Observer Design Pattern

Overview

The Observer Design Pattern defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. It is commonly used in event-driven systems.

Key Characteristics

• Defines a clear dependency between objects (subject and observers).
• Promotes loose coupling between the subject and its observers.
• Ensures that observers are notified automatically of changes in the subject.

Implementation

The following is an example of an Observer implementation in Java:

import java.util.ArrayList;
import java.util.List;

// Subject
interface Subject {
    void attach(Observer observer);
    void detach(Observer observer);
    void notifyObservers();
}

// Concrete Subject
class ConcreteSubject implements Subject {
    private List observers = new ArrayList<>();
    private String state;

    public void setState(String state) {
        this.state = state;
        notifyObservers();
    }

    public String getState() {
        return state;
    }

    @Override
    public void attach(Observer observer) {
        observers.add(observer);
    }

    @Override
    public void detach(Observer observer) {
        observers.remove(observer);
    }

    @Override
    public void notifyObservers() {
        for (Observer observer : observers) {
            observer.update(state);
        }
    }
}

// Observer
interface Observer {
    void update(String state);
}

// Concrete Observer
class ConcreteObserver implements Observer {
    private String name;

    public ConcreteObserver(String name) {
        this.name = name;
    }

    @Override
    public void update(String state) {
        System.out.println(name + " received update: " + state);
    }
}

// Demo
public class ObserverDemo {
    public static void main(String[] args) {
        ConcreteSubject subject = new ConcreteSubject();

        Observer observer1 = new ConcreteObserver("Observer 1");
        Observer observer2 = new ConcreteObserver("Observer 2");

        subject.attach(observer1);
        subject.attach(observer2);

        subject.setState("State 1");
        subject.setState("State 2");
    }
}
    

When to Use

• When multiple objects need to be notified about changes in the state of another object.
• When you want to establish a loosely coupled communication between objects.

Advantages

• Promotes loose coupling between the subject and observers.
• Ensures all dependent objects are updated automatically.

Disadvantages

• Can lead to unexpected behavior if not carefully managed (e.g., circular updates).
• May introduce performance overhead if there are many observers.

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