Validates a swedish social security number

SSNValidatorUtils.java

/**
 * Validates a Swedish social security number.
 *
 * @author Johan Stenberg
 */
public class SSNValidatorUtils extends AbstractUtils {

    private SSNValidatorUtils() {
    }

    public static boolean validateSSN(String ssn) {
        if (ssn == null || ssn.length() != 13 || ssn.indexOf('-') != 8) {
            return false;
        }

        String pattern = "^[12][90][0-9]{6}-[0-9]{4}";
        if (!ssn.matches(pattern)) {
            return false;
        }

        int year = Integer.parseInt(ssn.substring(0, 4));
        int month = Integer.parseInt(ssn.substring(4, 6));
        int day = Integer.parseInt(ssn.substring(6, 8));

        boolean validMonthAndDate = validateMonthAndDate(month, day, isLeapYear(year));
        if (!validMonthAndDate) {
            return false;
        }

        int checkSumDigit = Integer.parseInt(ssn.substring(ssn.length() - 1, ssn.length()));

        String ssnModifiedForCheckSum = ssn.substring(2, 8) + ssn.substring(9, ssn.length() - 1);
        return validCheckSum(ssnModifiedForCheckSum, checkSumDigit);
    }

    private static boolean validateMonthAndDate(int month, int day, boolean isLeapYear) {
        switch (month) {
            case 1:
            case 3:
            case 5:
            case 7:
            case 8:
            case 10:
            case 12:
                return day >= 1 && day <= 31;
            case 4:
            case 6:
            case 9:
            case 11:
                return day >= 1 && day <= 30;
            case 2:
                return day >= 1 && day <= (isLeapYear ? 29 : 28);
            default:
                return false;
        }
    }

    private static boolean isLeapYear(int year) {
        return (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0);
    }

    private static boolean validCheckSum(String ssnModifiedForCheckSum, int checkSumDigit) {
        int sum = 0;
        for (int i = 0; i < ssnModifiedForCheckSum.length(); ++i) {
            int j = Character.getNumericValue(ssnModifiedForCheckSum.charAt(i));
            j *= (i % 2 == 0) ? 2 : 1;

            sum += j - (j > 9 ? 9 : 0);
        }

        int calcedCheckSum = 10 - (sum % 10);

        return (calcedCheckSum == 10 ? 0 : calcedCheckSum) == checkSumDigit;
    }
}