Java Generics — Assigning a list of subclass to a list of superclass

Question

I have a basic question regarding assignment of a list of subclass to a list of superclass.

So I have something like the following:

Class B extends          


        

Answer 1

List<B> is not List<A>:

Through example: let say you have class B1 extends A{} and class B2 extends A{} then (if you would be able to do that:

List<B1> b1 = new AList<B1>();
List<A> a = b1;

List<B2> b2 = new AList<B2>();

by the hypothesis, you should be able to do a.add(new B2()) but this is wrong.

If you try the same thing but using arrays instead of lists, it will compile and throw exception in runtime.

We say that arrays are covariant and generics are invariant.

to make the code compile you have the wite it:

List<? extends A> a = b;

this says that a is a list of some subtype of A. _But you don know which one. Because of that you can't do a.put(X)