2006/8/29, Spark Shen <[EMAIL PROTECTED]>:
Alexey Varlamov 写道:
> 2006/8/29, Spark Shen <[EMAIL PROTECTED]>:
>> Krzysztof Sobolewski 写道:
>> > Spark Shen wrote:
>> >
>> >
>> >> Did you (refer to thread [app] ant with ecj) put
>> >>
>> >> Eclipse compiler JAR on Ant's
>> >> execution classpath to execute the javac task.
>> >>
>> >> If so, may be that's why It always gives you "unchecked cast"
>> warning,
>> >> no errors.
>> >>
>> >
>> > Well, I created a new EnumSet class in completly unrelated
>> environment... So
>> > it is perfectly possible that there's a classpath conflict
>> somewhere. I
>> > can't verify that because don't have the Harmony build here :)
>> >
>> > BTW: another experiment revealed that
>> > EnumSet<? extends Enum<E>> set = (EnumSet<? extends
>> Enum<E>>)collection;
>> > compiles without warnings. No matter how much I think I understand
>> generics,
>> > there's always something that surprises me ;)
>> >
>> Conceptually speaking, I think EnumSet<? extends Enum<E>> still has
>> potential pitfall:
>>
>> In the signature of addAll(Collection<? extends E> collection) method, ?
>> denotes subclass of E.
>> While in EnumSet<? extends Enum<E>>, ? denotes subclass of Enum<E>. But
>> consider the relationship
>> between E and Enum<E>, E is subclass of Enum<E>. So, we can not
>> guarantee that subclass of E is definitely
>> subclass of Enum<E>.
>
> Hmm, as E is a subclass of Enum<E>, subclasses of E definitely have
> Enum<E> among ancestors ;)
Yes, you are right. And I made a wrong judgement here. :-P
The actual statement I want to judge is that:
E is subclass of Enum<E>. So, we can not guarantee that subclasses of
E(? extends E) is definitely
compatible with subclasses of Enum<E>(? extends Enum<E>).
Since the input collection is of type <? extends E>, cast it to <?
extends Enum<E>> seems suspicious.
Nothing suspicious actually - the cast just lifts upper bound up.
Taking into account cyclic declaration of E, the bound in fact remains
the same. So I believe this is correct way in this particular case.
--
Alexey
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