Comparator

[Marc Petit-Huguenin]

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On 07/15/2013 10:11 AM, Maurizio Cimadamore wrote:
> On 15/07/13 18:06, Marc Petit-Huguenin wrote: On 07/15/2013 09:16 AM, Henry
> Jen wrote:
>>>> Or use the new static method,
>>>> 
>>>> .sorted(Map.Entry<String, Integer>.comparingByValue())
> Nice!  But it did not compile as it:
> 
> .sorted(Map.Entry.comparingByValue())
>> Does the above works? I think Henry pasted wrong snippet, as you can't
>> really pass type-argument to a receiver in a static method call.

Yes, "Map.Entry.comparingByValue()" works, but
"Map.Entry.comparingByValue().reverse()" does not, so I changed it to
".sorted(Map.Entry.<String, Integer>comparingByValue().reversed())"

> 
>> Maurizio
> 
> worked, but I needed to reverse the order, so I had to use
> 
> .sorted(Map.Entry.<String, Integer>comparingByValue().reversed())
> 
> I am a little bit worried that finding the right syntax relies on try and
> fail.
> 
>>>> Cheers, Henry
>>>> 
>>>> On Jul 15, 2013, at 9:09 AM, Maurizio Cimadamore 
>>>> <maurizio.cimadamore at oracle.com> 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))
>>>>>> 
>>>>>> 
>>>>>> 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
> 

- -- 
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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