Java Generics WildCard Question: List<? extends A>

Question

Let's say I have these classes : Vehicle, Car and Spaceship:

class Vehicle{

    void rideVehicle(Vehicle v){
        System.out.println("I am riding a vehicle!");
    }

}

class Car extends Vehicle{
    void rideVehicle(Vehicle c){
        System.out.println("I am riding a car!");
    }
}


class SpaceShip extends Vehicle{
    void rideVehicle(Vehicle c){
        System.out.println("I am riding a spaceship!");
    }

}

and I write this method addCars:

private static void addCars(List<? extends Vehicle> vcls){
        vcls.add(new Car());
        vcls.add(new Car());
        vcls.add(new Car());

    }

Why do I get a compile time error?? I understand that List is a supertype of List for any X that extends Vehicle. right?

Thanks

Edit: the error I get (compile-time) : The method add(capture#2-of ? extends Vehicle) in the type List is not applicable for the arguments (Car).

Answer 1

Method arguments are contravariant in the subtype, and by the definition of the wildcard, for every type T that extends Vehicle Foo<T> is a subtype of Foo<* extends Vehicle>. The implication of this is that wildcards are great when you only care about the return type, but dont work in situations like this when you want to pass a value of the type to a method.

The problem is that a user might try to call

List<SpaceShip> l = ...
addCars(l);

if your code were to compile, l would then be a list of spaceships containing 3 cars. Clearly no good.

Answer 2

Here's a pointer to why you're getting a compile error. Specifically,

List is an example of a bounded wildcard. The ? stands for an unknown type, just like the wildcards we saw earlier. However, in this case, we know that this unknown type is in fact a subtype of Shape. (Note: It could be Shape itself, or some subclass; it need not literally extend Shape.) We say that Shape is the upper bound of the wildcard.

There is, as usual, a price to be paid for the flexibility of using wildcards. That price is that it is now illegal to write into shapes in the body of the method. For instance, this is not allowed:

public void addRectangle(List<? extends Shape> shapes) {
     shapes.add(0, new Rectangle()); // Compile-time error! 
} 

You should be able to figure out why the code above is disallowed. The type of the second parameter to shapes.add() is ? extends Shape-- an unknown subtype of Shape. Since we don't know what type it is, we don't know if it is a supertype of Rectangle; it might or might not be such a supertype, so it isn't safe to pass a Rectangle there.