Why this simple assingment is not possible in Generics ?

Question

Hi
Just curious to know why this simple conversion does not work and requires casting ?
List<? super Integer> list = new ArrayList<Integer>();
List<? super Number> listN = list; //Requires Casting
List<? extends Number> listExNu = new ArrayList<Number>();
List<? extends Integer> listExIn = listExNu; //Requires CastingAs per my understanding ,
A List<? super Number> can fit inside a List<? super Integer> ...
So what really happens here to force casting ? Any ideas ?

Answer 1

ejp wrote:
List<? super Integer> list = new ArrayList<Integer>();The only two classes that are superclasses of Integer are Number and Object.Not quite. Please note that the clause *? super Integer* is inclusive, i.e. it also matches List<Integer>.
Additionally, it matches all interfaces implemented by Integer. Thus, the following types can be assigned to list:
List<Integer>
List<Serializable>
List<Comparable<Integer>>
List<Comparable<? extends Integer>>
List<Comparable<? super Integer>>
List<Comparable<? extends Number>>
List<Comparable<? extends Serializable>>
List<Comparable<?>>
List<Number>
List<Object>And the following types can be assigned to listN:
List<Serializable>
List<Number>
List<Object>Since the latter is a genuine subset of the former, an implicit conversion such as listN = list is impossible.