Где ошибка? В "хотелке" валидатора сделать через TreeMap?
public class Solution {
    public static void main(String[] args) throws Exception {
        try (BufferedReader reader = new BufferedReader(new InputStreamReader(System.in))) {
            try (FileInputStream stream = new FileInputStream(reader.readLine())) {
                ArrayList<Integer> bytesList = new ArrayList<>();

                while (stream.available() > 0) {
                    if (!bytesList.contains(stream.read())) {
                        bytesList.add(stream.read());
                    }
                }

                Collections.sort(bytesList);

                for (Integer b : bytesList) {
                    System.out.print(b + " ");
                }
            }
        }
    }
}