source: src/main/java/agents/anac/y2017/simpleagent/CartesianCalculator.java

package agents.anac.y2017.simpleagent;

import java.util.ArrayList;
import java.util.List;

public class CartesianCalculator {

        public <T> List<List<T>> calculate(List<List<T>> input) {
                List<List<T>> res = new ArrayList<>();
                if (input.isEmpty()) { // if no more elements to process
                        res.add(new ArrayList<T>()); // then add empty list and return
                        return res;
                } else {
                        process(input, res); // we need to calculate the cartesian product
                                                                        // of input and store it in res variable
                }
                return res; // method completes , return result
        }

        private <T> void process(List<List<T>> lists, List<List<T>> res) {
                List<T> head = lists.get(0); // take first element of the list
                List<List<T>> tail = calculate(lists.subList(1, lists.size())); // invoke
                                                                                                                                                // calculate
                                                                                                                                                // on
                                                                                                                                                // remaining
                                                                                                                                                // element,
                                                                                                                                                // here
                                                                                                                                                // is
                                                                                                                                                // recursion

                for (T h : head) { // for each head
                        for (List<T> t : tail) { // iterate over the tail
                                List<T> tmp = new ArrayList<>(t.size());
                                tmp.add(h); // add the head
                                tmp.addAll(t); // and current tail element

                                res.add(tmp);
                        }
                }
        }
}