Observer Design Pattern

The Observer Design Pattern is a behavioral design pattern used in software engineering to establish a one-to-many dependency between objects. In this pattern, when the state of one object (called the subject) changes, all its dependent objects (called observers) are automatically notified and updated.

Key Components:

Subject: The object being observed. It maintains a list of observers and notifies them of any state changes.
Observer: The object that wants to be notified of changes in the subject’s state.
ConcreteSubject: A concrete implementation of the subject that holds the state and sends notifications when the state changes.
ConcreteObserver: A concrete implementation of an observer that reacts to changes in the subject’s state.

Step 1: Define the Observer Interface

interface Observer {
    void update(String state);
}

Step 2: Define the Subject Interface

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

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

Step 3: Implement the ConcreteSubject Class

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

    public String getState() {
        return state;
    }

    public void setState(String state) {
        this.state = state;
        notifyObservers();  // Notify observers when state changes
    }
}

Step 4: Implement the ConcreteObserver Class

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 state change: " + state);
    }
}

Step 5: Usage Example (Main Method)

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

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

        // Attach observers to the subject
        subject.attach(observer1);
        subject.attach(observer2);

        // Change state and notify observers
        subject.setState("New State 1");
        
        // Detach one observer and change state again
        subject.detach(observer1);
        subject.setState("New State 2");
    }
}