645

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();

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.

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6
  • 5
    possible duplicate of How to create an infinite Stream<E> out of an Iterator<E>? Commented Jul 1, 2014 at 13:10
  • 3
    @DmitryGinzburg euh i don't want to create an "Infinite" Stream. Commented Jul 1, 2014 at 13:13
  • 2
    @DmitryGinzburg Stream.generate(iterator::next) works ? Commented Jul 1, 2014 at 13:16
  • 1
    @DmitryGinzburg That won't work for a finite iterator. Commented Jul 1, 2014 at 13:19
  • 8
    See stackoverflow.com/questions/23114015/… Commented Jul 1, 2014 at 17:00

13 Answers 13

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731

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);

A potentially more readable alternative 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);
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22 Comments

Stream are lazy: the code is only linking the Stream to the Iterator but the actual iteration won't happen until a terminal operation. If you use the iterator in the meantime you won't get the expected result. For example you can introduce a sourceIterator.next() before using the stream and you will see the effect (the first item will not be seen by the Stream).
@assylias, yeah it is really nice ! Maybe you could explains for future readers this quite magic line Iterable<String> iterable = () -> sourceIterator;. I have to admit that it took me some time to understand.
I should tell what I found. Iterable<T> is a FunctionalInterface which has only one abstract method iterator(). So () -> sourceIterator is a lambda expression instantiating an Iterable instance as an anonymous implementation.
Again, () -> sourceIterator; is a shortened form of new Iterable<>() { @Override public Iterator<String> iterator() { return sourceIterator; } }
@JinKwon It is not really a shortened form of an anonymous class (there are a few subtle differences, such as scope and how it is compiled) but it does behave similarly in this case.
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183

Since version 21, Guava library provides Streams.stream(iterator)

It does what @assylias's answer shows.

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3 Comments

Javadoc: static.javadoc.io/com.google.guava/guava/21.0/com/google/common/…
Far better to use this consistently until such time as the JDK supports a native one-liner. It will be far simpler to find (hence refactor) this in future than the pure-JDK solutions shown elsewhere.
This is excellent but… how does Java have native iterators and streams… but no built-in, straightforward way to go from one to the other!? Quite an omission in my opinion.
112

Great suggestion! Here's my reusable take on it:

public class StreamUtils { public static <T> Stream<T> asStream(Iterator<T> sourceIterator) { return asStream(sourceIterator, false); } public static <T> Stream<T> asStream(Iterator<T> sourceIterator, boolean parallel) { Iterable<T> iterable = () -> sourceIterator; return StreamSupport.stream(iterable.spliterator(), parallel); } }

And usage (make sure to statically import asStream):

List<String> aPrefixedStrings = asStream(sourceIterator) .filter(t -> t.startsWith("A")) .collect(toList());
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Comments

97

This is possible in Java 9.

Stream.generate(() -> null) .takeWhile(x -> iterator.hasNext()) .map(n -> iterator.next()) .forEach(System.out::println);
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5 Comments

Unfortunately these don't seem to work with .parallel() streams. They also appear a bit slower than going over Spliterator, even for sequential use.
Also, the first method throws up if the iterator is empty. The second method does work for now, but it does violate the requirement of the functions in map and takeWhile to be stateless, so I'd hesitate to do that in production code.
Really, this should be an accepted answer. Even though parallel might be funky the simpleness is amazing.
You really shouldn't be working with side effects and mutation.
While takeWhile() is generally a cheap operation on sequential stream pipelines, it can be quite expensive on ordered parallel pipelines. Reference: docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/…
16 
import com.google.common.collect.Streams;

and use Streams.stream(iterator) :

Streams.stream(iterator) .map(v-> function(v)) .collect(Collectors.toList());
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12

Create Spliterator from Iterator using Spliterators class contains more than one function for creating spliterator, for example here am using spliteratorUnknownSize which is getting iterator as parameter, then create Stream using StreamSupport

Spliterator<Model> spliterator = Spliterators.spliteratorUnknownSize( iterator, Spliterator.NONNULL); Stream<Model> stream = StreamSupport.stream(spliterator, false);
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Comments

7

Stream.iterate does the job for you. Simply iterate while it has elements and map them to next.

public <X> Stream<X> toStream(Iterator<X> iter) { return Stream.iterate(iter, Iterator::hasNext, UnaryOperator.identity()).map(Iterator::next); }
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1 Comment

Great option for Java 9+.
5

I think that one answer using Apache Commons Lang is missing.

Iterator<String> sourceIterator = Arrays.asList("A", "B", "C").iterator(); Stream<String> targetStream = org.apache.commons.lang3.stream.Streams.of(sourceIterator);

This works since Apache Commons Lang 3.13.0 (July 2023).

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Comments

2

If you are using Spring (Data) then there is a utility class StreamUtils which has two methods:

createStreamFromIterator(Iterator<T> iterator) createStreamFromIterator(CloseableIterator<T> iterator)

https://docs.spring.io/spring-data/commons/docs/3.0.0/api/org/springframework/data/util/StreamUtils.html#createStreamFromIterator(java.util.Iterator)

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Comments

2

1 assylias's solution wrapped in a method:

public static <T> Stream<T> toStream(Iterator<T> iterator) { return StreamSupport.stream(((Iterable<T>)() -> iterator).spliterator(), false); }

2 guava Streams implementation (marked with @Beta):

public static <T> Stream<T> stream(Iterator<T> iterator) { return StreamSupport.stream(Spliterators.spliteratorUnknownSize(iterator, 0), false); }
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1

I prefer two static methods
1. For Iterator object

public static <T> Stream<T> of(Iterator<T> iterator) { return StreamSupport.stream(Spliterators.spliteratorUnknownSize(iterator, Spliterator.ORDERED | Spliterator.IMMUTABLE), false);}


2. For iterable object

 public static <T> Stream<T> of(Iterable<T> iterable) { return StreamUtil.of(iterable.iterator());}
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Comments

-1

if iteration size it's known this is possible:

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.**limit(3)**.forEach(System.out::println); }
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Comments

-6

Use Collections.list(iterator).stream()...

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5 Comments

While short, this is very underperforming.
This will unfold the whole iterator to java object then convert it to stream. I dont suggest it
This seems to be only for enumerations, not iterators.
Not a terrible answer in general, useful in a pinch, but the question mentions performance and the answer is not performant.
Iterators and Streams are both lazy. Collecting the whole thing into a list before processing begins seems to miss the whole point.

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