Java Generic List>

Question

How come in java we cannot do:

List> aList = new ArrayList>();

Even though thi

Answer 1

Your statement does not compile because List is not the same type as List. The former is a supertype of the latter.

Have you tried this? Here I'm expressing that the List is covariant in its type argument, so it will accept any subtype of List (which includes List).

List> aList = new ArrayList>();

Or even this. Here the type parameter for the ArrayList on the right-hand side is the same as the type parameter on the left-hand side, so variance is not an issue.

List> aList = new ArrayList>();

You should be able to just say

List> aList = new ArrayList>();

I tend to avoid the ? type wildcard whenever possible. I find that the expense incurred in type annotation is not worth the benefit.