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

问题描述

我有一个列表，并希望将其减少为单个值（函数式编程术语fold，Ruby术语 inject ），如

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

当我感染了函数式编程思想（Scala）时，我正在寻找一种更简单的方法来编写它

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




解决方案

寻找是一个字符串 join（）方法，Java自8.0以来。请尝试以下方法之一。

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

    集合< String& source = Arrays.asList（a，b，c）; 
    String result = String.join（，，source）; 
     




2. Stream 接口支持与Scala的 foldLeft 函数非常相似的折叠操作。请查看以下连结收集器：

    集合< 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

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





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"

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

解决方案

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

1. Static method String#join(delimiter, elements):

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

2. 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(","));
   

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

   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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