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
}

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

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

public static void main(String[] args) {
    List<? extends Mammal> mammals = getMammals();
    Mammal mammal = mammals.get(0);
}

class Animal {
    public Animal getAnimal() {return this;}
}

class Mammal extends Animal {
    public Mammal getAnimal() {return this;}
}

class Giraffe extends Mammal {
    public Giraffe getAnimal() {return this;}
}