supposed we know that ViewGroup extends View.
Further we have a generic, parametrized class A<T extends View>
Question:
Why wont method C.add() accept new A<ViewGroup>() as parameter?
Shouldn't it work, because of polymorphism?
SOLUTION:
Singning add with ? extends View lets add accept new A<ViewGroup>() as a parameter.
You signed your add method as:
static void add(A<View>)
but you probably meant:
static void add(A<? extends View> a)
new A<ViewGroup>() is invalid.
new ArrayList<ViewGroup>().
add(A<? extends View> a) lets add() accept new A<ViewGroup>() thnx. Sorry for my initial typo.Not quite because add() might be using a method of View that is not found in ViewGroup.
Summary: View is a ViewGroup, however ViewGroup is not a View. Polymorphism is where you can assign a View object to a ViewGroup declaration.
First of all, your question isn't very clear because the UML diagram contradicts your text. You say that View extends ViewGroup, but the diagram shows the reverse : ViewGroup extends View.
Now, a List<Car> doesn't extend a List<Vehicle>. If it were the case, you could do:
List<Car> listOfCars = new ArrayList<Car>();
List<Vehicle> listOfVehicles = listOfCars;
listOfVehicles.add(new Bicycle());
// now the list of cars contain a bicycle. Not pretty.
ViewGroup extends View here.First of all you said View extends ViewGroup, but the diagram says ViewGroup extends View (which I assume to be right).
Secondly, you are not allowed to pass a List<ViewGroup> as a List<View>. This is a compile time protection to prevent someone from adding an AnotherView into this list and compromise type-safety of generics.
List<ViewGroup> is not a subtype of List<View>, but it is a subtype of List<? extends View>. So you can modify your method to accept a List<? extends View> instead, but be aware that you can't add to the list passed to the method this way.
There is also another syntax called lower bound wildcard (List<? super ViewGroup>), as opposed to the upper bound wildcard mentioned above, which enables you to add to the list but you can olny pass in a list of ViewGroup or its parents.
More about wildcards in generics can be found here: http://download.oracle.com/javase/tutorial/java/generics/wildcards.html
