Re: Generics error messages

Roedy Green <look-on@xxxxxxxxxxxxxxxxxxxx> wrote:
> Is there distinction between <E> and <? extends E>?
>

Yes, but it's kind of subtle.

Let's say I've got a class Automobile and two classes Car and Truck that
both derive from Automobile. A Collection<Automobile> can contain
instances of Automobile, or Car, or Truck. I can, for example, add one
of each and then read them back out into references of type Automobile,
and this is all safe to do. A Collection<Car>, on the other hand, may
only contain Cars. It is an error to try to add a Truck to such a
collection. Note that the two generic classes Collection<Automobile>
and Collection<Car> are NOT related by inheritance, even though their
type parameters are related.

Note that up to this point, we've only been talking about the (pre-
erasure) runtime class of various collection objects. The references
used to refer to those collection objects can have a range of other
types. A reference of type Collection<Automobile> is somewhat limited.
It can ONLY refer to a Collection<Automobile>, and can NOT refer to a
Collection<Object> or a Collection<Car>. (Of course, an instance of
Collection<Automobile> can contain objects of class Car, but that's not
the same as having an object of class Collection<Car>.)

There are also wildcard types. A Collection<? extends Automobile> is a
reference type that can refer to any of a Collection<Automobile>,
Collection<Car>, or Collection<Truck>. Even though the three classes
are not related to each other by inheritance, they are all assignment
compatible with that wildcard type.

So what's the practical difference? Well, a reference of type
Collection<? extends Automobile> MIGHT be referring to an object of
class Collection<Truck>, so you can't safely add a Car to it. It also
might be referring to an object of Collection<Car>, so you can't add a
Truck to it. In fact, it turns out you can never add anything to a
collection of a lower-bound wildcard type, because you can never
guarantee that the object you're adding is compatible with the object's
actual type parameter. This is quite different from the simpler
Collection<Automobile>, where you know that the type parameter is
Automobile and therefore it's definitely safe to add a Car or Truck
object.

(Note that this can sometimes be counter-intuitive. Many people
initially expect that you would be able to add a Car object to a
Collection<? extends Automobile> but would not be able to add it to a
Collection<Automobile>. That's an unfortunate confusion of words, and
it's almost exactly backwards from the truth.)

> If the underlying arrayList has a method addAll( Collection ) types
> aside, why would not my method override it.?

Because someone might pass a Collection<Car> to addAll, and then your
method would not be appropriate. Essentially, you're writing a more
specific method than the superclass; one that is pickier about its
parameters. You can't do that without an overload, and you can't
overload without changing the type erasure.

--
www.designacourse.com
The Easiest Way To Train Anyone... Anywhere.

Chris Smith - Lead Software Developer/Technical Trainer
MindIQ Corporation
.

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