Can you explain what the "observer" pattern is and in what cases it is used?

The "Observer" pattern is a behavioral pattern that allows an object (the subject) to notify other objects (observers) about changes in its state.

It is used when implementing a subscription mechanism, so that when data changes, related components are automatically updated without tight coupling.

Example: In a GUI, when the data model changes, all related views (widgets) receive notifications and update.

Example in Java:

interface Observer {
    void update();
}

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

    public void attach(Observer o) {
        observers.add(o);
    }

    public void setState(int state) {
        this.state = state;
        notifyAllObservers();
    }

    private void notifyAllObservers() {
        for (Observer o : observers) {
            o.update();
        }
    }
}

class ConcreteObserver implements Observer {
    @Override
    public void update() {
        System.out.println("State changed, observer notified.");
    }
}