Question

I am just trying to understand the extends keyword in Java Generics.

List<? extends Animal> means we can stuff any object in the List which IS A Animal

then won’t the following also mean the same thing:

List<Animal>

Can someone help me know the difference between the above two? To me extends just sound redundant here.

Thanks!

Answer 1

List<Dog> is a subtype of List<? extends Animal>, but not a subtype of List<Animal>.

Why is List<Dog> not a subtype of List<Animal>? Consider the following example:

void mySub(List<Animal> myList) {
    myList.add(new Cat());
}

If you were allowed to pass a List<Dog> to this function, you would get a run-time error.

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EDIT: Now, if we use List<? extends Animal> instead, the following will happen:

void mySub(List<? extends Animal> myList) {
    myList.add(new Cat());     // compile error here
    Animal a = myList.get(0);  // works fine 
}

You could pass a List<Dog> to this function, but the compiler realizes that adding something to the list could get you into trouble. If you use super instead of extends (allowing you to pass a List<LifeForm>), it’s the other way around.

void mySub(List<? super Animal> myList) {
    myList.add(new Cat());     // works fine
    Animal a = myList.get(0);  // compile error here, since the list entry could be a Plant
}

The theory behind this is Co- and Contravariance.