Observer Pattern

Publisher and subscribers

A subject keeps a list of observers and notifies all of them when its state changes, without knowing what each one does. This decouples the event source from the reactions — new observers can be added with zero changes to the subject.

A minimal listener interface

This is the same shape as Swing's ActionListener, Spring's ApplicationListener<OrderPlaced>, or any pub/sub broker — a fixed callback contract plus a list of subscribers.

interface OrderListener { void onPlaced(Order o); }

class OrderService {
    private final List<OrderListener> listeners = new ArrayList<>();
    void subscribe(OrderListener l) { listeners.add(l); }

    void place(Order o) {
        // ... persist order ...
        listeners.forEach(l -> l.onPlaced(o));
    }
}

service.subscribe(order -> emailService.sendReceipt(order));

Pattern check

Quick check: What problem does the Observer pattern primarily solve?

• Guaranteeing only one instance of a class exists
• Letting an object notify many interested parties about a change without depending on their concrete types
• Building complex objects step by step
• Choosing which concrete class to instantiate at runtime