BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
    String fileName = reader.readLine();
    FileInputStream inputStream = new FileInputStream(fileName);
    Map<Integer, Integer> map = new HashMap<>();
    int data;
    Integer max = 0;
    while (inputStream.available() > 0) {
        data = inputStream.read();
        if (map.containsValue(data)) map.replace(data, map.get(data) + 1);
        else map.put(data, 1);
        if (map.get(data) > max) max = map.get(data);
    }
        inputStream.close();
    for (Map.Entry<Integer, Integer> entry : map.entrySet()){
        if(entry.getValue() > max) {
            System.out.print(entry.getKey() + "/t");
        }
    }

    }
}

if (map.containsValue(data)) ...

int counterForData = 0;
if (map.contains(data)) {
  counterForData = map.get(data);
}
map.put(data, counterForData+1);

if (map.containsValue(data)) map.replace(data, map.get(data) + 1);
else map.put(data, 1);

map.compute(data, (k, v) -> (v == null) ? 1 : v + 1);

Map<Byte, Integer> map = new HashMap<>();
int maxCount = 0;
//byte maxByte = 0;
int data;
while ((data = inputStream.read()) >= 0) {
    byte b = (byte)data;
    int count = map.getOrDefault(b, 0) + 1;
    map.put(b, count);
    if (count > maxCount) {
        maxCount = count;
        //maxByte = b;
    }
}
for (Map.Entry<Integer, Integer> entry : map.entrySet()){
    if (entry.getValue() == maxCount) {
        System.out.print(entry.getKey() + "t");
    }
}
System.out.println();