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

I'm late to the party but I followed Shiomi's link and translated the functions into Java. The result is an easy to follow and understand algorithm (I may be a little slow since I had a hard time understanding Bart Kiers' solution).

Here it is (the key is an int, replacing to String should be straightforward):

Usage

    public void testProduct(){
        Map> data =   new LinkedHashMap>(){{                
            put(0, new ArrayList(){{
                add("John"); add("Sarah");                      
            }});                
            put(1, new ArrayList(){{
                add("Red"); add("Green"); add("Blue"); add("Orange");
            }});
            put(2, new ArrayList(){{
                add("Apple"); add("Tomatoe"); add("Bananna");                   
            }});
    }};

        List product =  GetCrossProduct(data);
        for(String[] o : product)
            System.out.println(Arrays.toString(o));

    }

Result

[John, Red, Apple]
[John, Red, Tomatoe]
[John, Red, Bananna]
[John, Green, Apple]
[John, Green, Tomatoe]
[John, Green, Bananna]
[John, Blue, Apple]
[John, Blue, Tomatoe]
[John, Blue, Bananna]
[John, Orange, Apple]
[John, Orange, Tomatoe]
[John, Orange, Bananna]
[Sarah, Red, Apple]
[Sarah, Red, Tomatoe]
[Sarah, Red, Bananna]
[Sarah, Green, Apple]
[Sarah, Green, Tomatoe]
[Sarah, Green, Bananna]
[Sarah, Blue, Apple]
[Sarah, Blue, Tomatoe]
[Sarah, Blue, Bananna]
[Sarah, Orange, Apple]
[Sarah, Orange, Tomatoe]
[Sarah, Orange, Bananna]

Cartesian Product Functions

    public static List GetCrossProduct(Map> lists)
    {
        List results = new ArrayList();
        GetCrossProduct(results, lists, 0, new String[(lists.size())]);
        return results;
    }

    private void GetCrossProduct(List results, Map> lists, int depth, String[] current)
    {
        for (int i = 0; i < lists.get(depth).size(); i++)
        {
            current[depth] = lists.get(depth).get(i);            
            if (depth < lists.keySet().size() - 1)
                GetCrossProduct(results, lists, depth + 1, current);
            else{
                results.add(Arrays.copyOf(current,current.length));                
            }
        }
    }