Comparator

[Marc Petit-Huguenin]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256

On 07/15/2013 09:18 AM, Maurizio Cimadamore wrote:
> On 15/07/13 17:15, Marc Petit-Huguenin wrote: On 07/15/2013 09:09 AM,
> Maurizio Cimadamore wrote:
>>>> On 15/07/13 16:53, Marc Petit-Huguenin wrote: Hi,
>>>> 
>>>> I am trying to use a comparator on a Map.Entry<String, Integer>
>>>> stream, and I am not sure why this does not compile:
>>>> 
>>>> .sorted(Comparator.comparing(Map.Entry::getValue))
>>>>> This is as expected - the method reference Map.Entry is defined on
>>>>> a generic class with unknown parameter types (some Map<K, V>) -
>>>>> therefore the compiler needs the info from the target (in a
>>>>> top-down way) in order to decide what the method reference looks
>>>>> like. But the target type info is incomplete as it depends on
>>>>> inference variables on its own. So the compiler ends up applying a
>>>>> simpler (arity-based only) check during overload resolution - which
>>>>> means the compiler can't really pick the right comparing() method
>>>>> ahead of the target-type. A shorter way to get this to compile
>>>>> should be this: .sorted(Comparator.comparing(Map.Entry<String,
>>>>> Integer>::getValue))
> I forgot to tell that I tried this too.  It fails with a "error: reference
> to comparing is ambiguous", but not with the new "inference loop" error.
> 
> Note that ".sorted(Comparator.comparing((Map.Entry<String, Integer> entry)
> -> entry.getValue()))" (i.e. without the .intValue()) fails with a similar
> error.
>> Are you using the official JDK 8 or the experimental lambda-repo
>> compiler? Only the latter contains a fix for this.

I built it yesterday from the lambda repository:

 .:5faccb5b660e corba:08e436473edb hotspot:7b772f9e0365 jaxp:ae467ce203a8
jaxws:f2e56f100ecf jdk:00b08a855e2e langtools:f75d9c517541 nashorn:dd371db92a91

> 
>> Maurizio
> 
>>>> 
>>>>> Let me know if that works for you. Maurizio
>>>> error: reference to comparing is ambiguous error: incompatible
>>>> types: Cannot instantiate inference variables T because of an
>>>> inference loop
>>>> 
>>>> (interestingly the second error was not present when using a build
>>>> from last week)
>>>> 
>>>> These two compile:
>>>> 
>>>> .sorted(Comparator.comparing((ToIntFunction<Map.Entry<String, 
>>>> Integer>>)Map.Entry::getValue))
>>>> 
>>>> .sorted(Comparator.comparing((Map.Entry<String, Integer> entry) -> 
>>>> entry.getValue().intValue()))
>>>> 
> -- Marc Petit-Huguenin Email: marc at petit-huguenin.org Blog:
> http://blog.marc.petit-huguenin.org Profile:
> http://www.linkedin.com/in/petithug
> 

- -- 
Marc Petit-Huguenin
Email: marc at petit-huguenin.org
Blog: http://blog.marc.petit-huguenin.org
Profile: http://www.linkedin.com/in/petithug
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.12 (GNU/Linux)
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=iQ8u
-----END PGP SIGNATURE-----