public InvalidCreditCardNumberException(CreditCard creditCard) {
   super(
       creditCard,
       "Credit card number " + (creditCard != null ? creditCard.toString() : "") + " is invalid.");
 }
 @Test
 public void testTOString() {
   CreditCard creditCard = createCreditCard();
   Assert.assertEquals(creditCard.toString().length() == 0, false);
 }