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)
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.
Provide an interface for the creation of objects.
Allow subclasses to “decide” which class to instantiate.
Povide an interface for creating families of related
or dependent objects. A factory for factories.
Seperate the construction process (how) of a complex object
from the concrete representations (what).
Ensure a class only has one instance, and provide a global
point of access to it.
Oh, I’m so
●● JFactory: a class with static methods to instantiate objects
like JDatabase, JUser, JDocument, JTemplate, etc.
●● most of those methods are singletons
●● KFactory: any class can be instantiated
●● get() = singleton, tmp() = any instantiation
(free to Johan Cruyff,
Dutch Football Pattern Designer
Make variations on copies of a basic-object.
Structural design patterns
●● Adapter: Adapt an interface to an expected
●● Bridge: Decouple an interface from its
●● Composite: Create a tree structure for
●● Decorator: Extend functionality dynamically.
●● Facade: Simplify usage by defining a high-level
●● Flyweight: Support fine-grained objects
efficiently by sharing.
●● Proxy: Represent an object with another object
for access control.
Adapter (= Wrapper)
c Adapt an interface to an expected interface.
●● new in 1.6: JAdapter and JAdapterInstance
●● JUpdateAdapter extends JAdapterInstance
JUpdaterExtension & JUpdaterExtension
wanted: documentation and examples!
what does it adapt?
Decouple an abstraction from its implementation.
1 LITER 1 LITER
1 LITER 1 LITER
Create a tree structure for part-whole hierarchies. A node is also a
(part of a) tree. Recursive:
Add extra functionallity (at runtime),
while keeping the interface the same.
●● KPatternDecorator: a general decorator
●● e.g. extended by KDecoratorJoomlaApplication
Provide a general (simpler) interface for a set of interfaces.
Use one instance of a class to provide many
Provide a surrogate or placeholder for another object
to control access to it.
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-
●● 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
●● 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
●● Visitor: Define a new operation on a class without changing it.
Encapsulate a command request in an object.
Chain of Responsibility
Define a method of passing a request among a chain of objects.
+ KCommandContext, KCommandEvent, KCommandHandler and
●● 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....
Domain -> (little) language -> grammar -> objects
HÉ! he means:
do this, do that,
and after finishing it,
Enable sequential access to collection elements, without showing
the underlying data-structures (array, list, records, etc)
c Layer in between: communication via one object.
Save and restore the internal state of an object.
Notify “subscribers” of changes.
●● JObserver and JObservable
●● JObserver extended by JEvent and that by JPlugin
●● KPatternObserver and KPatternObservable
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.....
When something can be done in several ways, make those
The skeleton of an algorithm is fixed, but parts can be filled in
Make a kind of plugin-possibility for methods: in that way me-
thods can be added in runtime.
●● 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....
●● encapsulate what varies, OCP
●● 1class, 1 responsibility, SRP
●● program against an interface, not against an imple-
●● prefer composition over inheritance
●● loose coupling, modular black boxes
GOF: 23 “classical” patterns:
Ailments of Unsuitable Project-Disoriented Software
Could be improved by studying design patterns and applying