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.