Comparator

[Maurizio Cimadamore]

On 15/07/13 16:53, Marc Petit-Huguenin wrote:
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> 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))


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()))
>
> Thanks.
>
> - -- 
> 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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