为什么 Stream<T>不实现Iterable<T>?

2022-01-22 00:00:00 iterable java-8 java java-stream

在 Java 8 中，我们有类 Stream<T>，奇怪的是有一个方法

In Java 8 we have the class Stream<T>, which curiously have a method

Iterator<T> iterator()

所以你会期望它实现接口 Iterable<T>，正是需要这种方法，但事实并非如此.

So you would expect it to implement interface Iterable<T>, which requires exactly this method, but that's not the case.

当我想使用 foreach 循环遍历 Stream 时，我必须执行类似的操作

When I want to iterate over a Stream using a foreach loop, I have to do something like

public static Iterable<T> getIterable(Stream<T> s) {
    return new Iterable<T> {
        @Override
        public Iterator<T> iterator() {
            return s.iterator();
        }
    };
}

for (T element : getIterable(s)) { ... }

我错过了什么吗?

推荐答案

人们已经问过同样的问题 在邮件列表中 ☺.主要原因是Iterable也有可重迭代的语义，而Stream没有.

People have already asked the same on the mailing list ☺. The main reason is Iterable also has a re-iterable semantic, while Stream is not.

我认为主要原因是 Iterable 意味着可重用性，而 Stream 是只能使用一次的东西——更像是一个 Iterator.

I think the main reason is that Iterable implies reusability, whereas Stream is something that can only be used once — more like an Iterator.

如果 Stream 扩展了 Iterable 则现有代码在收到抛出 Exception 的 Iterable 时可能会感到惊讶这他们第二次 for (element : iterable).

If Stream extended Iterable then existing code might be surprised when it receives an Iterable that throws an Exception the second time they do for (element : iterable).

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