Java Itertools Impl

public static class Itertools {
        
        // Generate a range of integers
        public static List<Integer> range(int start, int end, int step) {
            List<Integer> result = new ArrayList<>();
            for (int i = start; i < end; i += step) {
                result.add(i);
            }
            return result;
        }
        
        // Generate all permutations of a list
        public static <T> List<List<T>> permutations(List<T> list) {
            if (list.size() == 0) return new ArrayList<>();
            if (list.size() == 1) return Collections.singletonList(list);
            
            List<List<T>> result = new ArrayList<>();
            for (int i = 0; i < list.size(); i++) {
                T current = list.get(i);
                List<T> remaining = new ArrayList<>(list);
                remaining.remove(i);
                
                for (List<T> perm : permutations(remaining)) {
                    List<T> temp = new ArrayList<>();
                    temp.add(current);
                    temp.addAll(perm);
                    result.add(temp);
                }
            }
            return result;
        }
        
        // Generate all combinations of a list
        public static <T> List<List<T>> combinations(List<T> list, int r) {
            List<List<T>> result = new ArrayList<>();
            combinationsHelper(list, r, 0, new ArrayList<>(), result);
            return result;
        }
        
        private static <T> void combinationsHelper(List<T> list, int r, int start, List<T> current, List<List<T>> result) {
            if (current.size() == r) {
                result.add(new ArrayList<>(current));
                return;
            }
            for (int i = start; i < list.size(); i++) {
                current.add(list.get(i));
                combinationsHelper(list, r, i + 1, current, result);
                current.remove(current.size() - 1);
            }
        }
        
        // Generate the Cartesian product of multiple lists
        public static <T> List<List<T>> product(List<List<T>> lists) {
            List<List<T>> result = new ArrayList<>();
            result.add(new ArrayList<>());
            
            for (List<T> list : lists) {
                List<List<T>> temp = new ArrayList<>();
                for (List<T> r : result) {
                    for (T t : list) {
                        List<T> newList = new ArrayList<>(r);
                        newList.add(t);
                        temp.add(newList);
                    }
                }
                result = temp;
            }
            return result;
        }

        // Generate combinations with replacement
        public static <T> List<List<T>> combinations_with_replacement(List<T> list, int r) {
            List<List<T>> result = new ArrayList<>();
            combinationsWithReplacementHelper(list, r, 0, new ArrayList<>(), result);
            return result;
        }
        
        private static <T> void combinationsWithReplacementHelper(List<T> list, int r, int start, List<T> current, List<List<T>> result) {
            if (current.size() == r) {
                result.add(new ArrayList<>(current));
                return;
            }
            for (int i = start; i < list.size(); i++) {
                current.add(list.get(i));
                combinationsWithReplacementHelper(list, r, i, current, result);
                current.remove(current.size() - 1);
            }
        }
    }

    // Example usage of Itertools methods
    public static void itertoolsExamples() {
        System.out.println("Range: " + Itertools.range(0, 10, 2));
        
        List<Integer> numbers = Arrays.asList(1, 2, 3);
        System.out.println("Permutations: " + Itertools.permutations(numbers));
        
        System.out.println("Combinations (n=3, r=2): " + Itertools.combinations(numbers, 2));
        
        List<List<String>> lists = Arrays.asList(
            Arrays.asList("A", "B"),
            Arrays.asList("X", "Y"),
            Arrays.asList("1", "2")
        );
        System.out.println("Cartesian product: " + Itertools.product(lists));

        System.out.println("Combinations with replacement (n=3, r=2): " + Itertools.combinations_with_replacement(numbers, 2));
    }
}