A brief overview of common design patterns including code samples as presented to Women Who Code Belfast on 26th Nov 2013

2. Colleen Crangle
• Woman in tech throughout the 70s and 80s
• PhD in Philosophy (Logic and the Philosophy of
Language and Science), in follow-up to a BSc
cum laude and an MSc in Computer Science and
Mathematics from South Africa
• Affiliated scholar with the Centre for the Study
of Language and Information at Stanford
University and a partner in a small R&D
business in Silicon Valley, California

3. Introduction to Design Patterns
Jackie Pollock

4. Who am I?
• Technical Consultant @ Kainos Software
• Joined June 2006 as Graduate Software
Engineer
• Specialities:
• Document Management
• Workflow
• Scanning
• Integration

5. Overview
• What are design patterns?
• Why are they useful
• Types of design pattern
• Creational
• Structural
• Behavioural

6. What is a design pattern?
• General repeatable solution to
commonly occurring problem
• Not a finished design to be transformed
directly into code
• Relationships and interactions between
classes or objects without
implementation specifics

7. Where did they come from?
• Architectural concept by Christopher
Alexander in 1977
• Ward Cunningham and Kent Beck 1987
“Using pattern languages for objectorientated programmers”
• 1990s Gamma, Helm, Johnson, and
Vlissides. AKA: Gang of Four (GoF)
book “Design Patterns: Elements of
Reusable Object-Oriented Software”

8. Benefits
• Design reuse
• Uniform design vocabulary
• Enhance understanding, restructuring, &
team communication
• Basis for automation
• Transcends language-centric biases
• Abstracts away from many unimportant
details

9. Drawbacks
• Require significant tedious & error-prone
human effort to handcraft pattern
implementations / design reuse
• Can be deceptively simple
• May limit design options
• Leaves some important details
unresolved

10. Creational Patterns

11. Factory
Creating objects without having to specify
the exact class of object that will be
created – will be determined at runtime

12. Abstract Factory
Creates family of objects at runtime – each
of which will likely be a Factory in its own
right

13. Builder
Separates construction of the various
elements from the representation of the
whole therefore same process can create
different variants of theme

14. Singleton
Only one instance of object across
application. Used to provide single point of
access. Should not be used to replace
global variables

15. Prototype
Create complex objects by cloning existing ones
and changing required characteristics. Useful when
cost of new objects is prohibitive

16. Structural Patterns

17. Adapter
Converts an existing interface into another
that your application can use.

18. Composite
Treat a group of objects the same way as
a single instance of object. Tree structures
with a has-a relationship

19. Decorator
Add behaviour to object without affecting
behaviour of other objects from the same
class

20. Facade
Simplified interface to larger body of code.
Reduces dependencies on inner class
objects, wraps complex product APIs in
manageable layer

21. Proxy
Placeholder for another object, controlling
access. Used for lazy loading resource
hungry objects

22. Behavioural Patterns

23. Command
Turns requests into objects, separating out
the code making the request from the
object performing it

24. Iterator
Access elements of collection sequentially
without focusing on underlying object
types

25. Observer
When one object changes state, all
dependent objects notified and updated.
View element of Model-View-Controller
pattern

26. State
Object alters behaviour when internal state
changes

27. Word of Caution
Always go for simplistic approach rather
than applying patterns to every situation
If you force-fit a pattern into a situation
where the pattern doesn’t apply, you
are, by definition, not following the pattern

28. Resources
• Books
• Design Patterns
• Head First Design Patterns
• Design Patterns for Dummies
• Online
• Slideshare
• Pluralsight
• SourceMaking
• Wikipedia
• Image Credits
• Nelleke Verhoeff (Yepr)

29. Thanks for listening
Any Questions?
j.pollock@kainos.com
@kievia
jackiepollock

30. Finish
• Any queries?
• WomenWhoCodeBelfast@Gmail.com
• Twitter
• #WomenWhoCodeBelfast
• @WWCBelfast, @nirushika
Looking forward to seeing you at our next event!