Animal Shelter - CC150

https://github.com/mirandaio/interview-questions/blob/master/stacks_and_queues/AnimalQueue.java
An animal shelter holds only dogs and cats, and operates on a strictly “first in, first out” basis. People must adopt either the “oldest” (based on arrival time) of all animals at the shelter, or they can select whether they would prefer a dog or a cat (and will receive the oldest animal of that type). They cannot select which specific animal they would like. Create the data structures to maintain this system and implement operations such as enqueue, dequeueAny, dequeueDog and dequeueCat. You may use built-in LinkedList data structure.

public class AnimalQueue {
    LinkedList<Dog> dogs = new LinkedList<Dog>();
    LinkedList<Cat> cats = new LinkedList<Cat>();
    private int order = 0;

    public void enqueue(Animal a) {
        a.setOrder(order);
        order++;

        if(a instanceof Dog)
            dogs.addLast((Dog) a);
        else if(a instanceof Cat)
            cats.addLast((Cat) a);
    }

    public Animal dequeueAny() {
        if(dogs.size() == 0) {
            return cats.poll();
        } else if(cats.size() == 0) {
            return dogs.poll();
        }

        Dog dog = dogs.peek();
        Cat cat = cats.peek();

        if(dog.isOlderThan(cat))
            return dogs.poll();

        return cats.poll();
    }

    public Dog dequeueDog() {
        return dogs.poll();
    }

    public Cat dequeueCat() {
        return cats.poll();
    }
}