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

Alex Buckley Alex.Buckley at Sun.COM
Mon Apr 21 12:35:46 PDT 2008


Please don't bring JLS changes to compiler-dev. OpenJDK is about the 
Java platform's implementation, not its spec. As the spec lead for the 
Java language and VM, I do not subscribe to compiler-dev and only 
occasionally review its archives.

You can file spec defects and RFEs at with the Submit A Bug 

   Java Platform Standard Edition (JDK/JRE)
   Java Language Spec. and Java Virtual Machine Spec. (specification)
Operating System:
   // Whatever you like

See also bug 6480391.


Rémi Forax wrote:
> Hi all,
> the JLS currently considers that List<? extends Object> is not 
> equivalent to List<?>.
> List<?> is reified but not List<? extends Object> so they behave 
> differently.
> 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<?>.
> cheers,
> Rémi

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