Inheritance and association relationships in UML

Question

I was creating a class model using inheritance, and have a situation I don't know how to represent.

For this question I built an example diagram in which I have a class Person with some attributes.

I also have a class Class1 that needs to use objects of a new class with Person's attributes plus sex and birthDate attributes, so I create a Person2 class which inherits from Person.

But I also have another class Class2 which needs to use objects with only the Person attributes, not Person2. I can't use an association relationship because Person is an abstract class.

How should I handle this situation?

Answer 1

First of all, your diagram does not correspond to the narrative:

• In your diagram, Person inherits from Person2. So Person2 would be an anonymous person without any name. Not very useful.

  To letPerson2 inherit from Person (in UML-speak we say that Person2 is a specialization of Person), you must have the triangle arrow head on the side of Person. Note that Person2 has a name, but it's not directly accessible: it can only be accessed via public methods of Person, since it's private.

• Your diagram does not give any clue about Person being abstract. The usual way is to write the name of abstract classes in italic.

After correction, it would look like:

Class2 may perfectly be associated with an abstract class. The only thing that Class2 may not do in this case, is to instantiate (i.e. create an object of) the Person. But it may very well get the Person object from somewhere else. For example: since Person2 is a Person, a Class1 object may invoke a method of a Class2 object passing person as a Person parameter. Or Class2 could use an injected factory that would create instances of concrete specializations of Person.

There are plenty of practical uses of associations with abstract classes. For example, it's used for making observers, or the more complex interpreter pattern.