UML Distilled: A Brief Guide to the Standard Object Modeling Language, Second Edition

Reference Objects and Value Objects

One of the common things said about objects is that they have identity. This is true, but it is not quite as simple as that. In practice, you find that identity is important for reference objects, but not so important for value objects.

Reference objects are things like Customer. Here, identity is very important, because you usually want only one software object to designate a customer in the real world. Any object that references a Customer object will do so through a reference or pointer; all objects that reference this Customer will reference the same software object. That way, changes to a Customer are available to all users of the Customer.

If you have two references to a Customer and you wish to see whether ...