gontard gontard - 7 months ago 69
Java Question

How to convert an iterator to a stream?

I am looking for a concise way to convert an

to a
Stream
or more specifically to "view" the iterator as a stream.

For performance reason, I would like to avoid a copy of the iterator in a new list:

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





Based on the some suggestions in the comments, I have also tried to use
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);
}


However, I get a
NoSuchElementException
(since there is no invocation of
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)


I have looked at
StreamSupport
and
Collections
but I didn't find anything.

Answer

One way is to create a Spliterator from the Iterator and use that as a basis for your stream:

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

An alternative which is maybe more readable is to use an Iterable - and creating an Iterable from an Iterator is very easy with lambdas because Iterable is a functional interface:

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

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