当我们已经有了 StringBuilder 时，为什么要使用 StringJoiner?

问题描述

我最近遇到了一个 Java 8 类 StringJoiner 它使用分隔符添加字符串并为其添加前缀和后缀，但我无法理解此类的需要，因为它也使用 StringBuilder 在后端，也执行非常简单的追加操作字符串.

I recently encountered with a Java 8 class StringJoiner which adds the String using the delimiters and adds prefix and suffix to it, but I can't understand the need of this class as it also uses StringBuilder at the backend and also performs very simple operation of appending the Strings.

我是否因为没有真正理解这门课的真正目的而错过了什么?

Am I missing something by not actually understanding the real purpose of this class?

推荐答案

StringJoiner 非常有用，当你需要在 Stream 中加入 Strings 时.

StringJoiner is very useful, when you need to join Strings in a Stream.

例如，如果您必须遵循字符串列表:

As an example, if you have to following List of Strings:

final List<String> strings = Arrays.asList("Foo", "Bar", "Baz");

使用起来更简单

final String collectJoin = strings.stream().collect(Collectors.joining(", "));

就像使用 StringBuilder 一样:

final String collectBuilder =
    strings.stream().collect(Collector.of(StringBuilder::new,
        (stringBuilder, str) -> stringBuilder.append(str).append(", "),
        StringBuilder::append,
        StringBuilder::toString));

6 年后编辑正如评论中所指出的，现在有更简单的解决方案，例如 String.join(", ", strings)，当时还没有这些解决方案.但是用例还是一样的.

EDIT 6 years laterAs noted in the comments, there are now much simpler solutions like String.join(", ", strings), which were not available back then. But the use case is still the same.

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