Observer

import java.util.*;
interface PaymentSuccessEventSubscriber{//has common method called notify
    void notify(int order_id);
}

class EmailService implements PaymentSuccessEventSubscriber{ 
    public void notify(int order_id){
        sendEmail(order_id);
    }
    public void sendEmail(int order_id){
        System.out.println("In email service");
    }
}
class SmsService implements PaymentSuccessEventSubscriber{
    public void notify(int order_id){
        sendSms(order_id);
    }
    public void sendSms(int order_id){
        System.out.println("In sms service");
    }
}
//--------------------------
class Flipkart{
    private List<PaymentSuccessEventSubscriber> subscribers = new ArrayList<>(); //list of subscribers
    public void subscribe(PaymentSuccessEventSubscriber event){ //to add
        subscribers.add(event);
    }
    public void unsubscribe(PaymentSuccessEventSubscriber event){ //to remove
        subscribers.remove(event);
    }
    public void paymentSuccess(int order_id){ //when success call all the subscribers
        for(PaymentSuccessEventSubscriber subscriber : subscribers){ 
            subscriber.notify(order_id);
        }
    }
}

//---------------------------
public class Client{
    //create event subscribers
    private static PaymentSuccessEventSubscriber emailService = new EmailService();
    private static PaymentSuccessEventSubscriber smsService = new SmsService();
    
    public static void main(String[] args){
        Flipkart flipkart = new Flipkart(); //make flipkart object
        flipkart.subscribe(emailService); //add event subscribers
        flipkart.subscribe(smsService);
        
        flipkart.paymentSuccess(123); //on payment success, all subscribers gets called
        flipkart.paymentSuccess(23);
    }
}

//output
In email service
In sms service
In email service
In sms service