Optional.or() doesn't use a wildcard in its signature

[Stefan Zobel]

2015-11-01 15:41 GMT+01:00 Vitaly Davidovich <vitalyd at gmail.com>:
> One could argue that this is perfectly sound to do given Optional is
> readonly, we just can't express this variance cleanly in java (i.e. without
> upcast).
>

Sure, as in Scala's Option[+A].
I should have said "... from a **Java** type perspective ".

Regards,
Stefan


>
> On Nov 1, 2015 7:31 AM, "Stefan Zobel" <spliterator at gmail.com> wrote:
>>
>> 2015-11-01 1:12 GMT+01:00 Michael Nascimento <misterm at gmail.com>:
>> > Hi Vitaly,
>> >
>> > Exactly, I was just trying to point out the method signature seems
>> > broken
>> > anyway.
>> >
>> > Regards,
>> > Michael
>>
>>
>> Hi Michael,
>>
>> I don't think the signature is broken.
>>
>> An Optional<StringBuilder> is not a subtype of Optional<CharSequence>.
>> So even if the signature were
>>
>> public <R extends T> Optional<T> or(Supplier<Optional<R>> supplier)
>>
>> we'd have to upcast the result of supplier.get() to Optional<T>
>> which appears to be wrong from a type perspective.
>>
>>
>> Regards,
>> Stefan
>>
>>
>>
>> >
>> > On 31 Oct 2015 11:59, "Vitaly Davidovich" <vitalyd at gmail.com> wrote:
>> >>
>> >> This would require Supplier<Optional<? extends T>>, not Supplier<?
>> >> extends
>> >> Optional<T>>.
>> >>
>> >> sent from my phone
>> >>
>> >> On Oct 31, 2015 2:49 PM, "Michael Nascimento" <misterm at gmail.com>
>> >> wrote:
>> >>>
>> >>> If this instance is an Optional<CharSequence> ,  passing an
>> >>> Optional<StringBuilder> will fail to compile.
>> >>>
>> >>> Regards,
>> >>> Michael
>> >>> On 31 Oct 2015 11:21, "Stefan Zobel" <spliterator at gmail.com> wrote:
>> >>>
>> >>> > 2015-10-31 19:11 GMT+01:00 Remi Forax <forax at univ-mlv.fr>:
>> >>> >
>> >>> > > Hi all, hi Paul,
>> >>> > >
>> >>> > > I've just seen that Optional.or is declared as
>> >>> > >   public Optional<T> or(Supplier<Optional<T>> supplier) {
>> >>> > > instead of
>> >>> > >   public Optional<T> or(Supplier<? extends Optional<T>> supplier)
>> >>> > > {
>> >>> > >
>> >>> > > regards,
>> >>> > > Rémi
>> >>> > >
>> >>> >
>> >>> >
>> >>> > I don't get it. Optional is final anyway. Can you explain?
>> >>> >
>> >>> > Thanks,
>> >>> > Stefan