How do I implement the Observer pattern in Java?

How do I implement the Observer pattern in Java?

To implement the Observer pattern in Java, you need to define a subject and observers. The subject maintains a list of observers and notifies them of any state changes.

Can you show me a simple code example?

Sure! Here's a basic example:

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 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!");
    }
}

What are some use cases for the Observer pattern?

The Observer pattern is commonly used in event handling systems, like GUI frameworks, and in implementing publish-subscribe mechanisms.