Question

i have these classes

class SuperParent {
        @Override
        public String toString() {

            return "SuperParent";
        }
    }

    class A extends SuperParent {

        @Override
        public String toString() {

            return "A";
        }
    }

    class B extends SuperParent {

        @Override
        public String toString() {

            return "B";
        }

    }

and i want to have a List of ArrayLists , so i can add to ArrayList of B’s and A’s
i tried

List<ArrayList<?extends SuperParent>> mList = new ArrayList<ArrayList<SuperParent>>();

but does not compile

when i tried this,

ArrayList<ArrayList<SuperParent>> mList = new ArrayList<SuperParent>>();

it compiles but i cant add ArrayLists of As or ArrayLists Bs

i understand java types and inheritance too but when it comes to Generics i miss with it .
Any reference or a declaration would help me .

Answer 1

This is how you would declare a list of lists where the elements of outer list can be a list of A or list of B:

ArrayList<ArrayList<? extends SuperParent>> lists = 
    new ArrayList<ArrayList<? extends SuperParent>>();
lists.add(new ArrayList<A>(Arrays.asList(new A(), new A())));
lists.add(new ArrayList<B>(Arrays.asList(new B(), new B())));

Generally, it is recommended to use interface types than concrete types when declaring or accepting as method parameters. That way, the above snippet would look like below:

List<List<? extends SuperParent>> lists = 
    new ArrayList<List<? extends SuperParent>>();
lists.add(Arrays.asList(new A(), new A()));
lists.add(Arrays.asList(new B(), new B()));