How to convert an Iterator to a Stream?
JavaIteratorJava 8Java Problem Overview
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.
Java Solutions
Solution 1 - Java
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);
Solution 2 - Java
Since version 21, Guava library provides Streams.stream(iterator)
It does what @assylias's answer shows.
Solution 3 - Java
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());
Solution 4 - Java
This is possible in Java 9.
Stream.generate(() -> null)
.takeWhile(x -> iterator.hasNext())
.map(n -> iterator.next())
.forEach(System.out::println);
Solution 5 - Java
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);
Solution 6 - Java
import com.google.common.collect.Streams;
and use Streams.stream(iterator)
:
Streams.stream(iterator)
.map(v-> function(v))
.collect(Collectors.toList());
Solution 7 - Java
Another way to do this on Java 9+ using Stream::iterate(T, Predicate, UnaryOperator)
:
Stream.iterate(iterator, Iterator::hasNext, UnaryOperator.identity())
.map(Iterator::next)
.forEach(System.out::println);
Solution 8 - Java
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);
}
Solution 9 - Java
Use Collections.list(iterator).stream()...