java递归问题如何实现如下算法?

public class test {
public static List> source;

public static void main(String[] args) {
    source = new ArrayList<>();

    List<String> a = new ArrayList<String>();
    a.add("黑色");
    a.add("白色");
    List<String> b = new ArrayList<String>();
    b.add("64G");
    b.add("128G");
    List<String> c = new ArrayList<String>();
    c.add("中国联通");
    c.add("中国移动");
    source.add(a);
    source.add(b);
    source.add(c);
    ArrayList<String> result = new ArrayList<>();
    recursion(result, source.get(0), 0, "");
    System.out.println(result);
}

public static void recursion(List<String> result, List<String> para, int num, String choose) {

    for (int i = 0; i < para.size(); i++) {
        if (source.size() == num + 1) {
            result.add(choose + "/" + para.get(i));
        } else {
            recursion(result, source.get(num + 1), num + 1, choose + "/" + para.get(i));
        }
    }
}

}

就是个数组的数组，双层遍历，不算递归吧