为什么这个 java Stream 被操作两次?

问题描述

Java 8 API 说:

管道源的遍历直到管道的终端操作执行完毕后才开始.

那么为什么下面的代码会抛出:

So why the following code throws :

java.lang.IllegalStateException: 流已经被操作过或关闭

Stream<Integer> stream = Stream.of(1,2,3);
stream.filter( x-> x>1 );
stream.filter( x-> x>2 ).forEach(System.out::print);

根据 API 的第一个过滤操作不应该对 Stream 进行操作.

The first filtering operation according to the API is not supposed to operate on the Stream.

推荐答案

发生这种情况是因为您忽略了 filter 的返回值.

This happens because you are ignoring the return value of filter.

Stream<Integer> stream = Stream.of(1,2,3);
stream.filter( x-> x>1 ); // <-- ignoring the return value here
stream.filter( x-> x>2 ).forEach(System.out::print);

Stream.filter 返回一个 new Stream ，由与给定谓词匹配的此流的元素组成.但需要注意的是，它是一个新的 Stream.当过滤器被添加到它时，旧的已经被操作.但新的不是.

Stream.filter returns a new Stream consisting of the elements of this stream that match the given predicate. But it's important to note that it's a new Stream. The old one has been operated upon when the filter was added to it. But the new one wasn't.

引用自 Stream Javadoc:

Quoting from Stream Javadoc:

一个流应该只操作一次(调用中间或终端流操作).

在这种情况下，filter 是对旧 Stream 实例进行操作的中间操作.

In this case, filter is the intermediate operation that operated on the old Stream instance.

所以这段代码可以正常工作:

So this code will work fine:

Stream<Integer> stream = Stream.of(1,2,3);
stream = stream.filter( x-> x>1 ); // <-- NOT ignoring the return value here
stream.filter( x-> x>2 ).forEach(System.out::print);

正如 Brian Goetz 所指出的，您通常会将这些调用链接在一起:

---

As noted by Brian Goetz, you would commonly chain those calls together:

Stream.of(1,2,3).filter( x-> x>1 )
                .filter( x-> x>2 )
                .forEach(System.out::print);

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