Java 8流中的arg max？ [英] arg max in Java 8 streams?

问题描述

根据产生double或int值的标准的最大化，我经常需要集合的最大元素。 Streams有max（）函数，需要我实现一个比较器，我觉得很麻烦。是否有更简洁的语法，例如 names.stream（）。argmax（String :: length）在以下示例中？

I often need the maximum element of a collection according to the maximization of a criterion which produces a double or int value. Streams have the max() function which requires me to implement a comparator, which I find cumbersome. Is there a more concise syntax, such as names.stream().argmax(String::length) in the following example?

import java.util.Arrays;
import java.util.Comparator;
import java.util.List;

public class ArgMax
{
    public static void main(String[] args)
    {
        List<String> names = Arrays.asList("John","Joe","Marilyn");
        String longestName = names.stream().max((String s,String t)->(Integer.compare(s.length(),t.length()))).get();
        System.out.println(longestName);
    }
}

推荐答案

使用

String longestName = names.stream().max(Comparator.comparing(String::length)).get();

来比较某些属性上的元素（可能比这更复杂，但不必） 。

to compare elements on some property (can be more complex than that, but doesn't have to).

正如Brian在评论中建议的那样，使用可选＃get（）这样做是不安全的，如果有可能流为空。您更适合使用其中一种更安全的检索方法，例如 可选＃orElse（对象） 如果没有最大值，它将为您提供一些默认值。

As Brian suggests in the comments, using Optional#get() like this is unsafe if there's a possibility that the Stream is empty. You'd be better suited to use one of the safer retrieval methods, such as Optional#orElse(Object) which will give you some default value if there is no max.

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