Generics cast issue

Question

Here\'s my issue: given these classes

class A {}
class B extends A {}

This code compiles:

    List

        

Answer 1

Damn good question, and I don't know the answer, but here's a work around:

List<Class<? extends A>> numbers = new ArrayList<Class<? extends A>>(Arrays.asList(B.class));

Answer 2

Arrays.asList(B.class);

is generified as

List<Class<B>> numbers = Arrays.asList(B.class);

Since it has only 1 attribute of type T that match the parameterized type (in this case, B.class).

Answer 3

This is a problem of Covariance/Contravariance in Generics. The following SO question should help you understand and solve the problem:

Demonstrate covariance and contravariance in Java?

How would contravariance be used in Java generics?

Answer 4

This will compile:

List<Class<? extends A>> numbers = Arrays.<Class<? extends A>>asList(B.class);

Answer 5

Just to add, we can rewrite the code as below

List<Class<? extends A>> asListA = Arrays.asList(B.class, A.class);
List<Class<B>> asListB = Arrays.asList(B.class);

List<Class<? extends A>> numbers = asListA;
List<Class<? extends A>> numbers2 = asListB; // it will fail here

And @Inerdia explained the details already.

Answer 6

In the first example, the inferred type of the Arrays.asList() call is List<Class<? extends A>>, which is obviously assignable to a variable of the same type.

In the second example, the type of the right side is List<Class<B>>. While Class<B> is assignable to Class<? extends A>, List<Class<B>> is not assignable to List<Class<? extends A>>. It would be assignable to List<? extends Class<? extends A>>.

The reason for this is the same one as why a List<B> isn't assignable to List<A>. If it was, it would make the following (not-typesafe) code possible:

List<Class<B>> bb = new ArrayList<B>();
List<Class<? extends A>> aa = bb;
aa.add(A.class);