如何在Java 1.8的reduce方法内使用StringBuilder? [英] How to use StringBuilder inside the reduce method in java 1.8?

问题描述

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

每个单词的预期输出首字母，即 afy .

Expected output first letter of each word, i.e. afy.

但是在 acc.append 处出错， acc 被视为 String .

But getting error at acc.append, acc is treated to be a String.

推荐答案

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，该重载还应为 StringBuilder 使用一个二进制运算符:

Your use of reduce is incorrect. The overload you intend to call is the one with 3 parameters, which should also take a binary operator for StringBuilder:

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

如果要在并行流上使用它，请执行可变还原(使用stream.collect)，因为初始的身份字符串生成器对象可能会意外地从多个线程附加到:

If you're to use this on a parallel stream, please perform a mutable reduction (with stream.collect) as the initial, identity string builder object may otherwise be appended to unpredictably from multiple threads:

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

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