public static char getMax(String s) {

char maxappearchar = ' ';
int counter = 0;
int[] ascii_count = new int[128];  // fast path for ASCII
HashMap<Character,Integer> nonascii_count = new HashMap<Character,Integer>();

for (int i = 0 ; i < s.length() ; i++)
{
    char ch = s.charAt(i);  // This does appear to be the recommended way to iterate over a String
    // alternatively, iterate over 32bit Unicode codepoints, not UTF-16 chars, if that matters.
    if (ch < 128) {
        ascii_count[ch]++;
    } else {
        // some code to set or increment the nonascii_count[ch];
    }
}

// loop over ascii_count and find the highest element
// loop over the keys in nonascii_count, and see if any of them are even higher.
return maxappearchar;

}

aus https://stackoverflow.com/questions/31990103/more-effective-method-for-finding-the-most-common-character-in-a-string