Question

Here’s my issue: given these classes

class A {}
class B extends A {}

This code compiles:

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

And this does not:

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

What gives?

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UPDATE: This code compiles in Java 8. Apparently, due to ‘Improved Type Inference’.

Answer 1

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);