为什么 Comparator.comparing 不适用于 String::toLowerCase 方法参考? [英] Why Comparator.comparing doesn't work with String::toLowerCase method reference?

问题描述

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 it.

Here is my attempt :

package experimentations.chapter02;

import java.util.Arrays;
import java.util.Comparator;
import java.util.stream.Collectors;

public class StringStream {

    public static void main(String[] args) {
        sortStrings();
    }

    public static void sortStrings(){
        String[] stringsArray = "The quick brown fox has a dirty ladder".split("\s+");
        System.out.println(
                Arrays.stream(stringsArray)
                .sorted(Comparator.comparing(String::toLowerCase).reversed())
                .collect(Collectors.toList())
        );
    }

}

The problem here is that String::toLowerCase is not accepted in the static method Comparator.comparing.

Meanwhile, I managed to sort the array, but modifying it:

public static void sortStrings(){
        String[] stringsArray = "The quick brown fox has a dirty ladder".split("\s+");
        System.out.println(
                Arrays.stream(stringsArray)
                .map(String::toLowerCase)
                .sorted(Comparator.reverseOrder())
                .collect(Collectors.toList())
        );
}

So, what is the simpliest workaround?

解决方案

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()));

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