cross join

CrossJoiner.java

package com.company;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

import static java.util.Arrays.asList;

/**
 * @author eutkin
 */
public class CrossJoiner {

    @SafeVarargs
    public static List<List<Object>> crossJoin(Collection<Object>... nullableCollections) {
        if (nullableCollections == null || nullableCollections.length == 0) {
            return Collections.emptyList();
        }
        List<Collection<Object>> collections = Stream
                .of(nullableCollections)
                .filter(collection -> collection != null && !collection.isEmpty())
                .collect(Collectors.toList());

        List<List<Object>> left = collections.get(0).stream().map(elem -> {
            List<Object> buffer = new ArrayList<>();
            buffer.add(elem);
            return buffer;
        }).collect(Collectors.toList());
        for (int i = 1; i < collections.size(); i++) {
            final int current = i;
            left = left.stream().flatMap(l -> collections.get(current).stream().map(r -> {
                List<Object> buffer = new ArrayList<>();
                buffer.addAll(l);
                buffer.add(r);
                return buffer;
            })).collect(Collectors.toList());
        }
        return left;
    }
}