observer.java

import java.util.ArrayList;

class DogEvent {
    public String message;
}


@FunctionalInterface
interface DogObserver {
    void update(DogEvent event);
}


class Dog {
    private ArrayList<DogObserver> observers = new ArrayList<>();

    public Dog() {

    }

    public void hey() {
        DogEvent event = new DogEvent();
        event.message = "I'm Ready" + Math.random();
        notifyDogObservers(event);
    }

    public void addDogObserver(DogObserver observer) {
        observers.add(observer);
    }

    private void notifyDogObservers(DogEvent event) {
        observers.forEach((DogObserver observer) -> {
            observer.update(event);
        });

    }
}


class Person {
    public Person() {
        Dog dog = new Dog();
        dog.addDogObserver((DogEvent event) -> {
            System.out.println("Person: " + event.message);
        });

        dog.hey();
        dog.hey();
    }
}


class Alien {
    public Alien() {
        Dog dog = new Dog();
        dog.addDogObserver((DogEvent event) -> {
            System.out.println("Alien: " + event.message);
        });
        dog.hey();
    }
}


public class Observer {

    public static void main(String[] args) {
        new Person();
        new Alien();
    }

}