Is List<Double> a subtype of List<? extends Number> and why?

Question

Here is what I know:

1. Double is a subtype of Number and List<Double> is not a subtype of List<Number>.
2. List<Dog> is not a subtype of List<Animal> because you can add Cat to List<Animal> but you can't do that with List<Dog>.
3. List<? extends Number> means this list can store variables of type Number and variables of subtype of Number. List<Double> means this list can store variables of type Double.

Please correct me if anything above is wrong and then Is List<Double> a subtype of List<? extends Number> and why?

Answer 1

All your items are correct.

1. Double is a subtype of Number and List<Double> is not a subtype of List<Number>.

2. List<Dog> is not a subtype of List<Animal> because you can add Cat to List<Animal> but you can't do that with List<Dog>.

That's correct. Generics aren't covariant (but arrays are!). Here's some follow up reading: Why are arrays covariant but generics are invariant?

3. List<? extends Number> means this list can store variables of type Number and variables of subtype of Number. List<Double> means this list can store variables of type Double.

This is true, but there's an important difference between List<Number> and List<? extends Number>. You can think of List<? extends Number> as a list of a specific Number-subtype (that is one of List<Double>, List<Integer>, List<Long>, ...) and a List<Number> as a list that can potentially contain a mix of Double, Integer, ...

As for your final question:

Is List<Double> a subtype of List<? extends Number>...

Yes, you can have for instance

List<Double> doubles = new ArrayList<>();
List<? extends Number> numbers = doubles;

... and why?

This is just the way subtyping is defined.

As for the motivation, suppose you have a method that accepts a list of numbers. If you let the parameter have the type List<Number> you won't be able to pass a List<Double> to it. (Your second item in your question explains why!) Instead, you can let the parameter have type List<? extends Number>. Since List<Double> is a subtype of List<? extends Number> it will work out.