Java : Cartesian Product of a List of Lists

Question

I have a problem that is really kind of a general programming question, but my implementation is in Java, so I will provide my examples that way

I have a class like

Answer 1

Recursive solution:

public  List> cartesianProduct(int i, List... a) {
    if(i == a.length ) {
        List> result = new ArrayList<>();
        result.add(new ArrayList());
        return result;
    }
    List> next = cartesianProduct(i+1, a);
    List> result = new ArrayList<>();
    for(int j=0; j < a[i].size(); j++) {
        for(int k=0; k < next.size(); k++) {
            List concat = new ArrayList();
            concat.add(a[i].get(j));
            concat.addAll(next.get(k));
            result.add(concat);
        }
    }
    return result;
}