JAVA Nested Type Generic: mix of super and extends-CodePudding

The following is a valid assignment:

List<? extends ArrayList<? extends Integer>> l1 = new ArrayList<ArrayList<Integer>>();

However, for the following ones, I get incompatible types error

List<ArrayList<? extends Integer>> l2 = new ArrayList<ArrayList<Integer>>();
List<? super ArrayList<? extends Integer>> l3 = new ArrayList<ArrayList<Integer>>();

What is the logic here?

Also, is there any valid construction for l2 and l3? (i.e. by changing the right side of the equation)

CodePudding user response：

Java's inner level generics are invariant. That means that embedded type parameter must match exactly. In this case, if you have ? extends Integer as your inner parameter on the left, it must be on the right also.

List<ArrayList<? extends Integer>> l2 = new ArrayList<ArrayList<? extends Integer>>();
List<? super ArrayList<? extends Integer>> l3 = new ArrayList<ArrayList<? extends Integer>>();

Or you can use the diamond operator.

List<ArrayList<? extends Integer>> l2 = new ArrayList<>();
List<? super ArrayList<? extends Integer>> l3 = new ArrayList<>();