How to collect objects from a doubly nested map in java 8?

Question

I'm planning to leverage the benefits of using parallel streams in my program and I want to replace a loop like this:

    for (int gridY = gridYFrom; gridY <= gridYTo; gridY++) {
        for (int gridX = gridXFrom; gridX <= gridXTo; gridX++) {
            coordinates.add(Coordinate.from(gridX, gridY));
        }
    }

with something like this:

    IntStream.rangeClosed(gridYFrom, gridYTo).parallel().map(y -> 
        IntStream.rangeClosed(gridXFrom, gridXTo).mapToObj(x -> {
            return Coordinate.from(x, y);
        }
    )).collect(Collectors.toSet());

My problem is that here I get a cyclic inference error. I understand that I am supposed to return an int from the inner map to be compatible with the outer one but I want to return a Coordinate object (thus the mapToObj call). Is it possible to do so using collect and without using a forEach construct?

Adam Arold · Accepted Answer · 2016-01-09 04:56:37Z

4

What you want to do is these things:

First of all, when you call map on an IntStream, it still returns an IntStream, which isn't what you want. Instead, also use mapToObj for the outer loop.

Second of all, the inner loop returns an incomplete Stream<Coordinate>, which I assume is also not what you want. So, you'll want to call .collect(Collectors.toSet()) on that as well.

Finally, you'll want to flatMap the Stream<Set<Coordinate>> into a single Stream<Coordinate>, and you can do that by using

stream.flatmap(Set::stream);

This all boils down to

IntStream.rangeClosed(0, 10).parallel().mapToObj(y ->
    IntStream.rangeClosed(0, 20).mapToObj(x -> 
        Coordinate.from(x,y)).collect(Collectors.toSet())
).flatMap(Set::stream).collect(Collectors.toSet());

EDIT: Actually, forget the inner collect. Just flatmap to Stream::sequential.

You'll end up with

IntStream.rangeClosed(0, 10).parallel().mapToObj(y -> 
    IntStream.rangeClosed(0, 20).mapToObj(x -> 
        Coordinate.from(x, y))).flatMap(Stream::sequential).collect(Collectors.toSet())

edited Jan 9, 2016 at 4:56

Adam Arold

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answered Jan 9, 2016 at 4:48

samczsun

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3 Comments

Adam Arold Over a year ago

Great, thanks! I figured out that I need to collect twice but the other mapToObj was still a problem.

samczsun Over a year ago

Please check my edit. I completely forgot about Stream::sequential.

Alexis C. Over a year ago

Why do use Stream::sequential as function to flat map the inner streams? Just use flatMap(s -> s).

Adam Arold · Accepted Answer · 2016-01-09 04:59:11Z

2

You don't need to collect twice. The problem is that IntStream isn't like any other stream in java, unfortunately. So, you have to convert it into a normal stream first, then you can do flatMap:

IntStream.rangeClosed(gridYFrom, gridYTo)
   .mapToObj( y -> Integer.valueOf(y) )
   .flatMap( y -> 
      IntStream.rangeClosed(gridXFrom, gridXTo)
         .mapToObj(x ->  Coordinate.from(x,y))
   )
   .collect(Collectors.toSet())

edited Jan 9, 2016 at 4:59

Adam Arold

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answered Jan 9, 2016 at 4:53

Dima

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1 Comment

Misha Over a year ago

You can use .boxed() instead of .mapToObj( y -> Integer.valueOf(y) )

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