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

2. Agenda – Lecture 3b Approaches to Design Exercise: smelly Farm.
Larman, GRASP.
Wirfs-Brock et. al., RDD.
Exercise: smelly Farm.
Applying GRASP via Refactoring.
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SOEN 343, © P.Chalin,

3. Generalization Super (or parent) class; subclass (or child).
+ aa
+ getAa ( )
Super (or parent) class; subclass (or child).
Sub is-a-kind-of Super
Means an object of Sub can be used anywhere an object of Super can.
Sub inherits properties of Super.
An operation (or attribute) in Sub with same signature overrides the operation (attr.) in Super.
Sub
+ getAa ( )
Operation override is one form of polymorphism.
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SOEN 343, © P.Chalin,

4. Dependency
A dependency is a using relationship that states that a change … of one thing may affect another that uses it. A B
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SOEN 343, © P.Chalin,

5. Association Is a structural relationship.
Specifies that objects of one thing are connected to objects of another.
Given an association you can navigate from an object of one class to an object of another and vice versa.
I.e. bidirectional navigability is implicit. A B
12/29/2018
SOEN 343, © P.Chalin,

6. Directed Association
Given an association you can navigate from an object of one class to an object of another (but not necessarily vice versa).
(… an A might be eventually reachable from a B.) A B
12/29/2018
SOEN 343, © P.Chalin,

7. Directed Association … with (role) names. A - b : B [a..b] A - b B
12/29/2018
SOEN 343, © P.Chalin,

8. Approaches to Design Larman’s approach (GRASP).
Wirfs-Brock et. al. Responsibility-Drive Design (RDD).
Both are based on responsibility assignment.
I.e. assigning responsibilities to classes / objects.
12/29/2018
SOEN 343, © P.Chalin,

9. Object Responsibilities
A responsibility is an obligation of an object in terms of its behavior.
(More on this later)
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SOEN 343, © P.Chalin,

10. Design: Larman’s Approach
Methodology based on
Responsibility assignment.
GRASP General Responsibility Assignment Software Patterns.
Input:
Use cases.
Domain model.
Operation contracts.
Larman, Chapter 16.
Principles
12/29/2018
SOEN 343, © P.Chalin,

11. GRASP Information Expert Creator Low Coupling High Cohesion Controller
General Responsibility Assignment Software Patterns.
Information Expert
Creator
Low Coupling
High Cohesion
Controller
Polymorphism …
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SOEN 343, © P.Chalin,

12. Information Expert
“… expresses the common ‘intuition’ that objects do things related to the information they have.”
Assign the responsibility for “knowing” [something] to the object (class) that has the information necessary to fulfill the responsibility.
12/29/2018
SOEN 343, © P.Chalin,

13. Creator
Assign to class B the responsibility to create an instance of class A if
B aggregates A objects.
B contains A objects.
B records instances of A objects.
B closely uses A objects.
B has the initializing data that will be passed to A when it is created.
12/29/2018
SOEN 343, © P.Chalin,

14. Farm Example
Exercise Set 3
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SOEN 343, © P.Chalin,

15. Farm Class Diagram
- animals
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SOEN 343, © P.Chalin,

16. Test Cases package farm.tests; … public class FarmTest extends …{
public void testGetNumLegs() {
Farm f = new Farm();
f.add(new Animal("Duck", "Donald"));
assertEquals(2, f.getNumLegs());
f.add(new Animal("Dog", "Pluto"));
assertEquals(6, f.getNumLegs()); }
12/29/2018
SOEN 343, © P.Chalin,

17. Animal Class public class Animal { private String name;
private String kind;
public Animal(String aKind, String aName) {
kind = aKind;
name = aName; }
public String getKind() {
return kind; …
12/29/2018
SOEN 343, © P.Chalin,

18. Farm Class, public int getNumLegs() {
int result = 0;
Iterator it = animals.iterator();
while(it.hasNext()) {
Animal a = (Animal) it.next();
if(a.getKind().equals("Duck")) {
result += 2;
} else if(a.getKind().equals(
"Dog")) {
result += 4;
} else { // ? }
return result;
12/29/2018
SOEN 343, © P.Chalin,

19. Bad Smell in Farm.getNumLegs()?
Can you fix it?
Hints:
By Information Expert, who should know about the number of legs of Animals, the Farm or … ?
Having cascaded if’s (or switch statements) over information from another class is usually a very bad smell in OO.
Apply Larman’s GRASP principles of Information Expert and Polymorphism.
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SOEN 343, © P.Chalin,

20. GRASP: Polymorphism Principle
Larman:
When related alternatives or behaviors vary be type (class), assign responsibility for the behavior—using polymorphic operations—to the types for which the behavior varies.
(This principle was illustrated last week with the Animal hierarchy.)
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SOEN 343, © P.Chalin,

21. Farm Example, Solution #1
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SOEN 343, © P.Chalin,

22. Farm Example, Solution #2
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SOEN 343, © P.Chalin,

23. Practice in Tutorial
Apply Larman’s GRASP principles of Information Expert and Polymorphism.
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SOEN 343, © P.Chalin,