How can I make Cartesian product with Java 8 streams?

Question

I have the following collection type:

Map> map;

I would like to create unique combinations of each o

Answer 1

Here is another solution, which does not use as many features from Streams as Tagir's example; however I believe it to be more straight-forward:

public class Permutations {

    transient List> perms;

    public List> list(Map> map) {

        SortedMap> sortedMap = new TreeMap<>();
        sortedMap.putAll(map);

        sortedMap.values().forEach((v) ->  perms = expand(perms, v));

        return perms;
    }

    private List> expand(List> list, Collection elements) {

        List> newList = new LinkedList<>();

        if (list == null) {
            elements.forEach((e) -> {
                SortedSet set = new TreeSet<>();
                set.add(e);
                newList.add(set);
            });
        } else {
            list.forEach((set) ->
                elements.forEach((e) -> {
                    SortedSet newSet = new TreeSet<>();
                    newSet.addAll(set);
                    newSet.add(e);
                    newList.add(newSet);
                }));
        }

        return newList;
    }
}

You can remove the Sorted prefix if you are not interested in ordering of elements; though, I think it's easier to debug if everything is sorted.

Usage:

Permutations p = new Permutations();
List> plist = p.list(map);
plist.forEach((s) -> System.out.println(s));

Enjoy!