# Purpose

Establishes a subscription relation where objects (observers) are automatically notified of any change in the state of another object (subject).

# Problem

• Subject-Observer Coupling: Need to notify changes without the subject knowing the concrete receiving classes.

• Dynamic Update: Objects that need to monitor states that change over time.

# Solution

• Subscription: Observers voluntarily register or unregister.

• Automatic Notification: The subject iterates over its subscribers and calls their update method after a change.

# Structure

# Participants

• Subject: Interface for registration and notification.
• ConcreteSubject: The object of interest with changing state.
• Observer: Interface with the update() method.
• ConcreteObserver: The subscriber that reacts to changes.

# When to Use It

When a change in one object requires changes in others that are unknown a priori, or when the list of interested parties is dynamic.

# Example: Weather Alert

Weather station (Subject) notifying mobile Apps and news services (Observers).

# Java Code

interface Subject {
    void register(Observer o);
    void notifyObservers();
}

class WeatherData implements Subject {
    private List<Observer> obs = new ArrayList<>();
    public void notifyObservers() {
        for (Observer o : obs) o.update(temp, hum, press);
    }
}

# Mapping Participants

• WeatherData: ConcreteSubject.
• MobileApp: ConcreteObserver.
• Service: Client.

# Conclusions

It is the foundation of reactive systems and allows a highly extensible architecture against new monitoring requirements.

# Related Patterns