I don't understand what line 9 of the code does 
byteCountArray[fileInputStream.read()] += 1;
What does that operation do? i understand that the array takes the values of array + 1 but what does it add? the value at index fileInputStream.read() + 1? what is the purpose of that addition? 
public static void main(String[] args) throws Exception {
        BufferedReader bf = new BufferedReader(new InputStreamReader(System.in));
        String fileName = bf.readLine();
        bf.close();

        int[] byteCountArray = new int[256];
        try (FileInputStream fileInputStream = new FileInputStream(fileName)) {
            while (fileInputStream.available() > 0) {
                byteCountArray[fileInputStream.read()] += 1;
            }
            fileInputStream.close();
        }
        int maxCount = 0;
        for (int byteCount : byteCountArray) {
            if (byteCount > maxCount) maxCount = byteCount;
        }
        ArrayList<Integer> resultList = new ArrayList<>();
        for (int i = 0; i < byteCountArray.length; i++) {
            if (byteCountArray[i] == maxCount) resultList.add(i);
        }
        for (Integer resultItem : resultList) System.out.print(resultItem + " ");

    }
}