// ==================================================
// Observer Design Pattern
// ==================================================
//
// What is Observer Pattern?
// Observer Pattern defines a one-to-many dependency between objects
// so that when one object (Subject) changes state, all its dependents
// (Observers) are notified and updated automatically.
//
// It is also known as the Publisher-Subscriber model.
//
// When to use:
// - When a change in one object requires changing others
// - When you want loose coupling between the event source and its listeners
// - When you want to add or remove listeners without modifying the source
//
// Key parts:
// 1. Subject (Publisher)     → maintains list of observers and notifies them
// 2. Observer (Subscriber)   → interface with an update method
// 3. Concrete Observers      → classes that react to the notification
//
// Example used here: Student registration event
// When a student registers, multiple actions should happen:
// - Assign batch
// - Send email
// - Send Slack invite
//
// --------------------------------------------------
// Folder Structure (if kept as separate files):
//
// observer/
// ├── Observer.java
// ├── RegistrationSubject.java
// ├── BatchCreator.java
// ├── EmailService.java
// ├── SlackNotification.java
// └── ObserverDemo.java
// ==================================================

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

// ==================================================
// File: Observer.java
// Observer Interface
// All subscribers must implement this
// ==================================================
interface Observer {
    void update(String studentName, String eventType);
}

// ==================================================
// File: RegistrationSubject.java
// Subject (Publisher)
// Maintains list of observers and notifies them when an event occurs
// ==================================================
class RegistrationSubject {
    private final List<Observer> observers = new ArrayList<>();
    private String studentName;

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

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

    private void notifyObservers(String eventType) {
        for (Observer observer : observers) {
            observer.update(studentName, eventType);
        }
    }

    public void registerStudent(String studentName) {
        this.studentName = studentName;
        System.out.println("Student registered: " + studentName);
        notifyObservers("STUDENT_REGISTERED");
    }
}

// ==================================================
// File: BatchCreator.java
// Concrete Observer 1
// ==================================================
class BatchCreator implements Observer {
    @Override
    public void update(String studentName, String eventType) {
        if ("STUDENT_REGISTERED".equals(eventType)) {
            System.out.println("[BatchCreator] Assigned batch to " + studentName);
        }
    }
}

// ==================================================
// File: EmailService.java
// Concrete Observer 2
// ==================================================
class EmailService implements Observer {
    @Override
    public void update(String studentName, String eventType) {
        if ("STUDENT_REGISTERED".equals(eventType)) {
            System.out.println("[EmailService] Sent welcome email to " + studentName);
        }
    }
}

// ==================================================
// File: SlackNotification.java
// Concrete Observer 3
// ==================================================
class SlackNotification implements Observer {
    @Override
    public void update(String studentName, String eventType) {
        if ("STUDENT_REGISTERED".equals(eventType)) {
            System.out.println("[SlackNotification] Sent Slack invite to " + studentName);
        }
    }
}

// ==================================================
// File: ObserverDemo.java
// Client / Usage
// (public removed so it can compile in the same file)
// ==================================================
class ObserverDemo {
    public static void main(String[] args) {
        RegistrationSubject registration = new RegistrationSubject();

        Observer batchCreator = new BatchCreator();
        Observer emailService = new EmailService();
        Observer slack = new SlackNotification();

        registration.addObserver(batchCreator);
        registration.addObserver(emailService);
        registration.addObserver(slack);

        registration.registerStudent("Sai Harsha");

        System.out.println("\n--- Removing Email Service ---");
        registration.removeObserver(emailService);

        registration.registerStudent("Another Student");
    }
}