你可以收集（加入（））而不映射到字符串吗？ [英] Can you collect(joining()) without mapping to string?

问题描述

我正在阅读java 8，并且作者说如果你有一个覆盖toString方法的类，那么在进行collect（joined（））时你不需要将流映射到字符串。例如：

I am reading java 8 in action, and the author says that if you have a class that overrides the toString method, you don't need to map the stream to Strings when doing collect(joining()). An example:

    public static void main(String... args) {
        List<Person> people =
        Arrays.asList(
                new Person(23, "Paul"),
                new Person(23, "John"),
                new Person(23, "Greg"),
                new Person(24, "Greg"),
                new Person(25, "Paul")
        ); // Person overrides toString

        String peopleString = people
                .stream()
                .collect(Collectors.joining());

        System.out.println(peopleString);

    }

然而，这不起作用，只有这样：

However, this doesn't work, and only this:

String peopleString = people
                .stream()
                .map(Person::toString)
                .collect(Collectors.joining());

有效，所以这本书错了吗？此外，为什么他说（我改变了一点措辞）：

works, so the book is wrong then? Besides, why does he say (I changed the wording a little):

还要注意，如果一个类有一个toString方法返回一个字符串，您将获得相同的结果，而无需使用提取名称的函数映射原始流。

Also note that if a class had a toString method returning a string, you’d obtain the same result without needing to map over the original stream with a function extracting the name.

当每个对象都是应该从对象继承 toString ？

When every object is supposed to inherit toString from Object?

推荐答案

无论这本书说错了，你的解释是对的（除非这一点完全不同而且你没有得到它）​​

Whatever the book says is wrong and your interpretation is right (unless the point is entirely different and you did not get it)

people.stream()

将生成 Stream< People> ，而 Collectors.joining 的定义为：

public static Collector<CharSequence, ?, String> joining()

显然这不能用作人不是 CharSequence 的实例。

obviously this can't work as Person is not an instance of CharSequence.

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