Comparator

[Marc Petit-Huguenin]

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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.

> 
> 
>> 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
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