Generics in Action

Question

Given a method declared as(ignore implementation):
public static <E extends Number> List<? super E> process(List<E> nums)
The above method is called as follows from some method in the same class:-
ArrayList<Integer> input = null;
List<Number> output = null;
output = process(input);
The above invocation is not allowed by the compiler. WHY?

Answer 1

I think i got now why compiler was not allowing the following:-
1. List<? super Integer> n = new ArrayList<Number>();
2. List<Number> output = n; //=> "Type mismatch: cannot convert from List<capture-of ? super Integer> to List<Number>"
The reason is when the above code will be compiled by the compiler it is going to erase all the generic code and place appropriate cast where ever required to ensure type safety. Now here compiler will remove <Number> and <? super Number> in line 1
i.e. List n = new ArrayList();
and now while processing the line 2 it will not be able to ensure the type safety as it doesn't know during compile time what type of collection is assigned to "List<? super Integer> n"
i.e. List output = n; // Can't ensure the type safety. i.e. it cant ensure whether "n" is going to be of type List<Number>
I hope i am thinking in the right direction. :)