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

2. Agenda – Lecture 10b Review & renew. GRASP: GoF:
Solutions to Exercise Set 9: PCV-TS-TDG.
GRASP:
Polymorphism (review).
Pure Fabrication.
GoF:
Singleton (review)
Composite.
From Requirements to design.
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SOEN 343, © P.Chalin,

3. Opening Remarks
Bakary’s requestion: “in an exam can we invent classes like Person”.
Person is an instance of the Domain Model pattern.
Applications will have instances of the Domain Model pattern, not a class named “DomainModel”.
Rene: for a definition of Controller, see Section 16.10, p. 237, first 13 lines.
Chris: “delete” is not a reserved word in Java.
Exams & memory.
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SOEN 343, © P.Chalin,

4. Do-it-all Servlet Redesign solutions: lect09.ppt SOEN 343, © P.Chalin,
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SOEN 343, © P.Chalin,

5. GRASP … Polymorphism. Pure Fabrication. Indirection.
Protected Variations.
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SOEN 343, © P.Chalin,

6. GRASP: Polymorphism Principle
Front Controller gives us an applied example of Larman’s polymorphism principle.
Larman:
When related alternatives or behaviors vary by type (class), assign responsibility for the behavior—using polymorphic operations—to the types for which the behavior varies.
4/23/2019
SOEN 343, © P.Chalin,

7. Code With Cascaded If’s
cmd = request.getParameter("command");
if(cmdName.equals(“AddStudent”)) { // Do addition …
} else if( ... “RemoveStudent”)) { // Do removal …
} ... else { // Error } …
FrontControllerServlet
# processRequest ( )
+ getUrlBase ( )
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SOEN 343, © P.Chalin,

8. Front Controller (Fowler) Command (GoF) Patterns
FrontCommand
+ init ( )
+ processRequest ( )
RemoveStudentCommand
ViewStudInfoCommand
+ processRequest ( )
+ processRequest ( )
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SOEN 343, © P.Chalin,

9. GRASP: Pure Fabrication
Assign a highly cohesive set of responsibilities to an artificial or convenience class that does not represent a problem domain concept—something made up, to support high cohesion, low coupling, and reuse.
Can you think of an example from EA?
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SOEN 343, © P.Chalin,

10. Gang Of Four Gamma, Helm, Johnson, Vlissides SOEN 343, © P.Chalin,
Erich
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SOEN 343, © P.Chalin,

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

12. Converting C to a Singleton.
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SOEN 343, © P.Chalin,

13. Singleton Pattern Notice: Constructor is no longer public.
To access the instance use getUniqueInstance().
All other attribute and method declarations of C stay the same.
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SOEN 343, © P.Chalin,

14. «Singleton» C.getUniqueInstance()
public static C getUniqueInstance() {
if(uniqueInstance == null) {
uniqueInstance = new C(); }
return uniqueInstance;
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SOEN 343, © P.Chalin,

15. Singleton: Design Alternatives/Issues
Create a class with static attributes and methods.
Trade-offs: consider how easy the following are:
Adapting the design so that x instances can be created (where x > 1).
Subclassing.
Passing the instance as a parameter. …
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SOEN 343, © P.Chalin,

16. Singleton: Testing
How to accommodate for the possibility of creating a Mock of a singleton?
See martinfowler.com.
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SOEN 343, © P.Chalin,

17. Composite: Ex. Objects
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SOEN 343, © P.Chalin,

18. Composite: Ex. Class Diagram
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SOEN 343, © P.Chalin,