How to convert an Iterator to a Stream?

Question

I am looking for a concise way to convert an Iterator 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(); 

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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 1

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);