List

 public int[] solution(String my_string) {
        char[] original = my_string.toCharArray();
        List<Character> answer = new ArrayList<>();
        for(char c : original){
            if(48<=c&&c<=57){
                answer.add(c);
            }
        }
        
        Collections.sort(answer);

 

ArrayList를 만들고,   

Collections.sort(ArrayList변수); -> 오름차순 정렬

Collections.sort(list, Collections.reverseOrder()); -> 내림차순 정렬