package Maths;

public class ParseInteger {
 public static void main(String[] args) {
 assert parseInt("123") == Integer.parseInt("123");
 assert parseInt("-123") == Integer.parseInt("-123");
 assert parseInt("0123") == Integer.parseInt("0123");
 assert parseInt("+123") == Integer.parseInt("+123");
 }

 /**
 * Parse a string to integer
 *
 * @param s the string
 * @return the integer value represented by the argument in decimal.
 * @throws NumberFormatException if the {@code string} does not contain a parsable integer.
 */
 public static int parseInt(String s) {
 if (s == null || s.length() == 0) {
 throw new NumberFormatException("null");
 }
 boolean isNegative = s.charAt(0) == '-';
 boolean isPositive = s.charAt(0) == '+';
 int number = 0;
 for (int i = isNegative ? 1 : isPositive ? 1 : 0, length = s.length(); i < length; ++i) {
 if (!Character.isDigit(s.charAt(i))) {
 throw new NumberFormatException("s=" + s);
 }
 number = number * 10 + s.charAt(i) - '0';
 }
 return isNegative ? -number : number;
 }
}