Observer

Learn the Observer pattern.

The Observer pattern

The Observer lets an object notify its subscribers about state changes. The notifier is called the Subject, the receivers are Observers. This is a publish-subscribe relationship.

Example: when a temperature sensor (Subject) changes, a screen and a log (Observers) are informed. The Subject adds subscribers with attach and calls update on all of them with broadcast. Subscribers can be added at any time, keeping parts of the system independent.

Registration and notification flow

Observers are registered into a list with attach. When a change happens the Subject walks this list and calls update on each. The notification can carry a string or an object.

In Java a List<Observer> usually holds the subscribers. With many subscribers the order matters, so registration order is preserved. A detach method can be added to unsubscribe.

Exercise Notify sensor subscribers

Define an Observer interface with void update(String msg). A Screen class implements it and prints "screen: " + msg with System.out.println inside update. A Subject class has attach(Observer) and broadcast(String msg) methods; broadcast calls update on every Observer in the list. In main, attach a Screen and call broadcast("temp 30").

Show example solution

import java.util.*;

interface Observer { void update(String msg); }

class Screen implements Observer {
    public void update(String msg) { System.out.println("screen: " + msg); }
}

class Subject {
    private List<Observer> list = new ArrayList<>();
    public void attach(Observer o) { list.add(o); }
    public void broadcast(String msg) { for (Observer o : list) o.update(msg); }
}

public class ObserverDemo {
    public static void main(String[] args) {
        Subject s = new Subject();
        s.attach(new Screen());
        s.broadcast("temp 30");
    }
}

Test yourself

Match the Observer roles

Subject

Sends notifications
Observer

Receives notifications
attach

Adds a subscriber
update

Notification method

Complete the attach call

subject.____(new Screen());

Show answer

subject.attach(new Screen());

Who notifies the subscribers in the Observer pattern?
- Observer
- The interface
- Decorator
- Subject
Show answer

Subject — The Subject sends notifications to subscribed Observers.

Next lesson: Decorator

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