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;

}

}