UML Weekend Crash Course™ - To Parent Directory

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Saturday Afternoon
Fruit
Apple Watermelon Orange
RedDelicious
GrannySmith
Figure 11-5 Modeling generalization
To express the same thing in words, I might say, “A ‘Red Delicious’ is a type of apple, and
an apple is a type of fruit.” For this reason, you sometimes hear generalization called the
“is a” relationship (that is, every red delicious object “is an” apple object and every apple
object “is a” fruit object).
This unique relationship between classes in a generalization raises an interesting problem.
In Session 9, I described the concept of visibility. Visibility determines which other objects
can see the attribute or operation. Normally, attributes are set to private so that only objects
of the same class may see them. But what would happen to the RedDelicious class in Figure
11-5 if the attributes of the Apple class were set as private RedDelicious would not have
access to them so inheritance is in essence short-circuited. Another visibility, called protected
visibility, is defined to handle just this situation. The protected visibility allows only objects
of the same class or subclasses to see the element.
Note
There are variations in the implementation of the protected visibility
between programming languages. Be sure to check the language manual for
the correct interpretation.
Elements of generalization
Because the generalization (also called an inheritance) relationship is not a form of association,
there is no need for multiplicity, roles, and constraints. These elements are simply
irrelevant.
To draw a generalization relationship, we first need to define superclass, subclass,
abstract class, concrete class, and discriminator. A superclass is a class that contains some
combination of attributes, operations, and associations that are common to two or more
types of objects that share the same purpose. Fruit and Apple are examples of superclasses.
The term superclass reflects the concept of superset. The superset or superclass in this case
contains the traits that are common to every object in the set.