[tigershark]

You can always create an Email type or a CreditCartExpiry type that validate in the constructor if the string contains @ or if the date is in the future or in the past. In this way you are guaranteed that you cannot pass around an invalid email address or an invalid credit card. Obviously in Java nothing guarantees that you won't forget to handle the failed state, while in languages with exhaustive pattern matching is pretty much granted if you write idiomatic code.