Stream.collect方法中的组合器如何在java 8中工作？ [英] How does combiner in Stream.collect method work in java 8?

问题描述

我创建了简单的演示：

public static void main(String[] args) {
        List<String> list2 = Arrays.asList("adf", "bcd", "abc", "hgr", "jyt", "edr", "biu");

String collect = list2.stream().collect(String::new, (res, elem) -> {
              res=res.concat(" ").concat(elem);
//            System.out.printf("res=%s, elem=%s\n", res.isEmpty(), elem);

        }, (res1, res2) -> {
            System.out.printf("res1=%s, res2=%s\n", res1, res2);            
        });
        System.out.println("collect=" + collect);
}

问题在于 BiConsumer组合器 collect 的一部分根本不运行。

The problem is that BiConsumer combiner part of collect doesn't run at all.

如果我使用<$ c $它会运行c> parallelStream（）但是两个参数 res1 和 res2 等于supplier String :: new 。

It runs if I use parallelStream() but two arguments res1 and res2 are equal to supplier String::new.

如何使组合器工作收集方法？

推荐答案

首先，没有必要组合器在非并行流中执行，因为没有什么可以组合。

First off, there is no need for the combiner to be executed in a non-parallel stream since there is nothing to combine.

其次，你的问题源于使用 String :: new 和 String.concat 。累加器应该通过将第二个参数与它组合来修改第一个参数，但由于Java中的字符串是不可变的，因此代码将不会产生任何内容。

Secondly, your issue stems from using String::new and String.concat. The accumulator is supposed to modify the first argument by combining the second argument with it but since strings in Java are immutable your code will produce nothing.

          res=res.concat(" ").concat(elem);

将创建一个新字符串然后扔掉它。您希望使用StringBuilder，以便保留中间结果：

will create a new string and then throw away it. You want to use a StringBuilder instead so you can keep the intermediate results:

public static void main(String[] args) {
    List<String> list2 = Arrays.asList("adf", "bcd", "abc", "hgr", "jyt", "edr", "biu");

    String collect = list2.stream().collect(StringBuilder::new, (res, elem) -> {
        res.append(" ").append(elem);
    }, (res1, res2) -> {
        res1.append(res2.toString());
        System.out.printf("res1=%s, res2=%s\n", res1, res2);
    }).toString();
    System.out.println("collect=" + collect);
}

这对于并行流也能正常工作

This will also work correctly with a parallel stream

res1 = hgr jyt，res2 = jyt

res1 = bcd abc，res2 = abc

res1 = adf bcd abc ，res2 = bcd abc

res1 = edr biu，res2 = biu

res1 = hgr jyt edr biu，res2 = edr biu

res1 = adf bcd abc hgr jyt edr biu，res2 = hgr jyt edr biu

collect = adf bcd abc hgr jyt edr biu

res1= hgr jyt, res2= jyt
res1= bcd abc, res2= abc
res1= adf bcd abc, res2= bcd abc
res1= edr biu, res2= biu
res1= hgr jyt edr biu, res2= edr biu
res1= adf bcd abc hgr jyt edr biu, res2= hgr jyt edr biu
collect= adf bcd abc hgr jyt edr biu

这篇关于Stream.collect方法中的组合器如何在java 8中工作？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！