JavaAlgorithms/Maths/ParseInteger.java

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;
  }
}