Use of super in type parameters

[Martin Buchholz]

With the coming of lambda, it is more likely that people will be creating
APIs with "not quite correct" generic types, as we are doing in
CompletableFuture.  Which is pretty bad for a feature that is designed
specifically to provide compile time safety.  It would be nice to at least
have an acknowledgment that there is a problem here, even if it will not be
fixed for this release.


On Mon, Apr 15, 2013 at 12:50 PM, Zhong Yu <zhong.j.yu at gmail.com> wrote:

> I have encountered on stackoverflow.com several legit use cases of
> lower bound. And the other day Ali Lahijani raised the question that
> Stream.reduce(BinaryOperator) breaks convariance of Stream, and I
> thought that the root problem is lack of lower bound - the method
> would have had a better signature
>     Stream<T>
>         <U super T> Optional<U> reduce(BinaryOperator<U> accumulator)
> So I don't think there's a lack of use cases for lower bound. (But I
> have no idea how difficult it is to support it)
>
> Zhong Yu
>
>
> On Mon, Apr 15, 2013 at 2:37 PM, Martin Buchholz <martinrb at google.com>
> wrote:
> > CompletableFuture currently has a method like this:
> >
> >     public CompletableFuture<Void> acceptEither
> >         (CompletableFuture<? extends T> other,
> >          Consumer<? super T> block) {
> >         return doAcceptEither(other, block, null);
> >     }
> >
> > But that signature is not quite correct (not as general as it could be).
> >  The "correct" signature is
> >
> >     public <U super T> CompletableFuture<Void> acceptEither
> >         (CompletableFuture<? extends U> other,
> >          Consumer<U> block) {
> >         return doAcceptEither(other, block, null);
> >     }
> >
> > but that fails to compile, because type parameters can only be
> constrained
> > by extends, not super.  Is implementing this on the radar?  Angelika
> claims
> > "lower bounds for type parameters make no sense"
> >
> http://www.angelikalanger.com/GenericsFAQ/FAQSections/TypeParameters.html#Why%20is%20there%20no%20lower%20bound%20for%20type%20parameters
> > ?
> >
> > but I am finding that hard to believe.  Is she right? For comparison,
>  the
> > equivalent static method can be made to do what we want:
> >
> >     public static <U> CompletableFuture<Void> acceptEither
> >         (CompletableFuture<? extends U> f,
> >          CompletableFuture<? extends U> other,
> >          Consumer<U> block) {
> >         return ...
> >     }
>