UML Weekend Crash Course™ - To Parent Directory

Session 11—The Class Diagram: Aggregation and Generalization 123
A subclass is a class that contains some combination of attributes, operations, and associations
that are unique to a type of object that is partially defined by a superclass. In
Figure 11-5, Apple, Watermelon, Orange, RedDelicious and GrannySmith are all examples
of subclasses. Note that a class may be both a superclass and a subclass.
The term subclass reflects the concept of subset. The subset, or subclass, contains a
unique set of properties for only certain objects within the set. The GrannySmith class in
Figure 11-5 would only contain the properties that are unique to GrannySmith apples. A
GrannySmith object would get the rest of the information about the properties it shares
with all apples from the Apple superclass, and all the properties it has in common with all
fruits from the Fruit superclass. In other words, it actually takes three classes to generate a
single object representing a GrannySmith apple.
An abstract class is a class that cannot create objects (cannot be instantiated). Any
superclass that defines at least one operation that does not have a method is said to be
abstract, or lacking a complete definition. Only a superclass can be abstract.
A concrete class is a class that has a method for every operation, so it can create objects.
The methods may be defined in the class or inherited from a superclass. All classes at the
bottom of a generalization hierarchy must be concrete. Any superclass may be concrete.
A discriminator is an attribute or rule that describes how I choose to identify the set of
subclasses for a superclass. If I wanted to organize the information about types of cars, I
could discriminate based on price range, manufacturer, engine size, fuel type, usage, or any
number of other criteria. The discriminator I choose depends on the problem I am trying to
solve. In Figure 11-6, I use a very common practice. I identify a set of predefined types
(that is, type of fruit and varieties within types). Other times I might use a property of
the objects themselves like size, price range, capacity, or age.
Fruit
Discriminator
Fruit Type
Apple Watermelon Orange
Discriminator
RedDelicious
Fruit Variety
GrannySmith
Figure 11-6 Modeling generalization with discriminators
The composition of a class reveals the possible discriminating properties. Classes define
properties of objects such as attributes, operations, and association. These are the first
three possible discriminating properties. If objects of the class share the same attributes,