Generics, Wildcards and Covariant Return

[T Vergilio]

I don't understand why the code below compiles:

    <T extends Number> List<? extends T> method () {
        return new ArrayList<T>();
    }

but this doesn't:

    <T extends Number> List<? extends T> method () {
        return new ArrayList<Number>(); //does not compile, wants a List<? extends T>
    }

Surely, if I'm saying T extends Number, I should be able to return a List<Number>?

[T Vergilio]

Never mind, I can see it now... Number is not a subclass of T, and I need to return T or any subclasses. That was a silly question

[Ankit Garg]

Hi Vergilio, the code won't compile even if you try to return ArrayList<Integer>. The problem is that within the method you can't decide what the type of the returned ArrayList will be. Even if you simplify the return type you won't be able to return ArrayList<Number>

<T extends Number> List<T> method () {
    return new ArrayList<Number>();  // won't compile
}

[T Vergilio]

Hi Ankit,

Yeah, that's a bit of a mess and doesn't make much sense on its own (I was trying to test an edge case and may have gone a bit too far there). You're right, the code needs to decide what T is. Something like this would have been better:

<T extends Number> List<? extends T> method (T t) {
        return new ArrayList<T>();
    }

So T is whatever is passed in as an argument. I could then call it using:

List<? extends Number> arrayList = method(n);