Question

I am currently working on making my code contain more generics. I encountered a compilation error which looks quite complicated but which I was able to reduce to an equivalent error in the following code:

List<List<?>> a = new ArrayList<List<Integer>>();

Why this happens? What can I do to fix it?

Answer 1

Instances of a generic class with different type parameters are not related, i.e. even though String is a subtype of Object, List<String> is not a subtype of List<Object>, and even though List<Integer> is a subtype of List<?>, List<List<Integer>> is not a subtype of List<List<?>>.

Perhaps you are looking for

List<? extends List<?>> a = new ArrayList<List<Integer>>();