How do I implement the Observer pattern in Java?

How do I implement the Observer pattern in Java?

The Observer pattern is a behavioral design pattern that allows an object, known as the subject, to maintain a list of its dependents, called observers, and notify them of any state changes. Would you like a code example?

Yes, please provide a code example.

Sure! Here's a simple implementation:

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

// Subject
class Subject {
    private List<Observer> observers = new ArrayList<>();

    public void addObserver(Observer observer) {
        observers.add(observer);
    }

    public void removeObserver(Observer observer) {
        observers.remove(observer);
    }

    public void notifyObservers(String message) {
        for (Observer observer : observers) {
            observer.update(message);
        }
    }
}

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

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

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

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

// Example usage
public class Main {
    public static void main(String[] args) {
        Subject subject = new Subject();
        ConcreteObserver observer1 = new ConcreteObserver("Observer 1");
        ConcreteObserver observer2 = new ConcreteObserver("Observer 2");

        subject.addObserver(observer1);
        subject.addObserver(observer2);

        subject.notifyObservers("Hello Observers!");
    }
}

This code defines a Subject that can notify its observers when a message is sent.

What are some use cases for the Observer pattern?

The Observer pattern is commonly used in scenarios like event handling systems, implementing publish-subscribe mechanisms, and in GUI frameworks where UI components need to update in response to changes in data.