Replace nested loop with Java 8 flatmap

Question

I'm trying to use flatmap to make a nested loop with the Stream API, but I can't seem to figure it out. As an example, I want to recreate the following loop:

List<String> xs = Arrays.asList(new String[]{ "one","two", "three"});
List<String> ys = Arrays.asList(new String[]{"four", "five"});

System.out.println("*** Nested Loop ***");
for (String x : xs)
    for (String y : ys)
        System.out.println(x + " + " + y);

I can do it like this, but this seems ugly:

System.out.println("*** Nested Stream ***");
xs.stream().forEach(x ->
    ys.stream().forEach(y -> System.out.println(x + " + " + y))
);

Flatmap looks promising, but how can I access the variable in the outer loop?

System.out.println("*** Flatmap *** ");
xs.stream().flatMap(x -> ys.stream()).forEach(y -> System.out.println("? + " + y));

Output:

*** Nested Loop ***
one + four
one + five
two + four
two + five
three + four
three + five
*** Nested Stream ***
one + four
one + five
two + four
two + five
three + four
three + five
*** Flatmap *** 
? + four
? + five
? + four
? + five
? + four
? + five

Possible duplicate of How to iterate nested for loops referring to parent elements? — px06
– px06, Commented Nov 18, 2016 at 14:01

Flown · Accepted Answer · 2016-11-18 14:02:09Z

12

You have to create your desired elements in the flatMap stage, like:

xs.stream().flatMap(x -> ys.stream().map(y -> x + " + " + y)).forEach(System.out::println);

answered Nov 18, 2016 at 14:02

Flown

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2 Comments

wvdz Over a year ago

Thanks. It seems to me the nested Stream option would be easier to use though. In the given example you can quite easily move the combination operation to flatmap, but in a lot of other cases this will be more work, especially when adding more levels to the nesting.

Flown Over a year ago

@wvdz It always depends on the use case and what is more readable to you.

Andrew · Accepted Answer · 2016-11-18 15:14:32Z

2

Normally, there is no need of the flatMap:

xs.forEach(x -> ys.stream().map(y ->  x + " + " + y).forEach(System.out::println)); // X
xs.forEach(x -> ys.forEach(y -> System.out.println(x + " + " + y))); // V

as well as there is no need of Stream API here.

Yes, it looks beautiful, but only with such childish tasks. You create/close a new stream for each element only to merge them into the resulting stream. And all of that is just for printing out?

In contrast, the forEach provides a one-line solution without any performance costs (a standard foreach inside).

edited Nov 18, 2016 at 15:14

answered Nov 18, 2016 at 14:19

Andrew

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1 Comment

wvdz Over a year ago

This is equivalent to the nested stream example that I gave in the question.

Ievgen Anikushyn · Accepted Answer · 2016-11-20 22:37:11Z

Basically, this is a cartesian product for those lists. I would combine them first into one list:

List<String> xs = Arrays.asList(new String[]{ "one","two", "three"});
List<String> ys = Arrays.asList(new String[]{"four", "five"});
List<List<String>> input = Arrays.asList(xs, ys);

Then create a stream of lists and each of the lists will map to its own stream and hold this stuff into Supplier:

Supplier<Stream<String>> result = input.stream() // Stream<List<String>>
                .<Supplier<Stream<String>>>map(list -> list::stream) // Stream<Supplier<Stream<String>>>

Then reduce this stream of suppliers and produce a cartesian product for the streams of strings that are belong to suppliers like this:

.reduce((sup1, sup2) -> () -> sup1.get().flatMap(e1 -> sup2.get().map(e2 -> e1 + e2)))

Reduce returns optional, so to handle absent value I will return an empty string stream:

.orElse(() -> Stream.of(""));

After all we just need to get the supplier value (which will be a stream of strings) and print it out:

s.get().forEach(System.out::println);

The whole method will look like:

public static void printCartesianProduct(List<String>... lists) {
        List<List<String>> input = asList(lists);
        Supplier<Stream<String>> s = input.stream()
                // Stream<List<String>>
                .<Supplier<Stream<String>>>map(list -> list::stream)
                // Stream<Supplier<Stream<String>>>
                .reduce((sup1, sup2) -> () -> sup1.get()
                        .flatMap(e1 -> sup2.get().map(e2 -> e1 + e2)))
                .orElse(() -> Stream.of(""));

        s.get().forEach(System.out::println);
    }

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