Ray

// Return a list of all the subsequences of `li' with length `n'.
// http://cadrlife.blogspot.com/2008/03/returning-considered-difficult.html

private static <T> List<List<T>> subn(int n, List<T> li) {
    if (n == 0) {
        return Collections.<List<T>>singletonList(new ArrayList<T>());
    }
    if (li.isEmpty()) {
        return new ArrayList<List<T>>();
    }
    List<List<T>> ret = new ArrayList<List<T>>();
    T x = li.get(0);
    List<T> xs = li.subList(1, li.size());
    for (List<T> sub : subn(n-1, xs)) {
        sub.add(0, x);
        ret.add(sub);
    }
    ret.addAll(subn(n, xs));
    return ret;
}