Question

I am currently coursing Computer Engineering and I remember a professor of a class called Introduction to Informational Systems saying that two classes related by a 1:1 cardinality does not make sense.

For example: I have the Client class and the Telephone class. Let’s supose that the client can only have one phone. The professor said that does not make sense creating the Telephone class, and telephone should be an attribute of the Client class. I absolutely agree with him.

But now I’m taking the Software Engineering class and the professor (not the same) did not make any comments about this issue, and now I’m really confused about this.

What is the correct approach?

Answer 1

I would say your Introduction to Information Systems professor was correct. And your SE professor, too (assuming his lack of comments makes him a contrarian). They are each right depending on your requirements and the domain you’re working with. But without any other details, it’s hard to model this for you, and I would lean towards what your CE professor had said. Keep in mind all those fun little principles you learned: KISS, DRY, etc., and apply them to your problem.

If Client will never ever possibly have more than one telephone number and no other entity in your domain needs a telephone number, then a separate Telephone class isn’t necessary. In the real world, if your requirements are vague, find out more information from your client.

If somebody down the road decides Clients can take on more than one telephone number, or another entity is introduced into your domain that needs a telephone number, this is a fairly easy refactoring to accomplish.

So with that in mind, let’s say your Client had a separate Address class that included the telephone number instead. Maybe that Address class gets re-used by another class, maybe Invoice or Shipment, where an Address could be shared or applied in both cases. In this example, you might want Address (Telephone) to be its own class.

In your example, Telephone might be a little too contrived. You’d want it to be a separate class for re-use if it had many properties (AreaCode, InternationalPrefix, Number, etc.), but if Client just needed a string-value called Telephone that a user would be typing in merely for reference, then it probably doesn’t make sense to be its own class.