如何在 java 1.8 中的 reduce 方法中使用 StringBuilder？

Question

String s = "apples for you";
StringBuilder returnString = new StringBuilder("");
Arrays.stream(s.split(" "))
        .reduce(returnString, (acc, str) -> acc.append(str.charAt(0)));

期望输出每个单词的第一个字母，即afy。

但在 acc.append 出现错误，acc 被视为 String。

Answer 1

您对 reduce 的使用不正确。您打算调用的重载是带有 3 个参数的重载，它还应该为 StringBuilder:

采用二元运算符

StringBuilder returnString = Arrays.stream(s.split(" "))
        .reduce(new StringBuilder(""), 
                (acc, str) -> acc.append(str.charAt(0)), 
                (sb1, sb2) -> sb1.append(sb2));

如果您要在并行流上使用它，请执行可变缩减（使用 stream.collect）作为初始标识字符串生成器对象，否则可能会从多个线程不可预测地附加到：

StringBuilder returnString = Arrays.stream(s.split(" "))
        .collect(StringBuilder::new, 
                 (acc, str) -> acc.append(str.charAt(0)), 
                 StringBuilder::append);

Answer 2

ernest_k's answer is correct, of course. It's worth adding, though, that it seems like you're using reduce to implement the joining自己收藏，可能想直接使用：

String result = 
  Arrays.stream(s.split(" ")).map(x -> x.substring(0, 1)).collect(Collectors.joining());

Answer 3

您可以将 reduce 方法与 String 一起使用，而不是 StringBuilder:

String str1 = "apples for you";
String str2 = Arrays
        // split string into an array of words
        .stream(str1.split("\s+"))
        // first character of a word
        .map(str -> str.charAt(0))
        // character as string
        .map(String::valueOf)
        // reduce stream of strings to a single
        // string by concatenating them
        .reduce((s1, s2) -> s1 + s2)
        // otherwise null
        .orElse(null);

System.out.println(str2); // afy

^{另请参阅：}

如何在 java 1.8 中的 reduce 方法中使用 StringBuilder？

How to use StringBuilder inside the reduce method in java 1.8?

java

string

stringbuilder

java-8

java-stream