Why is the following java code leads to compilation error

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, ie 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>>();

Answer 2

The two sides have to match for the inner List :

List<List<?>> foo = new ArrayList<List<?>>();
foo.add(new ArrayList<Integer>());

Though this is rather silly as you've just defined a List of "Lists that can hold anything", and unless you know exactly what each is in that outer List, there's no way to divine it later due to type erasure.