Illustrations to show the essence of the 23 "classical" Gang Of Four design patterns. Plus some general info about object oriented progr

1. Design Patterns
illustrated
Herman Peeren May 31st 2010
(DP-illustrations: Nelleke Verhoeff)

2. Design Patterns
●● recipes against common (OO-) programming problems
●● code reuse: no need to reinvent the wheel
●● common language
●● GOF: 23 “classical” patterns
classic,
The Book
very nice!

3. The one constant in software development:

4. The one constant in software development:
CHANGE!

5. The one constant in software development:
CHANGE!
I knew it ...

6. Ideal: code as modular black boxes

7. Wish list and OOP-principles
●● loose coupling: 1 change = ceteris paribus
●● code reuse (is not the same as copy/paste)
●● open for extension, closed for modification
●● encapsulate what varies
●● single responsibility principle
●● program against an interface not against an imple-
mentation. Dependency injection
●● prefer composition over inheritance
(in fact this is all the same with different words)

8. Avoid: tight coupling!

9. It might get you into trouble...

10. Beware of:
Code
smells!

11. some code smells:
1.● duplicate code: DRY
2.● long method, huge class: SRP
3.● combinatorial explosion (similar classes)
4.● conditional complexity, switch statements
5.● “exhibitionistic” classes

12. some code smells:
1.● duplicate code: DRY
2.● long method, huge class: SRP
3.● combinatorial explosion (similar classes)
4.● conditional complexity, switch statements
5.● “exhibitionistic” classes

13. Classic pattern categories
creational, structural and behavioral patterns:
●● creational: object instantiation
●● structural: larger structures of classes or objects
●● behavioral: interaction and distribution of responsibility

14. Creational design patterns
●● Factory Method: Allow subclasses to “decide”
which class to instantiate.
●● Abstract Factory: Encapsulate a set of analo-
gous factories that produce families of objects.
●● Builder: Encapsulate the construction of com-
plex objects from their representation; so, the
same building process can create various repre-
sentations by specifying only type and content.
●● Singleton: Ensure that only a single instance of
a class exists and provide a single method for
gaining access to it.
●● Prototype: Create an initialized instance for
cloning or copying.

15. Factory Method
Provide an interface for the creation of objects.
Allow subclasses to “decide” which class to instantiate.
c

16. Abstract Factory
Povide an interface for creating families of related
or dependent objects. A factory for factories.
c

17. Builder
Seperate the construction process (how) of a complex object
from the concrete representations (what).
c

18. Singleton
Ensure a class only has one instance, and provide a global
point of access to it.
Oh, I’m so
loooooooonly
c

19. Joomla!
●● JFactory: a class with static methods to instantiate objects
like JDatabase, JUser, JDocument, JTemplate, etc.
●● most of those methods are singletons
Nooku
●● KFactory: any class can be instantiated
●● get() = singleton, tmp() = any instantiation

20. “Every
advantage
has its
disadvantages”
(free to Johan Cruyff,
Dutch Football Pattern Designer
and Ajax-fan...)

21. Prototype
Make variations on copies of a basic-object.
COPY-SERVICE
c

22. Structural design patterns
●● Adapter: Adapt an interface to an expected
interface.
●● Bridge: Decouple an interface from its
implementation.
●● Composite: Create a tree structure for
part-whole hierarchies.
●● Decorator: Extend functionality dynamically.
●● Facade: Simplify usage by defining a high-level
interface.
●● Flyweight: Support fine-grained objects
efficiently by sharing.
●● Proxy: Represent an object with another object
for access control.

23. Adapter (= Wrapper)
c Adapt an interface to an expected interface.

24. Joomla!
●● new in 1.6: JAdapter and JAdapterInstance
●● JUpdateAdapter extends JAdapterInstance
JUpdaterExtension & JUpdaterExtension
extend JUpdateAdapter
wanted: documentation and examples!
what does it adapt?

25. Bridge
Decouple an abstraction from its implementation.
COLA
1 LITER 1 LITER
COLA
1 LITER 1 LITER
COLA
MILK
COLA
MILK COLA
MILK
c

26. Composite
Create a tree structure for part-whole hierarchies. A node is also a
(part of a) tree. Recursive:
c

27. Decorator
Add extra functionallity (at runtime),
while keeping the interface the same.
Matroushka’s...
c

28. Decorator
Nooku
●● KPatternDecorator: a general decorator
●● e.g. extended by KDecoratorJoomlaApplication

29. Facade
Provide a general (simpler) interface for a set of interfaces.
looks
simple
c

30. Flyweight
Use one instance of a class to provide many
“virtual” instances.
c

31. Proxy
Provide a surrogate or placeholder for another object
to control access to it.
c

32. Behavioral design patterns
●● Chain of Responsibility: Define a method of passing a
request among a chain of objects.
●● Command: Encapsulate a command request in an object.
●● Interpreter: Allow inclusion of language elements in an appli-
cation.
●● Iterator: Enable sequential access to collection elements.
●● Mediator: Define simplified communication between classes.
●● Memento: Save and restore the internal state of an object.
●● Observer: Define a scheme for notifying objects of changes to
another object.
●● State: Alter the behavior of an object when its state changes.
●● Strategy: Encapsulate an algorithm inside a class.
●● Template Method: Allow subclasses to redefine the steps of
an algorithm.
●● Visitor: Define a new operation on a class without changing it.

33. Command
Encapsulate a command request in an object.
YOU,DO YOUR
TASK!
TASK TASK
LIGHT LIGHT
ON OFF
c

34. Chain of Responsibility
c
Define a method of passing a request among a chain of objects.

35. Nooku
●● KCommandChain
+ KCommandContext, KCommandEvent, KCommandHandler and
KCommandInterface
●● N.B.: executes a series of commands
instead of passing a command to a series of handlers
(like in Tapestry e.g.) ...correct me if I’m wrong....

36. Interpreter
Domain -> (little) language -> grammar -> objects
(DSL)
HÉ! he means:
do this, do that,
and after finishing it,
go there!
c

37. Iterator
Enable sequential access to collection elements, without showing
the underlying data-structures (array, list, records, etc)
next
next
c

38. Mediator
c Layer in between: communication via one object.

39. Memento
Save and restore the internal state of an object.
ME
c

40. Observer
Notify “subscribers” of changes.
ME
NO ME
ME
WHO?
c

41. Joomla!
●● JObserver and JObservable
●● JObserver extended by JEvent and that by JPlugin
Nooku
●● KPatternObserver and KPatternObservable

42. State
Let an object show other methods after a change of internal
state (as if it changes it’s class).
in a.....hick......different state,
....hick....I behave differently....hick.....
c

43. Strategy
When something can be done in several ways, make those
ways interchangeable.
POSSI-
BILITIES
c

45. Template Method
The skeleton of an algorithm is fixed, but parts can be filled in
differently.
c

46. Visitor
Make a kind of plugin-possibility for methods: in that way me-
thods can be added in runtime.
printservice!
c

47. extra: Mixin
●● Used in Nooku (and a.o. in Ruby and Python)
●● a kind of abstract class that is mixed into another object
●● all methods of the mixin are now part of the object
●● handle with care: beware of tight coupling....

48. Golden OO-principles
●● encapsulate what varies, OCP
●● 1class, 1 responsibility, SRP
●● program against an interface, not against an imple-
mentation, DIP
●● prefer composition over inheritance
●● loose coupling, modular black boxes

49. Some books
GOF: 23 “classical” patterns:
very nice!
classic,
The Book
handy
examples

50. good start
Fowler:
extended
e.g. with
patterns
for web
Fowler: also
known from
refactoring
combi: re-
factoring
& patterns

51. Resign Patterns:
Ailments of Unsuitable Project-Disoriented Software

52. Joomla!
Could be improved by studying design patterns and applying
them.
Loose coupling...

53. Questions?
Contact info:
Herman Peeren
herman@yepr.nl
© Yepr
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