public class StringToInt { //no API public static void main(String[] args) { String stringNumber = "1234567"; char[] ch = stringNumber.toCharArray(); int sum = 0; //get ascii value for zero int zeroAscii = '0'; for (char c : ch) { sum = (sum * 10) + ((int) c - zeroAscii); } System.out.println(sum); } }