How do I convert a Java 8 IntStream to a List?

Question

I'm looking at the docs for the IntStream, and I see an toArray method, but no way to go directly to a List<Integer>

Surely there is a way to convert a Stream to a List?

Geophagy

Try this link:#14830813 – Requiem

@KarlRichter The other question doesn't give you a typed list. Also, this question was from four years ago, and has an answer with 300+ upvotes. Why are we trying to merge it now? – Geophagy

Answer 1

IntStream::boxed

IntStream::boxed turns an IntStream into a Stream<Integer>, which you can then collect into a List:

theIntStream.boxed().collect(Collectors.toList())

The boxed method converts the int primitive values of an IntStream into a stream of Integer objects. The word "boxing" names the int ⬌ Integer conversion process. See Oracle Tutorial.

Java 16 and later

Java 16 brought the shorter toList method. Produces an unmodifiable list. Discussed here.

theIntStream.boxed().toList() 

Leisha

@skiwi I mean, that all the other answers are unneeded after this one as they would be not so natural. – Ruthful

Thank you I hadn't seen the boxed method yet and it worked like a charm. – Cybil

Addition: I think this codes gets a little shorter, clearer and prettier if you use a static import of toList. This is done by placing the following among the imports of the file: static import java.util.stream.Collectors.toList;. Then the collect call reads just .collect(toList()). – Cleanly

In Eclipse it is possible to make the IDE add a static import for methods. This is done by adding the Collectors class in Preferences -> Java -> Editor -> Content Assist -> Favorites. After this, you only have to type toLi at hit Ctr+Space to have the IDE fill in toList and add the static import. – Cleanly

Was tearing my hair out about what was wrong with what I had tried, thank you for pointing out the boxed() part – Proscription

Don't forget to import java.util.stream.Collectors;. – Groceries

I was happy to get this answer -- and more broadly, these features in Java 8. Looking back after two years working in Python, and I'm so glad I don't have to deal with this nonsense. – Geophagy

java stream api is un-straightforward at so many levels.. – Leontina

Answer 2

You could also use mapToObj() on a Stream, which takes an IntFunction and returns an object-valued Stream consisting of the results of applying the given function to the elements of this stream.

List<Integer> intList = myIntStream.mapToObj(i->i).collect(Collectors.toList());

Daugava

In a stroke of irony, this is what boxed() is delegating to. – Boomer

Answer 3

You can use primitive collections available in Eclipse Collections and avoid boxing.

MutableIntList list = 
    IntStream.range(1, 5)
    .collect(IntArrayList::new, MutableIntList::add, MutableIntList::addAll);

Note: I am a contributor to Eclipse Collections.

Mandalay

Although Eclipse Collections are usually quite helpful, this does not look like it is making anything easier at all :) – Camarillo

Since EC 9.0, you can build a primitive list from a primitive Stream. MutableIntList list = IntLists.mutable.withAll(IntStream.range(1, 5)) – Smith

This is what I was looking for .. boxing the int stream to Integer or to object is different thing – Petey

Answer 4

You can use the collect method:

IntStream.of(1, 2, 3).collect(ArrayList::new, List::add, List::addAll);

In fact, this is almost exactly what Java is doing when you call .collect(Collectors.toList()) on an object stream:

public static <T> Collector<T, ?, List<T>> toList() {
    return new Collectors.CollectorImpl(ArrayList::new, List::add, (var0, var1) -> {
        var0.addAll(var1);
        return var0;
    }, CH_ID);
}

Note: The third parameter is only required if you want to run parallel collection; for sequential collection providing just the first two will suffice.

Spoony

That last sentence explains it all. – Newcomen

Answer 5

Find the folowing example of finding square of each int element using Java 8 :-

IntStream ints = Arrays.stream(new int[] {1,2,3,4,5});       
List<Integer> intsList = ints.map(x-> x*x)
          .collect(ArrayList<Integer>::new, ArrayList::add, ArrayList::addAll);

Petey