How to add item to a List with a bounded wildcard

Question

Hi,
Is there a way to add a subtype instance to a List with a bounded wildcard? For example, say I have an
abstract class called Car. Then I have 2 concrete subclasses--Ford and Toyota. Now say I have another
class that contains a prepopulated list and is wildcard bounded like this: List<? extends Car> carList. I
want to add a method where I pass in a Car subtype as a parameter and then add it into this list, e.g:
public void addCar(Car car) {
carList.add(car);
Trying this, I keep getting "cannot find symbol" compilation errors. When I went back to read the Generics
tutorial by Bracha, he mentions that explicitly adding a subtype, e.g. carList.add(new Ford()), can not be
done. But here I'm adding the "upper bound" of the wildcard, i.e. the Car type. Therefore there should be no
discrepancy between supertype and subtype an thus the carList.add(car) should be ok, IMO.
Am I misunderstanding something with this logic? Also, if carList.add(car) can not be done, then how can
I go about doing something similar which will allow me to add subtypes to a "generic" list?
Thanks in advanced!
-los

Answer 1

I get a compilation error expecting an Object instead of a Car. Of course you did. List<? super Car> is a list into which you can put cars -- and possibly other stuff. List<? extends Car> is a list out of which you can get cars (possibly of different makes).
Do you need a list that you can put cars in and get cars out? That's List<Car>
This isn't a linguistic problem you are having; this is a design problem. If you have a function that takes an argument which is a list that you can put any kind of car into and be sure of getting a car out of, it isn't reasonable to pass in a List<Ford> (because the function might put in a Chevy) or a List<Object> (but there might be motorcycles already in there). By the requirements you have, you need a List<Car> and nothing else.
Yes, you could use a cast, but all you are doing is circumventing the type system by saying "I know this List<Object> only has Cars in it."