soen343-week3 - SOEN 343 Software Design Section H Fall...

SOEN 343
Software Design
Section H Fall 2006
Dr Greg Butler
http://www.cs.concordia.ca/~gregb/home/soen343h-f06.html

Responsibilities, Principles,
Patterns
Design
Good Design, Smells, Evolutionary
Design
TDD (Test Driven Design)
RDD (Responsibility Driven Design)
GRASP Principles
Information Expert, Creator,
Polymorphism

What is Design?
Developing a blueprint (plan) for a
mechanism that performs the required
task,
… taking into account all the constraints, &
… making trade-offs between constraints
when they are in conflict.

Design vs.
Requirements and Implementation.
Requirements
Design
Implementation

Design Levels
•
Architectural design
.
.
.
•
Detailed design
.
.
.
•
Implementation (executable designs)

Two complimentary approaches to
design
•
Planned Design
vs.
Evolutionary Design
•
Can we get away with only planned
design?

Evolutionary Design
•
What is the probability that a S/W design
will need to be updated?
•
Change is inevitable , evolutionary design
recognizes this.
•
As software is changed, generally it
becomes more complex unless effort is
made to keep it simple .

Evolutionary Design
•
Practices
–
XP
–
Test driven development (TDD)
•
Do not add code unless you have a test failing …
•
Important characteristics
–
“Full” automated test suites
–
Refactoring
•
Help us learn detailed design idioms,
patterns, …

Prerequisites to Successful
Evolutionary Design?
•
Testing
–
… lots of automated testing.
•
Refactoring
–
… keeping the design simple.
•
Continuous integration
•
Actually, testing is a prerequisite to
refactoring.

Refactoring
•
A refactoring is a change made to the
internal structure of S/W to make it easier
to understand and less expensive to
modify, without changing its observable
behavior.

Can you pick out a good
design?
•
What is a good design?
–
Satisfies user needs.
–
Is a simple as possible. Kent Beck:
•
Runs all tests
•
Reveals intention.
•
No duplication.
•
Fewest number of classes or methods
•
… can you smell bad design choices?

“Bad Smells”
•
Duplication.