Java – Is there a function to find all subgroups from one main group by filtering certain subgroups?

I’m writing code in java, so it would be great if you could share code with java 🙂

Let’s say I have a group (1,2,3,4,5) and I want to create all subgroups of the group with a maximum size of a given natural number (e.g. 3).

I’ve found a code that returns all subgroups, however, in my project my group size can be up to 40, so I need to spend too much time calculating and it’s very problematic. I also prefer it to be a function rather than an object. Efficiency is also important, I can’t create all possible groups and then filter them out.

public static <T> Set<Set<T>> powerSet(Set<T> originalSet) {
        Set<Set<T>> sets = new HashSet<>();
        if (originalSet.isEmpty()) {
            sets.add(new HashSet<>());
            return sets;
        }
        List<T> list = new ArrayList<>(originalSet);
        T head = list.get(0);
        Set<T> rest = new HashSet<>(list.subList(1, list.size()));
        for (Set<T> set : powerSet(rest)) {
            Set<T> newSet = new HashSet<>();
            newSet.add(head);
            newSet.addAll(set);
            sets.add(newSet);
            sets.add(set);
        }
        return sets;
    }

I found it here:
Obtaining a powerset of a set in Java .

I looked at your code and tweaked it slightly.

You now enter the maximum size of the inner collection, which will be the size of the largest collection.

    public static <T> Set<Set<T>> powerSet(Set<T> originalSet, int size) {
        Set<Set<T>> sets = new HashSet<>();
        if (originalSet.isEmpty()) {
            sets.add(new HashSet<>());
            return sets;
        }
        List<T> list = new ArrayList<>(originalSet);
        T head = list.get(0);
        Set<T> rest = new HashSet<>(list.subList(1, list.size()));
        for (Set<T> set : powerSet(rest, size)) {
            if(set.size() <= size-1 ){
                Set<T> newSet = new HashSet<>();
                newSet.add(head);
                newSet.addAll(set);
                sets.add(newSet);
                sets.add(set);
            }

}

return sets;
    }