Core Java - OOPs - Guided Exercise: Notifiers - Interface-Based Polymorphism

[akash-coded]

🧪 Guided Exercise: Notifiers - Interface-Based Polymorphism

🎯 Objective:

Help learners internalize how interfaces enable polymorphism, type generalization, and flexible method dispatch.

---

🧩 Scenario:

You're designing a notification system that can send messages via different channels: Email, SMS, and Push Notification.

Each notification type:

• Implements a common Notifier interface

• Has its own version of send(String message) method

• Can be passed into a method that only expects a Notifier reference

• Will be used in a polymorphic collection

This is very close to real-world design patterns and Spring Boot behavior.

---

📦 Step-by-Step Plan

✅ Step 1: Define the Interface

public interface Notifier {
    void send(String message); // method to override
}

💡 Ask Learners:

• Why can’t we put method bodies here?

• Can we instantiate an interface?

---

✅ Step 2: Implement Concrete Classes

public class EmailNotifier implements Notifier {
    @Override
    public void send(String message) {
        System.out.println("📧 Email sent: " + message);
    }
}
public class SMSNotifier implements Notifier {

@Override

public void send(String message) {

System.out.println("📲 SMS sent: " + message);

}

}
public class PushNotifier implements Notifier {

@Override

public void send(String message) {

System.out.println("🔔 Push notification sent: " + message);

}

}

🧠 Discussion Prompt:

• Which method is being overridden?

• What’s common and what’s unique in each implementation?

---

✅ Step 3: Use Interface Reference in a Method

public class NotificationService {
// Method Overloading: One with message only
public void notifyUser(String message) {
    System.out.println("ℹ️ Default notification: " + message);
}

// Method Overloading: One with interface type
public void notifyUser(Notifier notifier, String message) {
    System.out.println("🔁 Sending via interface-based notifier...");
    notifier.send(message);
}

}

✍️ Ask Students to Implement:

• Add logic to call this method using all three types (EmailNotifier, SMSNotifier, etc.)

• Use both overloaded versions to see which one gets called

---

✅ Step 4: Polymorphic Collection

public class Main {
    public static void main(String[] args) {
        Notifier email = new EmailNotifier();
        Notifier sms = new SMSNotifier();
        Notifier push = new PushNotifier();
    NotificationService service = new NotificationService();

    service.notifyUser("Welcome to the system!");
    service.notifyUser(email, "Verify your email.");
    service.notifyUser(sms, "Your OTP is 123456.");
    service.notifyUser(push, "New message received!");

    System.out.println("\n🔁 Broadcasting to all via polymorphic collection:");
    
    Notifier[] allNotifiers = { email, sms, push };
    for (Notifier n : allNotifiers) {
        n.send("System maintenance at 10 PM.");
    }
}

}

---

📘 Concepts Reinforced

Concept	Example
Interface-based polymorphism	Notifier n = new EmailNotifier();
Method overriding	send(String) redefined in each class
Method overloading	notifyUser() used with and without Notifier
Dynamic method dispatch	n.send() calls the right send() based on object
Polymorphic collection	Notifier[] notifiers = { ... }

---

🧠 Reflective Questions for Learners

1. Why does n.send() in the loop behave differently for each object?

2. What if we added a new class WhatsAppNotifier? Do we have to change anything in NotificationService?

3. Why is Notifier interface better than making Notifier an abstract class here?

---

🏆 Bonus Challenges

• Add a method getNotifierType() to each class that returns "Email", "SMS", etc.

• Create a factory method to return a Notifier based on a string input ("email", "sms").

• Write a logger class that accepts any Notifier and logs delivery success.

[prasetha21-max]

PRASETHA N

package com.practice.problems.java;

public class Main {
	public static void main(String[] args) {
		Notifier email = new EmailNotifier();
		Notifier sms = new SMSNotifier();
		Notifier push = new PushNotifier();
		NotificationService service = new NotificationService();

		service.notifyUser("Welcome to the system!");
		service.notifyUser(email, "Verify your email.");
		service.notifyUser(sms, "Your OTP is 123456.");
		service.notifyUser(push, "New message received!");

		Notifier[] allNotifiers = { email, sms, push };
		for (Notifier n : allNotifiers) {
			n.send("System maintenance");
		}
	}
}

interface Notifier {
	void send(String message);
}

class EmailNotifier implements Notifier {
	@Override
	public void send(String message) {
		System.out.println("Email sent: " + message);
	}
}

class SMSNotifier implements Notifier {
	@Override
	public void send(String message) {
		System.out.println("SMS sent: " + message);
	}
}

class PushNotifier implements Notifier {
	@Override
	public void send(String message) {
		System.out.println("Push notification sent: " + message);
	}
}

class NotificationService {
	public void notifyUser(String message) {
		System.out.println("Default notification: " + message);
	}

	public void notifyUser(Notifier notifier, String message) {
		System.out.println("Sending via interface-based notifier...");
		notifier.send(message);
	}
}

[Lohithr1975]

Lohith R

class Main {
public static void main(String[] args) {

      Notifier email = new EmailNotifier();
    Notifier sms = new SMSNotifier();
    Notifier push = new PushNotifier();
NotificationService service = new NotificationService();

service.notifyUser("Welcome to the system!");
service.notifyUser(email, "Verify your email.");
service.notifyUser(sms, "Your OTP is 123456.");
service.notifyUser(push, "New message received!");

System.out.println("\n Broadcasting to all via polymorphic collection:");

Notifier[] allNotifiers = { email, sms, push };
for (Notifier n : allNotifiers) {
    n.send("System maintenance at 10 PM.");
}
System.out.println();

   Notifier emailNotifier = NotifierFactory("email");
   logger(emailNotifier,"hello via email");
   Notifier smsNotifier = NotifierFactory("sms");
   logger(smsNotifier,"hello via sms");

}

public static void logger(Notifier notifier, String message){
notifier.send(message);

    System.out.println("message send sucessefully");

}

public static Notifier NotifierFactory(String type){
if(type.equalsIgnoreCase("email")) return new EmailNotifier();
else if(type.equalsIgnoreCase("sms")) return new SMSNotifier();
else if(type.equalsIgnoreCase("push")) return new PushNotifier();
else return null;
}

}
interface Notifier {
void send(String message); // method to override
}
class EmailNotifier implements Notifier {
@OverRide
public void send(String message) {
System.out.println(" Email sent: " + message);
}
public String getNotifierType(){
return "emanil";
}
}
class SMSNotifier implements Notifier {
@OverRide
public void send(String message) {
System.out.println("SMS sent: " + message);
}
public String getNotifierType(){
return "SMS";
}
}

class PushNotifier implements Notifier {
@OverRide
public void send(String message) {
System.out.println(" Push notification sent: " + message);
}
public String getNotifierType(){
return "Push";
}
}
class NotificationService {
// Method Overloading: One with message only
public void notifyUser(String message) {
System.out.println("Default notification: " + message);
}

// Method Overloading: One with interface type
public void notifyUser(Notifier notifier, String message) {
System.out.println(" Sending via interface-based notifier...");
notifier.send(message);
}

}

[Sdey0907]

### Task 1 - Sayan Dey - Completed

interface Notifier {
void send(String message);
}

class EmailNotifier implements Notifier {
@OverRide
public void send(String message) {
System.out.println("Email sent: " + message);
}
}

class SMSNotifier implements Notifier {
@OverRide
public void send(String message) {
System.out.println("SMS sent: " + message);
}
}

class PushNotifier implements Notifier {
@OverRide
public void send(String message) {
System.out.println("Push notification sent: " + message);
}
}
class NotificationService {

public void notifyUser(String message) {
    System.out.println("Default notification: " + message);
}

public void notifyUser(Notifier notifier, String message) {
    System.out.println("Sending via interface-based notifier...");
    notifier.send(message);
}

}
public class Main {
public static void main(String[] args) {
Notifier email = new EmailNotifier();
Notifier sms = new SMSNotifier();
Notifier push = new PushNotifier();

    NotificationService service = new NotificationService();

    service.notifyUser("Welcome to the system!");

    service.notifyUser(email, "Verify your email.");
    service.notifyUser(sms, "Your OTP is 123456.");
    service.notifyUser(push, "New message received!");

    System.out.println("\nBroadcasting to all via polymorphic collection:");
    Notifier[] allNotifiers = { email, sms, push };

    for (Notifier n : allNotifiers) {
        n.send("System maintenance at 10 PM.");
    }
}

}

[vbhavana1]

public class Notificationservice {
public static void main(String[] args) {
Notifier email = new EmailNotifier();
Notifier sms = new SMSNotifier();
Notifier push = new PushNotifier();
NotificationService service = new NotificationService();

	service.notifyUser("Welcome to the system!");
	service.notifyUser(email, "Verify your email.");
	service.notifyUser(sms, "Your OTP is 123456.");
	service.notifyUser(push, "New message received!");

	Notifier[] allNotifiers = { email, sms, push };
	for (Notifier n : allNotifiers) {
		n.send("System maintenance");
	}
}

}

interface Notifier {
void send(String message);
}

class EmailNotifier implements Notifier {
@OverRide
public void send(String message) {
System.out.println("Email sent: " + message);
}
}

class SMSNotifier implements Notifier {
@OverRide
public void send(String message) {
System.out.println("SMS sent: " + message);
}
}

class PushNotifier implements Notifier {
@OverRide
public void send(String message) {
System.out.println("Push notification sent: " + message);
}
}

class NotificationService {
public void notifyUser(String message) {
System.out.println("Default notification: " + message);
}

public void notifyUser(Notifier notifier, String message) {
	System.out.println("Sending via interface-based notifier...");
	notifier.send(message);
}

}