Валидатор

package com.javarush.task.task18.task1803;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

/*
Самые частые байты
*/

public class Solution {
    public static void main(String[] args) throws Exception {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String fileName = br.readLine();
        br.close();
        FileInputStream fr = new FileInputStream(fileName);
        ArrayList<Integer> bayt = new ArrayList<>();
        HashMap<Integer, Integer> countbayt = new HashMap<>();


        while (fr.available() > 0){
            bayt.add(fr.read());
        }
        fr.close();

        int count = 0;


        for (int i = 0; i < bayt.size(); i++){
            for (int k = 0; k < bayt.size(); k++){

            if (bayt.get(i).equals(bayt.get(k))){
                count++;
            }

            }
            countbayt.put(bayt.get(i), count);

            count = 0;
        }

        int max = 1;
        for (Map.Entry<Integer, Integer> element : countbayt.entrySet()){
            if (element.getValue() > max) max = element.getValue();
            }

        for (Map.Entry<Integer, Integer> element : countbayt.entrySet()){
            if (element.getValue().equals(max)) System.out.print(element.getKey());
        }

    }
}