UML Weekend Crash Course™ - To Parent Directory

58
Saturday Morning
Customer
Withdraw Cash
Update Account
Withdraw Cash
with Overdraft
Protection
Protect Overdraft
Figure 5-9
dependency notation for the Use Case diagram
Generalization
Inheritance is a key concept in object-oriented programming, and OO analysis and design.
Inheritance tells us that one object has, at the time of its creation, access to all the properties
of another class, besides its own class. Thus, the created object incorporates all those
properties into its own definition. In layman’s terms, we say things like, “A Ford Explorer is
a car.” A car is a well-defined general concept. When you create a Ford Explorer, rather than
redefine all the car properties, you simply “inherit” or assimilate all the existing car properties,
then override and/or add any new properties to complete the definition of your new
Ford Explorer object.
The same idea, applied to actors and to Use Cases, is called generalization, and often goes
by the nickname, an “is a” relationship. A Senior Bank Teller is a Bank Teller with additional
authority and responsibilities. The “Withdraw Cash with Overdraft Protection” Use Case is a
more extensive requirement than the “Withdraw Cash” Use Case.
To model generalization, the UML uses a solid line with a hollow triangle. It looks a bit
like an arrow, but be careful not to confuse the two. The triangle is always on the end near
the item that is being inherited. In the examples mentioned earlier, the triangle would be
near “Bank Teller” and “Withdraw Cash,” as shown in Figure 5-10.
Customer
Withdraw Cash
Update Account
Withdraw Cash
with Overdraft
Protection
Protect Overdraft
Figure 5-10
Generalization notation for the Use Case diagram