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chaPter 10 • object-oriented systems analysis and design Using Uml 279

of the same method with the same name within a superclass/subclass relationship. The subclass
inherits a parent method but may add to it or modify it. The subclass may change the type of
data, or change how the method works. For example, there might be a customer who receives
an additional volume discount, and the method for calculating an order total is modified. The
subclass method is said to override the superclass method.
When attributes or methods are defined more than once, the most specific one (the lowest
in the class hierarchy) is used. The compiled program walks up the chain of classes, looking for
methods.
aBstraCt ClassEs. Abstract classes are general classes and are used when gen/spec is included
in the design. The general class becomes the abstract class. The abstract class has no direct
objects or class instances, and is only used in conjunction with specialized classes. Abstract
classes usually have attributes and may have a few methods.
Figure 10.20 is an example of a gen/spec class diagram. The arrow points to the general
class, or superclass. Often the lines connecting two or more subclasses to a superclass are joined
using one arrow pointing to the superclass, but these could be shown as separate arrows as well.
Notice that the top level is Person, representing any person. The attributes describe qualities that
all people at a university have. The methods allow the class to change the name and the address
(including telephone and email address). This is an abstract class, with no instances.
Student and Employee are subclasses because they have different attributes and methods.
An employee does not have a grade point average and a student does not have a salary. This is a

Person Figure 10.20
–lastName A gen/spec diagram is a refined
–firstName form of a class diagram.
–street
–apartment
–city
–state
–zip
–telephone
–cellPhone
–email
+changeAddress( )
+changeName( )
canbea canbea
isa isa

Student Employee
–studentNumber –employeeNumber
–creditsCompleted –salary
–gradePointAverage –YTDGross
–department –YTDWithholding
–major –dateHired
–minor –department
+changeStudent( ) +changeEmployee( )
+findStudent( ) +printTaxInformation( )
+graduateStudent( ) +producePaycheck( )
+initialize( )
+studentComplete( ) canbea canbea
+viewStudent( )
isa isakinda

Faculty Administrator
–degree –title
–position +changeEmployee( )
–webURL
+changePosition( )
+changeURL( )