Design Patterns illustrated

Herman Peeren May 31st 2010(DP-illustrations: Nelleke Verhoeff)

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!

The one constant in software development:

CHANGE!The one constant in software development:

CHANGE!The one constant in software development:

I knew it ...

Ideal: code as modular black boxes

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)

Avoid: tight coupling!

It might get you into trouble...

Code smells!

Beware of:

some code smells:1.● duplicate code: DRY2.● long method, huge class: SRP3.● combinatorial explosion (similar classes)4.● conditional complexity, switch statements5.● “exhibitionistic” classes

some code smells:1.● duplicate code: DRY2.● long method, huge class: SRP3.● combinatorial explosion (similar classes)4.● conditional complexity, switch statements5.● “exhibitionistic” classes

Classic pattern categories

creational, structural and behavioral patterns:

●● creational: object instantiation●● structural: larger structures of classes or objects ●● behavioral: interaction and distribution of responsibility

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.

Factory MethodProvide an interface for the creation of objects.Allow subclasses to “decide” which class to instantiate.

c

Abstract FactoryPovide an interface for creating families of related or dependent objects. A factory for factories.

c

BuilderSeperate the construction process (how) of a complex object from the concrete representations (what).

c

SingletonEnsure a class only has one instance, and provide a global point of access to it.

Oh, I’m soloooooooonly

c

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

“Every advantage

has its disadvantages”

(free to Johan Cruyff, Dutch Football Pattern Designer and Ajax-fan...)

PrototypeMake variations on copies of a basic-object.

COPY-SERVICE

c

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.

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

c

Joomla! ●● new in 1.6: JAdapter and JAdapterInstance

●● JUpdateAdapter extends JAdapterInstance JUpdaterExtension & JUpdaterExtension extend JUpdateAdapter

wanted: documentation and examples!what does it adapt?

BridgeDecouple an abstraction from its implementation.

c

COLA

COLA

COLA

COLA

1 LITER

1 LITER

COLA

MILK

MILK

1 LITER

1 LITER

MILK

CompositeCreate a tree structure for part-whole hierarchies. A node is also a (part of a) tree. Recursive:

c

DecoratorAdd extra functionallity (at runtime), while keeping the interface the same. Matroushka’s...

c

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

Decorator

FacadeProvide a general (simpler) interface for a set of interfaces.

lookssimple

c

FlyweightUse one instance of a class to provide many “virtual” instances.

c

ProxyProvide a surrogate or placeholder for another object to control access to it.

c

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.

CommandEncapsulate a command request in an object.

c

YOU,DO YOURTASK!

LIGHTOFF

LIGHTON

TASKTASK

Chain of ResponsibilityDefine a method of passing a request among a chain of objects.

c

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....

InterpreterDomain -> (little) language -> grammar -> objects(DSL)

he means:do this, do that,

and after finishing it, go there!

HÉ!HÉ!

c

IteratorEnable sequential access to collection elements, without showing the underlying data-structures (array, list, records, etc)

nextnext

next

c

MediatorLayer in between: communication via one object.

c

MementoSave and restore the internal state of an object.

ME

c

ObserverNotify “subscribers” of changes.

WHO?

ME

ME

MENO

c

Joomla!●● JObserver and JObservable●● JObserver extended by JEvent and that by JPlugin

Nooku●● KPatternObserver and KPatternObservable

StateLet 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

StrategyWhen something can be done in several ways, make those ways interchangeable.

POSSI-BILITIES

c

Template MethodThe skeleton of an algorithm is fixed, but parts can be filled in differently.

c

VisitorMake a kind of plugin-possibility for methods: in that way me-thods can be added in runtime.

printservice!

c

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....

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

Some booksGOF: 23 “classical” patterns:

classic, The Book

very nice!

handy examples

good start

Fowler: extended e.g. with patterns for web

Fowler: also known from refactoring

combi: re-factoring & patterns

Resign Patterns: Ailments of Unsuitable Project-Disoriented Software

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

Loose coupling...

Questions?

Contact info:Herman Peerenherman@yepr.nl

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