Finding Most Common Character In A String Using For Loop

import javax.swing.*;
public class mostFrequent{
public static void main(String args[]){
char index;
String s;
s = JOptionPane.showInputDialog("Enter String here");
int currFrequency = 0;
for(index = 0; index<s.length(); index = index++){
int i = 0;
for(i = 'A'; i<='Z'; i++){
if(i==s.charAt(index)){
currFrequency = currFrequency + 1;
}
}
}
System.out.println("end");
}
}
//my code so far