public static void main(String[] args) {
            String input = "123451212111";
            Integer integer = input.chars()
                    .boxed()
                    .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()))
                    .entrySet()
                    .stream()
                    .max(Comparator.comparing(Map.Entry::getValue))
                    .map(entry -> entry.getKey())
                    .get();

            System.out.println(Character.toChars(integer));
        }