为什么Comparator.comparing不能与String :: toLowerCase方法引用一起使用？ [英] Why Comparator.comparing doesn't work with String::toLowerCase method reference?

问题描述

我试图按逆序（忽略大小写）排序一个字符串数组，而不修改它，只打印它。所以我使用的是Java8流。但我无法做到。

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.

这是我的尝试：

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())
        );
    }

}

这里的问题是<$静态方法 Comparator.comparing 中不接受c $ c> String :: toLowerCase 。

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

与此同时，我设法对数组进行排序，但修改：

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())
        );
}

那么，最简​​单的解决方法是什么？

So, what is the simpliest workaround?

推荐答案

问题是，Java无法推断出某些复杂表达式的泛型类型。第一个语句有效，而第二个语句导致编译时错误：

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();

有几种方法可以解决这个问题。以下是其中三个：

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

将中间比较器存储在变量中：

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

使用 String.CASE_INSENSITIVE_ORDER ：

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

添加显式类型参数：

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

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