UML Weekend Crash Course™ - To Parent Directory

126
Saturday Afternoon
Dog
Discriminator
Breed
Collie Spaniel Setter
Discriminator
Sub-breed
English Springer
Spaniel
Dwarf
Spaniel
Japanese
Spaniel
Cocker
Spaniel
Figure 11-9
Step 4: Further specialization — subclassing a subclass
REVIEW
Associations identify the fact that types of objects can work together. Two specialized forms
of association, aggregation and composition, tighten the rules about how the objects work
together.
Aggregation models assemblies or configurations of objects. You identify an aggregation
association with a hollow diamond on the end of the association.
Composition is a form of aggregation. Composition says that the member objects
may not exist outside of the assembly. You identify a composition association with
a solid diamond on the end of the association.
Generalization is not a form of association. It is often called the “is a” relationship.
Generalization allows you to break apart a class definition according to the similarities
and differences in the objects that the class represents. You identify an inheritance
or generalization relationship with a hollow triangle at the superclass end of
the relationship.
A superclass contains shared properties, common to many types of objects within
the same class. A subclass contains only the properties unique to one type of object
within the class.
Abstract classes cannot create objects because their definition is missing a method
for at least one operation. Concrete classes can create objects because they have
access to a method for every operation.
The discriminator describes the attribute(s) or rule that you used to choose the subclasses.
You place the discriminator on the relationship line between the superclass
and the subclasses.