all sublists of a list

    public static <T extends Comparable<T>> List<List<T>> subsets(List<T> list)
    {
        List<List<T>> returnValue;
        if ( list.isEmpty() )
        {
            List<List<T>> lists =  new ArrayList<List<T>>();
            lists.add(list);
            returnValue = lists;

        } else if ( list.size() == 1 )
        {
            List<List<T>> lists =  new ArrayList<List<T>>();
            lists.add(list);
            lists.add(new ArrayList<T>());
            returnValue = lists;
        }
        else
        {
            List<T> firstElement = list.subList(0,1);
            List<T> rest = list.subList( 1, list.size() );
            returnValue = cartesian(subsets(firstElement), subsets(rest));

        }

        return returnValue;
    }

    public static  <T extends Comparable<T>>  List<List<T>> cartesian(List<List<T>> left, List<List<T>> right)
    {
        List<List<T>> returnValue = new ArrayList<>();
        if (left != null && right != null)
        {
            for ( List<T> leftElement : left )
            {
                for ( List<T> rightElement : right )
                {
                    returnValue.add(Stream.concat( leftElement.stream(), rightElement.stream() ).collect(Collectors.toList()));
                }
            }
        }
        return returnValue;
    }