public class NISSValidator {

    private NISSValidator() {
    }

    /**
     * Validates if a given string niss is valid.
     *
     * @param niss number to validate
     * @return true if the input is valid and false otherwise
     */
    public static boolean isValid(String niss) {
        return hasValidLength(niss) &&
            isNumeric(niss) &&
            hasValidFirstDigit(niss) &&
            hasValidControlDigit(niss);
    }

    private static boolean hasValidLength(String niss) {
        return !(niss == null || niss.length() != 11);
    }

    private static boolean isNumeric(String str) {
        for (char c : str.toCharArray()) {
            if (!Character.isDigit(c)) {
                return false;
            }
        }
        return true;
    }

    private static boolean hasValidFirstDigit(String niss) {
        return niss.charAt(0) == '1' || niss.charAt(0) == '2';
    }

    /**
     * Validates the niss intigrity.
     * Sums all the niss digits multiplied by factors array.
     * The niss is valid if 9 minus the reminder of the division
     * of niss factors by 10 is equal to the niss check digit.
     *
     * @param niss digits to validate
     * @return true if the control digit is valid and false otherwise.
     */
    private static boolean hasValidControlDigit(String niss) {
        int[] nissArray = convertToIntegerArray(niss);
        final int[] FACTORS = {29, 23, 19, 17, 13, 11, 7, 5, 3, 2};
        int sum = 0;
        for (int i = 0; i < FACTORS.length; i++) {
            sum += nissArray[i] * FACTORS[i];
        }
        return nissArray[nissArray.length - 1] == (9 - (sum % 10));
    }

    private static int[] convertToIntegerArray(String niss) {
        final int[] ints = new int[niss.length()];
        for (int i = 0; i < niss.length(); i++) {
            ints[i] = Integer.parseInt(String.valueOf(niss.charAt(i)));
        }
        return ints;
    }
}