List<?> is not equivalent to List<? extends Object>

Tom Hawtin Thomas.Hawtin at Sun.COM
Mon Apr 21 07:34:54 PDT 2008

Rémi Forax wrote:
> It's not a big problem in general, if you want a List<?>, you declare a 
> List<?>
> But (there is always a but), when the compiler infers a type argument
> it's more problematic, by example,
> HashSet<Class<?>> constantClassSet=new HashSet<Class<?>>();
> Collections.addAll(constantClassSet,
>  String.class, Object.class
> );
> emits a warning because the second line creates an array of
> Class<? extends Object> which is not safe.
> I think it's stupid, i propose to modify section (JLS3) to
> add a line saying that if an inferred type argument is Type<? extends 
> Bound>
> with Bound the bound of the type varaible of Type, it is inferred Type<?>.

Would that be useful in general?

Consider if you had used String.class and Integer.class. Then the 
inferred component type would be Class<? extends 
Serializable&Comparable<? extends Serializable&Comparable<?>> (or 
something like that!). So your rule wouldn't come in to play.

Tom Hawtin

(Disclaimer: I am not a language lawyer.)

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