java - 如何将迭代器转换为流？

Question

我正在寻找一种简洁的方法来转换 Iterator到 Stream或者更具体地说，将迭代器作为流“查看”。

出于性能原因，我想避免在新列表中复制迭代器:

Iterator<String> sourceIterator = Arrays.asList("A", "B", "C").iterator();
Collection<String> copyList = new ArrayList<String>();
sourceIterator.forEachRemaining(copyList::add);
Stream<String> targetStream = copyList.stream();

---

根据评论中的一些建议，我也尝试使用Stream.generate :

public static void main(String[] args) throws Exception {
    Iterator<String> sourceIterator = Arrays.asList("A", "B", "C").iterator();
    Stream<String> targetStream = Stream.generate(sourceIterator::next);
    targetStream.forEach(System.out::println);
}

但是，我得到一个 NoSuchElementException(因为没有调用 hasNext)

Exception in thread "main" java.util.NoSuchElementException
    at java.util.AbstractList$Itr.next(AbstractList.java:364)
    at Main$$Lambda$1/1175962212.get(Unknown Source)
    at java.util.stream.StreamSpliterators$InfiniteSupplyingSpliterator$OfRef.tryAdvance(StreamSpliterators.java:1351)
    at java.util.Spliterator.forEachRemaining(Spliterator.java:326)
    at java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:580)
    at Main.main(Main.java:20)

我看过 StreamSupport和 Collections但我什么也没找到。

Answer 1

一种方法是从 Iterator 创建一个 Spliterator 并将其用作流的基础:

Iterator<String> sourceIterator = Arrays.asList("A", "B", "C").iterator();
Stream<String> targetStream = StreamSupport.stream(
          Spliterators.spliteratorUnknownSize(sourceIterator, Spliterator.ORDERED),
          false);

另一种可能更具可读性的替代方法是使用 Iterable - 使用 lambdas 从 Iterator 创建 Iterable 非常容易，因为Iterable 是一个函数式接口(interface):

Iterator<String> sourceIterator = Arrays.asList("A", "B", "C").iterator();

Iterable<String> iterable = () -> sourceIterator;
Stream<String> targetStream = StreamSupport.stream(iterable.spliterator(), false);