Why is it not possible to call List not with List even if Integer extends Number?

Question

I was wondering why is it not possible to call List not with List> even Integer is an extended class of the abstract class

Answer 1

Because you could e.g. add an instance of a different subclass of Number to List<Number>, e.g., an object of type Double, but obviously you shouldn't be allowed to add them to List<Integer>:

public void myMethod(List<Number> list) {
    list.add(new Double(5.5));
}

If you were allowed to call myMethod with an instance of List<Integer> this would result in a type clash when add is called.

Answer 2

Generics are not co-varient like arrays. This is not allowed because of type erasure.

Consider classes Vehicle, Bike and Car.

If you make

public void addVehicle(List<Vehicle> vehicles){
    vehicles.add(new Car());
}

• This is possible, because a Car is of type Vehicle, you can add a Car into Vehicle because it passes IS-A test.
• But what if it was allowed to pass a List<Bike> to addVehicle(List<Vehicle> vehicles) ?

you would have added a Car to bike list. which is plain wrong. So generics doesn't allow this.

Read about Polymorphism with generics