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Answer 1

A:

No - you cannot cast directly from List<A> to List since both are different types - your only option is to iterate through List<A> and cast every object of type A into a object of type B and add it to List

Gambrinus 2009-10-30 16:56:46

That's simply wrong.

jarnbjo 2009-10-30 17:01:38

Answer 2

A:

Unfortunately, there's no magic way. Type erasure is sometimes a pity :(

Sinuhe 2009-10-30 16:57:20

Eh? If Java didn't have type erasure you wouldn't be able to do it. With erasure you can (but it is morally wrong).

Tom Hawtin - tackline 2009-10-30 18:33:39

That's true! My mistake.

Sinuhe 2009-10-30 22:47:09

Answer 3

A:

Sadly, no. Even though B is an A, List is not a List<A>.

So your iteration is the answer.

CPerkins 2009-10-30 16:57:54

Answer 4

A:

List<A> is not a subtype of List!

The JLS even mentions that explicitly:

Subtyping does not extend through generic types: T <: U does not imply that C<T> <: C.

Joachim Sauer 2009-10-30 16:59:26

Answer 5

+8 A:

You can cast through the untyped List interface:

List<A> a = new ArrayList<A>();
List<B> b = (List)a;

jarnbjo 2009-10-30 16:59:36

And loose all type safety in the process! If `A` is a base class of `B`, then the list can contain `A`, while the code that handles `b` expects it to return only `B` objects. That's the very reason why that case is not allowed in the first place.

Joachim Sauer 2009-10-30 17:00:52

@Joachim, I think that is the point of the original question. The only disadvantage of this approach over the traditional cast is that if you are wrong, the bug won't appear until much later and be harder to find, whereas an iteration would reveal your mistake.

Yishai 2009-10-30 17:03:04

@Joachim - you **are** losing type-safety; the compiler cannot guarantee that the List<A> really is a List which is why it won't let you do the cast in the first place. If you assert it yourself, that's fine, but this is *not* statically type-safe.

Andrzej Doyle 2009-10-30 17:05:02

"I happen to know only contains instances of B". So long as he is only using the resulting list to retrieve elements and not to insert them (which could trigger a runtime check for a `checkedList`, for example), he should be fine.

Pavel Minaev 2009-10-30 17:11:53

Pavel has it. The conditions of the question was that he was already convinced it only had instances of B. Without that condition, the question doesn't even make sense.

M1EK 2009-10-30 17:34:14

Answer 6

A:

You can try this :

List<A> a = new ArrayList<A>();
List<B> b = (List<B>) (List<?>) a;

It is based on the answer of jarnbjo, but on don't use raw lists.

Romain 2010-03-30 16:18:00

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