java - 您可以在不映射到字符串的情况下收集(joining())吗？

Question

我正在阅读java 8 in action，作者说如果你有一个重写toString方法的类，那么在执行collect(joining())时不需要将流映射到字符串。一个例子:

    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);

    }

但是，这不起作用，只能这样:

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

有效，那么这本书是错的？另外，他为什么这么说(我稍微改了一下措辞):

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.

什么时候每个对象都应该从 Object 继承 toString？

Answer 1

书上所说的都是错误的，而你的解释是正确的(除非观点完全不同并且你没有明白)

people.stream()

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

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

显然这不能像Person那样工作不是 CharSequence 的实例.