Why Comparator.comparing doesn't work with String::toLowerCase method reference?

Question

I am trying to sort an array of Strings by reverse order (ignoring case), without modifying it, and just printing it. So I am using Java8 stream. But I can\'t manage to do i

Answer 1

I found the solution :

 .sorted((String e) -> e.toLowerCase) 

I think the problem with the syntax

 .sorted(String::toLowerCase)

is that the compiler then expects to pass an Object to the instance method toLowerCase of String. So I need to make my own lambda method, without ignoring the type of the lambda parameter (String), otherwise the compiler still can't resolve it.

Answer 2

The problem is, that Java can not deduce the generic types for some complex expressions. The first statement works, whereas the second statement leads to a compile-time error:

Comparator<String> comparator = Comparator.comparing(String::toLowerCase);
Comparator<String> comparator = Comparator.comparing(String::toLowerCase).reversed();

There are several ways to solve the problem. Here are three of them:

Store the intermediate Comparator in a variable:

Comparator<String> comparator = Comparator.comparing(String::toLowerCase);
System.out.println(
            Arrays.stream(stringsArray)
            .sorted(comparator.reversed())
            .collect(Collectors.toList()));

Use String.CASE_INSENSITIVE_ORDER:

System.out.println(
            Arrays.stream(stringsArray)
            .sorted(String.CASE_INSENSITIVE_ORDER.reversed())
            .collect(Collectors.toList()));

Add explicit type parameters:

System.out.println(
            Arrays.stream(stringsArray)
            .sorted(Comparator.<String,String>comparing(String::toLowerCase).reversed())
            .collect(Collectors.toList()));