使用流将对象列表转换为从 toString 方法获得的字符串 [英] Using streams to convert a list of objects into a string obtained from the toString method

问题描述

Java 8 中有很多有用的新东西.例如，我可以用流迭代对象列表，然后对来自 Object 实例的特定字段的值求和.例如

There are a lot of useful new things in Java 8. E.g., I can iterate with a stream over a list of objects and then sum the values from a specific field of the Object's instances. E.g.

public class AClass {
  private int value;
  public int getValue() { return value; }
}

Integer sum = list.stream().mapToInt(AClass::getValue).sum();

因此，我在问是否有任何方法可以构建一个 String，将 toString() 方法的输出从实例连接到一行中.

Thus, I'm asking if there is any way to build a String that concatenates the output of the toString() method from the instances in a single line.

List<Integer> list = ...

String concatenated = list.stream().... //concatenate here with toString() method from java.lang.Integer class

假设 list 包含整数 1、2 和 3，我希望 concatenated 是 "123" 或 "1,2,3".

Suppose that list contains integers 1, 2 and 3, I expect that concatenated is "123" or "1,2,3".

推荐答案

一种简单的方法是将列表项附加到 StringBuilder

One simple way is to append your list items in a StringBuilder

List<Integer> list = new ArrayList<>();
list.add(1);
list.add(2);
list.add(3);

StringBuilder b = new StringBuilder();
list.forEach(b::append);

System.out.println(b);

你也可以试试:

String s = list.stream().map(e -> e.toString()).reduce("", String::concat);

说明:map 将整数流转换为字符串流，然后将其缩减为所有元素的串联.

Explanation: map converts Integer stream to String stream, then its reduced as concatenation of all the elements.

注意:这是在 O(n²)

Note: This is normal reduction which performs in O(n²)

为了获得更好的性能，请使用类似于 F. Böller 的答案的 StringBuilder 或 mutable reduction.

for better performance use a StringBuilder or mutable reduction similar to F. Böller's answer.

String s = list.stream().map(Object::toString).collect(Collectors.joining(","));

参考:流减少

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