Java 8 Stream Nested for loop

Question

How can I do the following nested loop using java stream?

for (int x = 0; x < 5; x++) {
    for (int y = 0; y < 5; y++) {
        System.out.println(x + ", " + y);
    }   
}

I can easily do one loop with IntStream.range(0, 5). Is this possible with streams?

EDIT: well I guess I can do this, but can it be done with flatMap?

IntStream.range(0, 5)
         .forEach(x -> IntStream.range(x, 5).forEach(y -> System.out.println(x + ", " + y)));

Is there any reason for using a stream? You do not have to migrate every loop to a stream operation just because you can. — luk2302
– luk2302, Commented Dec 29, 2016 at 21:08
I prefer your version over using flatMap. But I prefer two loops over any stream version. — Bohemian
– Bohemian ♦, Commented Dec 29, 2016 at 21:32

Andreas · Accepted Answer · 2016-12-29 21:26:09Z

4

Do you mean something like this?

IntStream.range(0, 5)
         .boxed()
         .flatMap(x -> IntStream.range(0, 5)
                                .mapToObj(y -> new int[] { x, y }))
         .map(xy -> xy[0] + ", " + xy[1])
         .forEach(System.out::println);

UPDATE

It would be better if the array is replaced with an actual object.

IntStream.range(0, 5)
         .boxed()
         .flatMap(x -> IntStream.range(0, 5)
                                .mapToObj(y -> new Coord(x, y)))
         .forEach(System.out::println);

public final class Coord {
    private final int x;
    private final int y;
    public Coord(int x, int y) {
        this.x = x;
        this.y = y;
    }
    public int getX() {
        return this.x;
    }
    public int getY() {
        return this.y;
    }
    @Override
    public String toString() {
        return this.x + ", " + this.y;
    }
}

edited Dec 29, 2016 at 21:26

answered Dec 29, 2016 at 21:17

Andreas

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2 Comments

user1738539 Over a year ago

Yes just like that. I couldn't figure out how to use flatMap() to create the tuple x,y

Alex Pakka Over a year ago

Too bad you cannot lift pairing function like comma into a pair of list monands in Java8.

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