如何从迭代器创建 Java 8 Stream？
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How to create a Java 8 Stream from an iterator?
提问by sdenham
Is it possible to create a Stream from an Iterator, in which the sequence of objects is the same as that generated by calling the iterator's next() method repeatedly? The specific case I am thinking of concerns the use of the iterator returned by TreeSet.descendingIterator(), but I can imagine other circumstances in which an iterator, but not the collection it references, is available.
是否可以从迭代器创建一个流，其中对象的序列与通过重复调用迭代器的 next() 方法生成的序列相同？我正在考虑的具体情况与 TreeSet.descendingIterator() 返回的迭代器的使用有关，但我可以想象在其他情况下迭代器可用，而不是它引用的集合。
For example, for a TreeSet<T> tsetwe can write tset.stream()...and get a stream of the objects in that set, in the set's sort order, but what if we want them in a different order, such as that available through using descendingIterator()? I am imagining something like tset.descendingIterator().stream()...or stream( tset.descendingIterator() )..., though neither of these forms are valid.
例如，对于 aTreeSet<T> tset我们可以tset.stream()...按照集合的排序顺序编写和获取该集合中的对象流，但是如果我们希望它们以不同的顺序（例如通过 using 可用的顺序）descendingIterator()怎么办？我正在想象类似tset.descendingIterator().stream()...或 的东西stream( tset.descendingIterator() )...，尽管这两种形式都无效。
采纳答案by andersschuller
For the particular example of NavigableSet.descendingIterator(), I think the simplest way is to use NavigableSet.descendingSet()instead.
对于 的特定示例NavigableSet.descendingIterator()，我认为最简单的方法是使用NavigableSet.descendingSet()。
But given you are probably interested in the more general case, the following seems to work:
但鉴于您可能对更一般的情况感兴趣，以下似乎有效：
import java.util.Iterator;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.TreeSet;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;
public class Streams {
public static void main(String... args) {
TreeSet<String> set = new TreeSet<>();
set.add("C");
set.add("A");
set.add("B");
Iterator<String> iterator = set.descendingIterator();
int characteristics = Spliterator.DISTINCT | Spliterator.SORTED | Spliterator.ORDERED;
Spliterator<String> spliterator = Spliterators.spliteratorUnknownSize(iterator, characteristics);
boolean parallel = false;
Stream<String> stream = StreamSupport.stream(spliterator, parallel);
stream.forEach(System.out::println); // prints C, then B, then A
}
}
In short, you have to create a Spliteratorfrom the Iteratorfirst using one of the static methods in Spliterators. Then you can create a Streamusing the static methods in StreamSupport.
总之，你必须创建一个Spliterator从Iterator第一使用的静态方法之一Spliterators。然后您可以Stream使用StreamSupport.
I don't have that much experience with creating Spliterators and Streams by hand yet, so I can't really comment on what the characteristics should be or what effect they will have. In this particular simple example, it didn't seem to matter whether I defined the characteristics as above, or whether I set it to 0 (i.e. no characteristics). There is also a method in Spliteratorsfor creating a Spliterator with an initial size estimate - I suppose in this particular example you could use set.size(), but if you want to handle arbitrary Iterators I guess this won't be the case. Again, I'm not quite sure what effect it has on performance.
我还没有太多手动创建 Spliterators 和 Streams 的经验，所以我无法真正评论应该具有什么特征或它们将产生什么效果。在这个特别简单的例子中，我是否如上定义特征，或者是否将其设置为 0（即没有特征）似乎并不重要。还有一种方法可以Spliterators用于创建具有初始大小估计的 Spliterator - 我想在这个特定示例中您可以使用set.size()，但是如果您想处理任意迭代器，我想情况并非如此。同样，我不太确定它对性能有什么影响。
回答by Karol Król
static <T> Stream<T> iteratorToFiniteStream(final Iterator<T> iterator) {
return StreamSupport.stream(Spliterators.spliteratorUnknownSize(iterator, 0), false);
}
static <T> Stream<T> iteratorToInfiniteStream(final Iterator<T> iterator) {
return Stream.generate(iterator::next);
}
回答by Jordan Shurmer
This doesn't create a stream, but Iteratoralso has a method called forEachRemaining:
这不会创建流，但Iterator也有一个名为的方法forEachRemaining：
someIterator.forEachRemaining(System.out::println);
someIterator.forEachRemaining(s -> s.doSomething());
//etc.
The argument you pass in is a Consumerwhich is the same thing you pass to Stream::forEach.
您传入的参数是一个Consumer，它与您传递给Stream::forEach 的内容相同。
Here are the docsfor that method. note that you can't continue the "chain"like you can with a stream. But I've still found this helpful the few times I've wanted a Stream from an Iterator.
这是该方法的文档。请注意，您不能像使用流一样继续“链”。但是我几次想要来自迭代器的 Stream 时，我仍然发现这很有帮助。
回答by lczapski
As it was written by Karol Królfor infinite stream you can use this:
正如Karol Król为无限流编写的那样，您可以使用它：
Stream.generate(iterator::next)
but you can also use it for finite stream with takeWhilesince Java 9
但您也可以将它用于有限流，takeWhile因为 Java 9
Stream.generate(iterator::next).takeWhile((v) -> iterator.hasNext())
