Struggling with understanding <? extends T> wildcard in Java

Question

I have a very basic question.

The code below doesn\'t compile (assume Apple Extends Fruit):

    List numbers = new ArrayList&l         


        

Answer 1

The best way to understand this is to think of the wildcard as saying something about the list, not the fruit. In other words:

List allBananas = getMyBananas();
enumerateMyFruit(allBananas);

static void enumerateMyFruit(List myFruit) {
    for (Fruit fruit : myFruit)
        System.out.println(fruit);
}

When we pass allBananas to enumerateMyFruit, inside the method we lose information about the original declared type of the list. In this example we can very clearly see why we shouldn't be able to e.g. put apples in a List, because we know that the list is actually a List. Again, the wildcard is telling us something about the declared type of the list.

List should be read as something like "a list originally declared to hold Fruit or some subtype of Fruit, but we don't know what that declared type is anymore". All that we know is that everything we pull out of the list is a Fruit.

Also, you are right, we could iterate the list and use instanceof to find out what is really in the list, but this wouldn't tell us the original declared type of the list. In the above code snippet we would find out that everything in the list turned out to be a Banana, but I could have just as easily declared allBananas as a List.

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You might also see why a List is not a List which explains some of this. The wildcard is how we have covariance among generic types. a List is not a List but it is a List. This comes with the restriction that we can't add to a List, because it might be a List, a List or something else. We don't know anymore.

There's also the ? super, which is the opposite. We can store Fruit in a List but we don't know what kinds of objects we will pull out of it. Its original declared type might actually be e.g. a List