Ultimately, this design is to be implemented into code to get the actual applications running.   Let us first talk about association. This here is a simple unidirectional association. ClassA has an association to ClassB but not vice versa. So ClassA has a reference of ClassB. By having a reference, it basically means that ClassB is an attribute of ClassA. Just remember that association, aggregation and composition are all “has a” relationship which is simply object composition.