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

2. Agenda – Lecture 4a Topics to be covered (though not in given order):
OO basics
Casting.
Design principles (GRASP)
Cohesion & coupling.
Design Models
UML
Sequence diagrams, …
Quiz 1.
11/11/2018
SOEN 343, © P.Chalin,

3. OO Basics: Static vs. Dynamic Typing
Why not declare all fields or variables of reference types as
Object
void * (C++)
What would we gain?
What would we loose?
11/11/2018
SOEN 343, © P.Chalin,

4. Type Casting (in Statically Typed OO Languages)
Purpose of Casting
Inform compiler of (assumed) subtype of an object.
Compiler can then perform better type checking.
Type cast
Like an assertion, it may fail; e.g.
Object i = new Integer(0); String s = (String) i; //  ClassCastException
11/11/2018
SOEN 343, © P.Chalin,

5. Design Principles Design for change
Subliminal message: (to be looked at next class)
Design for change
11/11/2018
SOEN 343, © P.Chalin,

6. GRASP Information Expert (wk 3) Creator (wk 3) Low Coupling
General Responsibility Assignment Software Patterns.
Information Expert (wk 3)
Creator (wk 3)
Low Coupling
High Cohesion
Controller
Polymorphism (wk 2-3) …
11/11/2018
SOEN 343, © P.Chalin,

7. Cohesion
Measure of the degree of “relatedness” that elements within a module share.
Degree to which the tasks performed by a single module are functionally related.
Brain storm:
Why put procedures/methods together within a module/class?
11/11/2018
SOEN 343, © P.Chalin,

8. Levels Of Cohesion
11/11/2018
SOEN 343, © P.Chalin,

9. Coupling
Measures the degree of dependency that exists between modules.
Brain storm:
Give examples of code that creates coupling.
11/11/2018
SOEN 343, © P.Chalin,

10. Design Model: Sequence Diagrams
Participants:
c : Customer,
r : Rental,
m : Movie
Each participant has a life line (vertical dashed line)
Synchronous message
Return result
Activation bar
11/11/2018
SOEN 343, © P.Chalin,