Is List<Dog> a subclass of List<Animal>? Why are Java generics not implicitly polymorphic?

Question

I'm a bit confused about how Java generics handle inheritance / polymorphism.

Assume the following hierarchy -

Animal (Parent)

Dog - Cat (Children)

So suppose I have a method doSomething(List<Animal> animals). By all the rules of inheritance and polymorphism, I would assume that a List<Dog> is a List<Animal> and a List<Cat> is a List<Animal> - and so either one could be passed to this method. Not so. If I want to achieve this behavior, I have to explicitly tell the method to accept a list of any subclass of Animal by saying doSomething(List<? extends Animal> animals).

I understand that this is Java's behavior. My question is why? Why is polymorphism generally implicit, but when it comes to generics it must be specified?

Answer 1

No, a List<Dog> is not a List<Animal>. Consider what you can do with a List<Animal> - you can add any animal to it... including a cat. Now, can you logically add a cat to a litter of puppies? Absolutely not.

// Illegal code - because otherwise life would be Bad List<Dog> dogs = new ArrayList<Dog>(); // ArrayList implements List List<Animal> animals = dogs; // Awooga awooga animals.add(new Cat()); Dog dog = dogs.get(0); // This should be safe, right? 

Suddenly you have a very confused cat.

Now, you can't add a Cat to a List<? extends Animal> because you don't know it's a List<Cat>. You can retrieve a value and know that it will be an Animal, but you can't add arbitrary animals. The reverse is true for List<? super Animal> - in that case you can add an Animal to it safely, but you don't know anything about what might be retrieved from it, because it could be a List<Object>.