input validation

/**
     * Confirms that input from user is valid for registration
     * @param invoice_address Input array
     * @return String with specific error message if invalid input, null if valid input
     */
    static public String validate_input (ArrayList<String> invoice_address) {

        // Here I assume that fields that hasn't been entered is considered empty Strings
        // rather than null objects. If that was the case, then null checking has to be performed:
        // If String is null, then set to empty String (which results in error message)

        int countryInt;
        String address = invoice_address.get(0);
        String zipCode = invoice_address.get(1);
        String countryID = invoice_address.get(2);
        String address2 = invoice_address.get(3);
        String city = invoice_address.get(4);

        // Since TechShip has customers from all over the world, with different languages and
        // address formats, I opted for simply checking the length of the String rather than
        // matching with regular expressions. One improvement would be allowing special
        // characters but no digits

        // Confirm that address is at least three characters long
        if (address.length() < 3 )
            return "Please enter valid primary address";

        // Confirms that zip code is only digits and at least three numbers
        if (!zipCode.chars().allMatch(Character::isDigit) || zipCode.length() < 3)
            return "Please enter valid zip code";

        // Parsing countryID String to int. If null, set to 0 (results in error message)
        try {
            if(countryID != null)
                countryInt = Integer.parseInt(countryID);
            else
                countryInt = 0;
        } catch (NumberFormatException e) {
            countryInt = 0;
        }

        // Confirms that a country in the list has been selected (range of 1-250)
        if (countryInt < 1 || countryInt > 250)
            return "Please select country";

        // Confirms that Address 2 field is either empty or at least three characters
        if (!address2.isEmpty() && address2.length() < 3)
            return "Please enter valid secondary address";

        // Confirms that city is at least three characters
        if (city.length() < 3)
            return "Please enter valid city";

        // Returns null if valid input
        return null;
    }
}