class StockTicker {
    private List<PriceListener> listeners = new ArrayList<>();
    void subscribe(PriceListener l) { listeners.add(l); }
    void updatePrice(double price) {
        for (PriceListener l : listeners) l.onPriceChange(price);
    }
}
StockTicker ticker = new StockTicker();
ticker.subscribe(price -> System.out.println("Dashboard sees: " + price));
ticker.updatePrice(142.50);

What's happening

StockTicker never imports or knows about any specific dashboard class — it only depends on the PriceListener interface.

This decoupling is exactly what makes event systems (GUI clicks, message queues, webhooks) extensible without modifying the publisher.