Re: [jboss-dev] Does this seem like a compiler bug?
The typical example of why List<?> is not a List<Object> is:
List<String> ls = ...;
List<Object> lo = ls;
lo.add(new Object());
String s = ls.get(0);
I guess the equivalent Class, Class<?> example is:
List<Class<String>> ls = ...;
List<Class> lc = ls;
lc.add(Object.class);
Class<String> cs = ls.get(0);
String s = cs.newInstance(); // CCE here
From that perspective I can see that Class<?> is expressing a
stricter
collection definition. However, for that case our usage of
Collection<Class<?>> is incorrect as the Collection is not bound to a
specific type, although unknown, Class.
Scott M Stark wrote:
By validating whether the parameter is used in any assignment
expressions in the class. I don't see how the Collection<Class> to a
Collection<Object> comparison is the same. This has a completely
different erasure for the collection element, where as Class<?> and
Class have the same erasure.
David M. Lloyd wrote:
> How would the compiler know that the parameter may be used for mutable
> operations? There's no way it could differentiate between the two
> cases.
>
> The "?" is treated differently from an unqualified type because an
> unqualified List can allow you to add any Object type, whereas "?"
> means there is *some* specific type required but you don't know what it
> is; therefore it is unsafe to make any assumptions regarding what can
> be accepted. This fails to compile for the same reason that casting
> from a Collection<Class> to a Collection<Object> fails. The compiler
> can't tell how you intend to make use of the parameter.
>
> I think this is an Eclipse bug, not a JDK bug.
>
> - DML
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