Comparator.reversed() type inference issue

[Attila Szegedi]

Hi folks,

I'm teaching Java lately, and while teaching streams I use that good old chestnut, the word count example. I'm baffled with some type inference woes, though… trying to specify a reverse comparator using Comparator.reversed() makes javac sad, e.g. this does not compile[1]:

       Map<String, Long> x = someMap();

       var rs1 = x.entrySet().stream().sorted(
           Map.Entry.comparingByValue().reversed()
       );

On the other hand, using Collections.reverseOrder does compile:

       var rs2 = x.entrySet().stream().sorted(
           Collections.reverseOrder(
               Map.Entry.comparingByValue()
           )
       );

Happens on both Java 11 and 14. It’s just baffling because based on type signatures, it seems reasonable the first form should work as well.

Thanks for any enlightenment!

Attila.

---
[1] -Xdiags:verbose on Java 14 says:
error: no suitable method found for sorted(Comparator<Entry<Object,V#1>>)
       var rs1 = x.entrySet().stream().sorted(

   method Stream.sorted() is not applicable
     (actual and formal argument lists differ in length)
   method Stream.sorted(Comparator<? super Entry<String,Long>>) is not applicable
     (argument mismatch; Comparator<Entry<Object,V#2>> cannot be converted to Comparator<? super Entry<String,Long>>)
 where V#1,V#2 are type-variables:
   V#1 extends Comparable<? super V#1>
   V#2 extends Comparable<? super V#2>