1. SOEN 343 Software Design
Computer Science and Software Engineering Department
Concordia University
Fall 2004
Instructor: Patrice Chalin

2. Agenda – Lecture 12a Review & renew. GRASP: protected variations. GoF:
Adapter
Façade
Factory
Strategy
Observer
2/28/2019
SOEN 343, © P.Chalin,

3. GRASP Information Expert. Creator. High Cohesion. Low Coupling.
Controller.
Polymorphism.
Pure Fabrication.
Indirection.
Protected Variations.
2/28/2019
SOEN 343, © P.Chalin,

4. GRASP Protected Variations
Problem: How to design objects, subsystems, and systems so that the variations or instability in these elements does not have an undesireable impact on other elements?
Solution: Identify points of predicted variation or instability; assign responsibility to create a stable interface around them.
2/28/2019
SOEN 343, © P.Chalin,

5. Core PV Mechanisms Encapsulation. Interfaces. Polymorphism.
Indirection, …
(Note: we are speaking of mechanisms, not principles)
2/28/2019
SOEN 343, © P.Chalin,

6. PV: Pick Your Battles
Beware not to try to overly “future-proof” your designs.
Actually, this is true of any principle …
2/28/2019
SOEN 343, © P.Chalin,

7. Gang Of Four Gamma, Helm, Johnson, Vlissides
Some patterns covered in Larman, Chap. 23,…
All patterns in XDE
As documentation.
As dynamic templates.
Erich
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SOEN 343, © P.Chalin,

8. GoF Pattern Summary (& Relationhips)
[Picutre (c) GoF CD]
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SOEN 343, © P.Chalin,

9. GoF Pattern Classification
Behavioral Patterns
Creational Patterns
Structural Patterns
2/28/2019
SOEN 343, © P.Chalin,

10. GoF Behavioral Patterns
Chain of Responsibility
Command
Interpreter
Iterator
Mediator
Memento
Observer
State
Strategy
Template Method
Visitor
2/28/2019
SOEN 343, © P.Chalin,

11. GoF Creational Patterns
Abstract Factory
Builder
Factory Method
Prototype
Singleton
2/28/2019
SOEN 343, © P.Chalin,

12. GoF Structural Patterns
Adapter
Bridge
Composite
Decorator
Facade
Flyweight
Proxy
2/28/2019
SOEN 343, © P.Chalin,

13. Adapter
Context / problem How to resolve incompatible interfaces, or provide a stable interface to similar components with different interfaces?
Solution: Convert the original interface of a component into another interface, through an intermediate adapter object.
2/28/2019
SOEN 343, © P.Chalin,

14. Adapter
Suppose we have a tax calculation class (or external library) but the interface is not well suited for our application.
2/28/2019
SOEN 343, © P.Chalin,

15. Adapter Adapter provides an interface suited to the application
GoodAsGoldTaxProAdapter
getTaxes( Sale ) : List of TaxLineItems
computeTax(…):double
GoodAsGoldTaxPro
Adapter provides an interface suited to the application
2/28/2019
SOEN 343, © P.Chalin,

16. Adapter (For More than One Class)
What if more than one class (library) needs to be adapted?
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SOEN 343, © P.Chalin,

17. Adapter
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SOEN 343, © P.Chalin,

18. Facade
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SOEN 343, © P.Chalin,

19. Facade
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SOEN 343, © P.Chalin,

20. Factory
Context / problem: Who should be responsible for creating objects when there are special considerations, such as complex creation logic, a desire to separate the creation responsibilities for better cohesion, and so forth?
Solution: Create a Pure Fabrication object called a Factory.
2/28/2019
SOEN 343, © P.Chalin,

21. Factory Example SOEN 343, © P.Chalin,
Have we seen code like this anywhere?
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SOEN 343, © P.Chalin,

22. Larman’s comment on prev. figure
Note that the factory methods return objects types to an interfacre rather than a class so that the factory can return any implementation of the interface.
Factory methods can also
2/28/2019
SOEN 343, © P.Chalin,

23. (Abstract) Factory Example (GoF)
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SOEN 343, © P.Chalin,

24. FrontControllerServlet RemoveStudentCommand
Factory (in EAs)
FrontControllerServlet
FrontCommand
# processRequest ( )
+ init ( )
- getCommand ( ) : FrontCommand
+ processRequest ( )
- getCommandClass ( )
RemoveStudentCommand
ViewStudInfoCommand
+ processRequest ( )
+ processRequest ( )
2/28/2019
SOEN 343, © P.Chalin,

25. Singleton (Larman Section 23.4) XDE Patterns Singleton
- uniqueInstance : Singleton
- attribute
# Singleton( )
+ getUniqueInstance ( ) : Singleton
+ getAttr( )
2/28/2019
SOEN 343, © P.Chalin,

26. Strategy
Context / problem: How to design for varying, but related, algorithms or policies? How to design for the ability to change (even dynamically) these algorithms or policies?
Solution: Define each algorithm/policy/strategy in a separate class with a common interface
2/28/2019
SOEN 343, © P.Chalin,

27. Strategy
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28. How Do We Create a Strategy?
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SOEN 343, © P.Chalin,

29. Composite Larman 23.7 (but explanation intertwined with other pattern)
Context / problem: How to treat an aggregate structure of objects the same way as an atomic (non-aggregate) object?
Solution: Define classes for composite and atomic objects that implement the same interface.
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SOEN 343, © P.Chalin,

30. Observer Pattern
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SOEN 343, © P.Chalin,

31. Observer How shall we have the display be updated?
Why not … have the Sale inform the display when it changes value.
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SOEN 343, © P.Chalin,

32. What is Wrong With This?
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33. Presentation Domain Data Source Layers: Dependencies Dependencies
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34. Layers: Functionality
Presentation
Domain
Functionality / services
Data Source
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SOEN 343, © P.Chalin,

35. Observer Pattern
Context / Problem: Different kinds of subscriber objects are interested in the state changes or events of a publisher object, and want to react in their own way when the publisher generates the event. …
2/28/2019
SOEN 343, © P.Chalin,

36. Observer Pattern
Solution: Define a “subscriber” or “listener” interface. Subscribers implement this interface. The publisher can dynamically register subscribers who are interested in an event, and notify them when an event occurs.
Clarification: Publisher can dynamically process registration requests from subscribers.
2/28/2019
SOEN 343, © P.Chalin,

37. Observers: Illustration
Change requests go down – e.g. one cell in the spread sheet might be changed.
Notifications go up (upward dependencies are not ok)
2/28/2019
SOEN 343, © P.Chalin,

38. Observer Class Diagram
(Shown in class)
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39. Sale Example
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40. Observer Pattern (GoF book)
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