Comparators.comparing overloads

Zhong Yu zhong.j.yu at
Sun Jul 21 14:25:47 PDT 2013

See another case where auto-unboxing causes the same problem

On Sun, Jul 21, 2013 at 4:16 PM, Richard Warburton
<richard.warburton at> wrote:
> Hi,
> We only mangle the name for return types, not argument types.
> Thanks for the prompt reply.
> Things still seem a little messy in this situation though.  Perhaps I've
> missed something but we're talking about needing a cast for
> any keyExtractor function that returns a number.  That seems to be a fairly
> common scenario for a keyExtractor.  Suppose I want to sort Strings by
> length, and I want to write:
> Comparator<String> comparator = comparing(String::length);
> I'll get an error telling me that the "reference to comparing is
> ambiguous".  Now in fact I can't even hint to the compiler using a return
> type cast as with the following snippet:
> Comparator<String> comparator = comparing(str -> (int) str.length());
> I presume this is because a cast to int can auto-box to Integer which
> subtypes Object, so its still ambiguous between Function and ToIntFunction.
>  Again, perhaps I've missed something here, but the way to get around this
> issue is by providing an explicit type for the keyExtractor function:
> ToIntFunction<String> lengthOfString = String::length;
> Comparator<String> comparator = comparing(lengthOfString);
> Which really does seem less elegant to me than having overloads with
> mangled names. As I say this isn't a weird corner case - numbers are an
> obvious thing to use as a comparator key.
> regards,
>   Dr. Richard Warburton
>   @RichardWarburto <>

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