Надо воспользоваться стринг билдером?
package com.javarush.task.task18.task1805;
import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.InputStreamReader;
import java.util.*;
/*
Сортировка байт
*/
public class Solution {
public static void main(String[] args) throws Exception {
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
try (FileInputStream fis = new FileInputStream(reader.readLine())){
List<Integer> list = new ArrayList<>();
while (fis.available() > 0){
list.add(fis.read());
}
Collections.sort(list);
for (Integer art: list) {
System.out.print(art + " ");
}
}
}
}