Composing comparators: method reference works, the lambda equivalent does not.

[Paulo Silveira]

While trying to compose comparators, I am having strange problems
using lambdas instead of method references.

This one compiles (I know the resulting comparator does not make sense):

Comparator<String> c = Comparator.comparing(String::toString)
                            .thenComparing(String::length);

This one does not compile:

Comparator<String> c = Comparator.comparing(s -> s.toString())
                              .thenComparing(s -> s.length());

(compilation error here: https://gist.github.com/peas/8267900)

It seems the compiler is expecting a Function<Object, String> instead
of Function<String, String>. The latest Goetz' state of lambda says
this should work, but it gives me the same compilation error:

Comparator<Person> c = Comparator.comparing(p -> p.getLastName())
                                 .thenComparing(p -> p.getFirstName());

My build is 1.8.0-ea-b121

Another quick question: when would I need to use
Comparator.comparingInt if Comparator.compare works just fine to
create a Comparator<Integer>? The only difference I can see is to
avoid NPEs through Integer.compare(...).

Regards
--
Paulo Silveira
www.caelum.com.br
www.casadocodigo.com.br
www.alura.com.br