Permutations of a String in Java

Permutations.java

static class Permutations {

        public static void main(String... args) {
            System.out.println("Permutations of 'abcd'");

            System.out.println(Permutations.of("abcd"));
        }

        public static List<String> of(String string) {
            if (string == null) {
                throw new IllegalArgumentException("string cannot be null.");
            }

            List<String> permutations = new LinkedList<String>();

            if (string.length() == 0) {
                permutations.add("");
                return permutations;
            }

            List<Character> chars = new ArrayList<Character>(string.length());

            for (char character : string.toCharArray()) {
                chars.add(character);
            }

            Character firstChar = string.charAt(0);
            String remainder = string.substring(1);

            List<String> permutationsOfRemainder = Permutations.of(remainder);

            for (String permutation : permutationsOfRemainder) {
                for (int i = 0; i <= permutation.length(); i++) {
                    permutations.add(insertCharAt(permutation, firstChar, i));
                }
            }

            return permutations;
        }

        private static String insertCharAt(String baseString,
                Character charToInsert, int index) {
            
            String start = baseString.substring(0, index);
            String end = baseString.substring(index);
            
            return start + charToInsert + end;
        }
 }