UML Weekend Crash Course™ - To Parent Directory

Session 12—Applying the Class Diagram to the Case Study 135
The next question is how to model the use of a pattern in the UML. The UML refers to a
pattern as a collaboration (not to be confused with a Collaboration diagram). In Figure 12-2,
the collaboration is rendered as an oval shape containing the pattern name. The classes are
drawn around it. Then you draw dashed lines from the collaboration to the classes that
implement the pattern. At the end of the dashed lines next to the classes, you place the
role that the class plays in the implementation of the pattern. In Figure 12-2, the Shipment
plays the role of observable and the Order plays the role of observer.
Observer
Pattern
observable
observer
Shipment
Figure 12-2
is observed by
0..* 1..1
Observer pattern example in a Class diagram
Order
Using Design Patterns in the Class Diagram
Another common problem in applications is managing complex state-specific behavior. If you
haven’t worked much with states, that may sound complicated. But you may have worked
around this problem without realizing it. Have you ever written a method in which you
found you had to use a lot of conditional logic like if statements or case statements You
couldn’t determine how the method should work until you first checked on the condition of
various attributes in the object. Well, an object’s state is nothing more than the current values
of its attributes. So when you checked the attribute values in the conditional logic, you
were actually checking the object’s state.
When the implementation of the methods depends on changes in the attribute values
(state changes), the code can quickly become very complicated to write and difficult to
maintain. The state design pattern offers a solution that both makes the initial writing
easier and substantially reduces the maintenance costs.
Table 12-3 describes the state design pattern.