Comparator

[Maurizio Cimadamore]

On 15/07/13 17:28, Marc Petit-Huguenin wrote:
> -----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

Ok - I'll dig it more and come back to you

Thanks
Maurizio


>
>>> 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)
>
> iQIcBAEBCAAGBQJR5CMqAAoJECnERZXWan7EQs8P/2Sh+duiYTm0hEbofYXocAWn
> ltYRk9ClO2lMEqJ0+WbT7qxqdbahORowkUH53UlNvMq3PP94M/AodKb4ap1B2O0M
> I1sYBc459hp8t4uw+FU/Cb1w3uy0ZR6P2g7c064y3kFXO6Caxl6mLmmwuOm68kwv
> ZtZM8pRQkdPDw3VAVNshzZqhm3ZiMMKGY2Q8l9SbLsKy2U+wxo3wluFP6jl+cXr5
> mJ8wx07fT4uJ/xmiO2pdCMONWOW7UC6tIt4NGGgOjt6K04rSsUeG21mjAfSS2cuW
> CLn/7BKBBJp8S7hRf8ieVsiYLYtTOYL2SxwiEG9pFa/Ftk4AxyBZlIjnE9+vOF3D
> 4CxUmEW/hRQZeoqcQ19Pi4AKl1OMbRE4O/yDxFmqgc/7bQEhItHtOfsFQbFWT8P1
> fDvigZiSUzu+cTu9U+QlRlQx7xkR4sOkb91qflIxs/W5EDMKtyCT0wgk4Ew0xXch
> 7ZXT/phohxUZTu40vB8TCIVq+PQjMSX6g79z1b5EXoxDH7yktxQ43HuLaQa5OZD/
> 83x4bMhqlIWMEP248pQIoBf93E3dHyW5k1080lsbT5jro9/mHsncf6E4hnA2SBWO
> /Xy6hzV7aLtGRQlvoWATTWAaQjXHroW9h1MfxBTWxE5sm+uL10chjaF/gEbC8Blz
> vrEGFEpJIDhnxdAEl717
> =iQ8u
> -----END PGP SIGNATURE-----