Casting between typed Lists?

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jagonzal@gmail.com - 18 Aug 2006 22:14 GMT

Hi,

I have an Interface B, which extends Interface A.

I have a method whose signature is:

public void doSomething(List<A> list){
...
}

However, when I try to invoke it like this:

List list2 = new ArrayList();
doSomething(list2);

the compiler complains that it can't take a List instead of a
List<A>. I say that, theoretically, it should work, since a B element
is also a A element due to interface inheritance. Casting a B variable
to A does not produce a ClassCastException. However, the compiler says
that it doesn't.

Is there any way to go around this (Other than working with non-typed
lists)? (casting the list? I tried that but it doesn't work :)

TIA,

JGN.

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Oliver Wong - 18 Aug 2006 22:33 GMT

> Hi,
>
[quoted text clipped - 19 lines]
> Is there any way to go around this (Other than working with non-typed
> lists)? (casting the list? I tried that but it doesn't work :)

Instead of "A" and "B", let's call them "Vehicule" and "Car". Car
extends Vehicule.

The doSomething(List<Vehicule> list) method does something to a list of
vehicules. One of the things it might do is add more vehicules to the list.
For example, it might add some boats to the list.

If you pass in a List<Car> instead of a List<Vehicule>, you'll get an
problems, because the doSomething method will try to add a Boat to a
List<Car>, and obvious a Boat is not a Car.

Solution A: change the method signature from doSomething(List<A> list)
to doSomething(List<? extends A> list).

Solution B: change the object declaration from List list2 = new
ArrayList() to List<A> list2 = new ArrayList<A>().

Which solution is better depends entirely on what it is your program is
supposed to do.

- Oliver

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jagonzal@gmail.com - 18 Aug 2006 22:47 GMT

> The doSomething(List<Vehicule> list) method does something to a list of
> vehicules. One of the things it might do is add more vehicules to the list.
[quoted text clipped - 3 lines]
> problems, because the doSomething method will try to add a Boat to a
> List<Car>, and obvious a Boat is not a Car.

Ah, I had not considered this scenario - thank you :)

> Solution A: change the method signature from doSomething(List<A> list)
> to doSomething(List<? extends A> list).

I think I'll go with this - I'm getting the list from another object,
and I can't change that return type.

> Which solution is better depends entirely on what it is your program is
> supposed to do.
>
> - Oliver

Thank you very much :)

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Casting between typed Lists?

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