如何在 Java 中实现列表折叠 [英] How to implement a list fold in Java

问题描述

我有一个 List 并且想将它减少到单个值(函数式编程术语折叠"，Ruby 术语 inject)，例如

I have a List and want to reduce it to a single value (functional programming term "fold", Ruby term inject), like

Arrays.asList("a", "b", "c") ... fold ... "a,b,c"

由于我被函数式编程思想 (Scala) 所感染，我正在寻找一种比代码更简单/更短的方法

As I am infected with functional programming ideas (Scala), I am looking for an easier/shorter way to code it than

sb = new StringBuilder
for ... {
  append ...
}
sb.toString

推荐答案

您正在寻找的是一个字符串 join() 方法，Java 从 8.0 开始就有.尝试以下方法之一.

What you are looking for is a string join() method which Java has since 8.0. Try one of the methods below.

1. 静态方法 String#join(delimiter, elements):

Collection<String> source = Arrays.asList("a", "b", "c");
String result = String.join(",", source);

流接口支持折叠操作，非常类似于 Scala 的 foldLeft 函数.看看下面的串联 Collector:

Stream interface supports a fold operation very similar to Scala’s foldLeft function. Take a look at the following concatenating Collector:

Collection<String> source = Arrays.asList("a", "b", "c");
String result = source.stream().collect(Collectors.joining(","));

您可能希望静态导入 Collectors.joining 以使您的代码更清晰.

You may want to statically import Collectors.joining to make your code clearer.

顺便说一下，这个收集器可以应用于任何特定对象的集合:

By the way this collector can be applied to collections of any particular objects:

Collection<Integer> numbers = Arrays.asList(1, 2, 3);
String result = numbers.stream()
        .map(Object::toString)
        .collect(Collectors.joining(","));

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