The Observer Design Pattern is a behavioral design pattern that defines a one-to-many dependency between objects so that when one object (the subject) changes state, all its dependents (observers) are notified and updated automatically.

It primarily deals with the interaction and communication between objects, specifically focusing on how objects behave in response to changes in the state of other objects.

· The pattern is concerned with defining a mechanism for a group of objects to interact based on changes in the state of one object (the subject). The observers’ behavior is triggered by changes in the subject’s state.

· It encapsulates the behavior of the dependent objects (observers) and allows for a clean separation between the subject and its observers. This separation promotes a more modular and maintainable design.

· The pattern promotes loose coupling between the subject and its observers. The subject doesn’t need to know the concrete classes of its observers, and observers can be added or removed without affecting the subject.

· The primary mechanism in the Observer Pattern is the notification of observers when a change occurs. This notification mechanism facilitates the dynamic and coordinated behavior of multiple objects in response to changes in the subject.

enum Event {
NEW_ITEM,
SALE
}

interface Listener {
void update(Event eventType);
}

record EmailMsgListener(String email) implements Listener {

@Override
public void update(Event eventType) {
// Actually send the mail
System.out.println("Sending mail to " + email + " concerning " + eventType);
    }

}

record MobileAppListener(String username) implements Listener {

@Override
public void update(Event eventType) {
// Actually send the push notification to username
System.out.println("Sending mobile app notification to " + username + " concerning " + eventType);
    }

}

class NotificationService {

private final Map<Event, List<Listener>> customers;

public NotificationService() {
customers = new HashMap<>();
Arrays.stream(Event.values()).forEach(event -> customers.put(event, new ArrayList<>()));

    }

public void subscribe(Event eventType, Listener listener) {
customers.get(eventType).add(listener);
    }

public void unsubscribe(Event eventType, Listener listener) {
customers.get(eventType).remove(listener);
    }

public void notifyCustomers(Event eventType) {
customers.get(eventType).forEach(listener -> listener.update(eventType));
    }

}

class Store {

private final NotificationService notificationService;

public Store() {
notificationService = new NotificationService();
    }

public void newItemPromotion() {
notificationService.notifyCustomers(Event.NEW_ITEM);
    }

public void salePromotion() {
notificationService.notifyCustomers(Event.SALE);
    }

public NotificationService getService() {
return notificationService;
    }

}

public class ObserverPattern {

public static void main(String[] args) {

Store store = new Store();
store.getService().subscribe(Event.NEW_ITEM, new EmailMsgListener("geekific@like.com"));
store.getService().subscribe(Event.SALE, new EmailMsgListener("geekific@like.com"));
store.getService().subscribe(Event.SALE, new EmailMsgListener("geekific@subs.com"));
store.getService().subscribe(Event.NEW_ITEM, new MobileAppListener("GeekificLnS"));

store.newItemPromotion();

System.out.println("==========================================");

store.salePromotion();

System.out.println("==========================================");

store.getService().unsubscribe(Event.SALE, new EmailMsgListener("geekific@like.com"));
store.salePromotion();

    }

}

Sending mail to geekific@like.com concerning NEW_ITEM

Sending mobile app notification to GeekificLnS concerning NEW_ITEM

==========================================

Sending mail to geekific@like.com concerning SALE

Sending mail to geekific@subs.com concerning SALE

==========================================

Sending mail to geekific@subs.com concerning SALE