Java - Write code to get the maximum occurring character in inputString

Requirements

Write code to get the maximum occurring character in inputString

Demo

//package com.book2s;

public class Main {
    public static void main(String[] argv) {
        String inputString = "book2s.com";
        System.out.println(getMaximumOccurringCharacter(inputString));
    }//from ww  w .ja  v  a 2s.  com

    /***
     * getMaximumOcurringCharacter(final String inputString)
     * get the maximum occurring character in inputString
     * @param inputString
     * @return
     */
    public static char getMaximumOccurringCharacter(final String inputString) {

        if (inputString != null && inputString.length() != 0) {
            final char[] inputStringArray = inputString.toCharArray();
            int countChar[] = new int[59];
            int maxOccurance = 0;
            int charToReturn = 0;
            int index = 0;
            for (char i : inputStringArray) {
                index = (int) i - 64;
                countChar[index] = (countChar[index]) + 1;
                if (maxOccurance < countChar[index]) {
                    maxOccurance = countChar[index];
                    charToReturn = index + 64;
                }
            }
            return (char) charToReturn;
        } else {
            throw new NullPointerException();
        }
    }
}