Java covariance

Question

I'm having a hard time trying to figure this out. Say I have the following code:

class Animal { }
class Mammal extends Animal { }
class Giraffe extends Mammal { }
...
public static List<? extends Mammal> getMammals() { return ...; }
...

public static void main(String[] args) {
    List<Mammal> mammals = getMammals(); // compilation error
}

Why does the assignment result in a compilation error? The error is something like:

Type mismatch: cannot convert from List<capture#4-of ? extends Mammal> to List<Mammal>

According to my understanding of covariance, the getMammals() method returns a list that will always contain Mammal objects so it should be assignable. What am I missing?

Answer 1

Because getMammals could return a List<Giraffe>, and if that was convertable to List<Mammal> then you'd be able to add a Zebra to it. You can't be allowed to add a Zebra to a list of Giraffe, can you?

class Zebra extends Mammal { }

List<Giraffe> giraffes = new List<Giraffe>();

List<Mammal> mammals = giraffes; // not allowed

mammals.add(new Zebra()); // would add a Zebra to a list of Giraffes