Cartesian product of arbitrary sets in Java

Edit: Previous solutions for two sets removed. See edit history for details.

Here is a way to do it recursively for an arbitrary number of sets:

public static Set<Set<Object>> cartesianProduct(Set<?>... sets) {
    if (sets.length < 2)
        throw new IllegalArgumentException(
                "Can't have a product of fewer than two sets (got " +
                sets.length + ")");

    return _cartesianProduct(0, sets);
}

private static Set<Set<Object>> _cartesianProduct(int index, Set<?>... sets) {
    Set<Set<Object>> ret = new HashSet<Set<Object>>();
    if (index == sets.length) {
        ret.add(new HashSet<Object>());
    } else {
        for (Object obj : sets[index]) {
            for (Set<Object> set : _cartesianProduct(index+1, sets)) {
                set.add(obj);
                ret.add(set);
            }
        }
    }
    return ret;
}

Note that it is impossible to keep any generic type information with the returned sets. If you knew in advance how many sets you wanted to take the product of, you could define a generic tuple to hold that many elements (for instance Triple<A, B, C>), but there is no way to have an arbitrary number of generic parameters in Java.

This is a pretty old question, but why not use Guava's cartesianProduct?

The method below creates the cartesian product of a list of list of strings:

protected <T> List<List<T>> cartesianProduct(List<List<T>> lists) {
    List<List<T>> resultLists = new ArrayList<List<T>>();
    if (lists.size() == 0) {
        resultLists.add(new ArrayList<T>());
        return resultLists;
    } else {
        List<T> firstList = lists.get(0);
        List<List<T>> remainingLists = cartesianProduct(lists.subList(1, lists.size()));
        for (T condition : firstList) {
            for (List<T> remainingList : remainingLists) {
                ArrayList<T> resultList = new ArrayList<T>();
                resultList.add(condition);
                resultList.addAll(remainingList);
                resultLists.add(resultList);
            }
        }
    }
    return resultLists;
}

Example:

System.out.println(cartesianProduct(Arrays.asList(Arrays.asList("Apple", "Banana"), Arrays.asList("Red", "Green", "Blue"))));

would yield this:

[[Apple, Red], [Apple, Green], [Apple, Blue], [Banana, Red], [Banana, Green], [Banana, Blue]]