java - How to increase charAt value inside a while loop -

i'm trying find way increase value using charat, want able update while inside while loop. below current code have. have tried recreating variable inside of loop , adding ++ end increase value, receive unexpected type errors.

i trying have take value enter (i.e. 3124) , concatenate each value , add them together. based on example, give value of 10.

public static void main(string[] args) {  int num, counter, i; string str1;   str1 = joptionpane.showinputdialog("enter number: "); num = str1.length();  counter = 0; = integer.parseint(character.tostring(str1.charat(0))); numtot = 0;  while (counter <= num) {   numtot = i;   counter++;   = integer.parseint(character.tostring(str1.charat(0++)));   }       system.exit(0); } 

some modifications program :

public static void main (string[] args) throws java.lang.exception {     int num, counter, i;     string str1;     int numtot = 0;      str1 = "3124";     num = str1.length();      counter = 0;     while (counter < num)     {           = integer.parseint(character.tostring(str1.charat(counter)));         numtot = numtot + i;         counter++;     }      system.out.println(numtot); } 

here's working version