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?

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() 

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());

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.

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.

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);