Design Patterns:

Elements of Reusable Object – Oriented Software

Web Apps and Services

Design Pattern Introduction: why a design pattern? History of design pattern What is a design pattern How we describe design pattern Classification of desing pattern Examples of design pattern Conclusions Bibliography

Why a Design Pattern

Reusability:one of Wasserman’s rules(1996)for an efficient and actual SE discipline

Helping new designers to have a more flexible and reusable design

Improving the documentation and maintenance of existing system by furnishing an explicit specification of class and object interactions and their intent

History of Design Pattern

1979:Christopher Alexander,architect, “The Timeless Way of Building”,Oxford Press

1987:OOPSLA (Object Oriented Programming System),Orlando, presentation of design pattern to the community OO by Ward Cunningham and Kent Beck

1995:Group of Four alias E.Gamma, R.Helm,R.Johnson and J.Vlissides : “Design Pattern:Elements of Reusable OO software”

What Is a Design Pattern A design pattern is a descriptions of communicating

objects and classes that are customized to solve a general design problem in a particular context

A pattern is made by four elements: name problem solution consequences

Name of Design Pattern

Describe a design problems and its solutions in a word or two

Used to talk about design pattern with our colleagues

Used in the documentation Increase our design vocabulary Have to be coherent and evocative

Problem

Describes when to apply the patterns

Explains the problem and its context

Sometimes include a list of conditions that must be met before it makes sense to apply the pattern

Have to occurs over and over again in our environment

Solution

Describes the elements that make up the design, their relationships, responsibilities and collaborations

Does not describe a concrete design or implementation

Has to be well proven in some projects

Consequences

Results and trade-offs of applying the pattern

Helpful for describe design decisions, for evaluating design alternatives

Benefits of applying a pattern

Impacts on a system’s flexibility,extensibility or portability

Description of Design Pattern

Pattern name and classificationPattern name and classification• contains the essence of pattern succinctly

• Become part of your design vocabulary

IntentIntent• What does the pattern do ?

• What particular problem does it address ?

Description of Design Pattern

MotivationMotivation• Illustrate a design problem and how the class and the object structures

solve the problem

ApplicabilityApplicability• In which situations the pattern can be applied?

• How can you recognize these situations?

Description of Design Pattern

StructureStructure• Graphical representation of the classes and their collaborations in the

pattern

ParticipantsParticipants• Class

• Objects

• Responsibilities

Description of Design Pattern CollaborationsCollaborations

• How the participants collaborate to carry out their responsibilities

ConsequencesConsequences• How does the pattern support its objectives?

• What are the trade-offs and results of using the pattern?

Description of Design Pattern

ImplementationImplementation

Sample CodeSample Code

Known UsesKnown Uses• Examples of the pattern found in real systems

Description of Design Pattern

Related PatternsRelated Patterns• What design patterns are closely related to this one?

• What are the important differences?

By purpose and by scope

Creational patternsCreational patterns

• Abstract the instantiation process

• Make a system independent to its realization

• Class Creational use inheritance to vary the instantiated classes

• Object Creational delegate instantiation to an another object

Classification of Design Pattern

Classification of Design Pattern Structural patternsStructural patterns

• Class Structural patterns concern the aggregation of classes to form largest structures

• Object Structural pattern concern the aggregation of objects to form largest structures

Classification of Design Pattern Behavioral patternsBehavioral patterns

• Concern with algorithms and assignment of responsibilities between objects

• Describe the patterns of communication between classes or objects

• Behavioral class pattern use inheritance to distribute behavior between classes

• Behavioral object pattern use object composition to distribute behavior between classes

Classification of design patterns ( a view)

InterpreterAdapterFactory Method

Visitor

Strategy

StateProxy

ObserverFlyweight

MementoFaçade

MediatorDecoratorSingleton

IteratorCompositePrototype

CommandBridgeBuilder

Chain of Responsibility

AdapterAbstract Factory

Template Method

BehavioralBehavioralStructuralStructuralCreationalCreational

ObjectObject

ClassClass

purpose

Scope

Creational Pattern

Singleton• Ensure a class only has one instance

• Provide a global point of access to it

Abstract Factory:• Provide an interface for creating families of related or dependent

objects without specifying their concrete classes

Creational Pattern Factory Method:

• Define an interface for creating an object but let subclasses decide which class to instantiate

• Lets a class defer instantiation to subclasses

Prototype • Specify the kinds of objects to create using a prototypical instance

• Create new objects by copying this prototype

Creational Pattern

Builder:• Separate the construction of a complex object from its representation so

that the same construction process can create different representations

Structural Pattern Composite

• Compose objects into tree structures to represent part-whole hierarchies

• Lets clients treat individual objects and compositions of objects uniformly

Decorator• Attach additional responsibilities to an object dynamically

• Provide a flexible alternative to subclassing for extending functionality

Structural Pattern

Adapter:• Convert the interface of a class into another interface clients expect

• Lets classes work together that couldn’t otherwise because of incompatible interfaces

Bridge:• Decouple an abstraction from its implementation so that the two can

vary independently

Structural Pattern Façade

• Provide a unified interface to a set of interfaces in a subsystem

• Defines an higher-level interface that makes the system easier to use

Flyweight• Use sharing to support large numbers of fine-grained objects

efficiently

Structural Pattern

Proxy• Provide a surrogate or placeholder for another object to control access to it

Behavioral Pattern Iterator

• Provide a way to access the elements of an aggregate object without exposing its representation

Command• Encapsulate a request as an object, thereby letting you parameterize

clients with different requests

Behavioral Pattern Interpreter

• Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language

Mediator• Define an object that encapsulate how a set of objects interact

• Promotes loose coupling by keeping objects from referring to each other explicitly

• Lets you vary their interaction independently

Behavioral Pattern Memento

• Capture and externalize an object’s internal state

Observer• Define a one-to-many dependency between objects so when one of

them change state all its dependents are updated automatically

Behavioral Pattern State

• Allow an object to alter its behavior when its internal state changes

• The object will appear to change its class

Visitor• Represent an operation to be performed on the elements of an object

structure

• Lets you define a new operation without changing the classes of the elements on which operates

Behavioral Pattern Strategy

• Define a family of algorithms

• Encapsulate each one

• Make them interchangeable

• Lets the algorithms vary independently from clients that use it

Chain of responsibilities• Avoid coupling the sender of a request to its receiver by giving more then

one object a chance to handle the request

• Chain the receiving objects and pass the request along the chain until an object handles it

Singleton Pattern Motivation

• we need to have exactly only one instance for a class (ex. Printer spooler)

• Make the class itself responsible for keeping track of its sole instance

• The class provide a way to access the instance

Applicability• There must be only one instance of a class accessible from a well-

known point

Singleton Pattern Structure

Singleton

Static Instance()SingletonOperation()GetSingletonData()

Static uniqueInstanceSingletonData

Return uniqueInstance

Singleton Pattern Participants

• Singleton class

Collaborations• Access only through Singleton’s instance operation

Consequences• Controlled access to sole instance

• Permits refinement of operation and representation

• More flexible than class operations

Example of Singleton use“Lotteria Algebrica” We had to have only one instance for class

Director. We simply solve our problem using Singleton PatternDirector

Static Instance()Given(n_ticket:int):void

Error():void

Static UniqueInstance

Decorator Pattern Motivation

• Add responsibilities to individual object not to an entire class

• conforming the interface of the component decorated

Decorator Pattern Structure Component

Operation()

ConcreteComponent

Operation()

Decorator

Operation()

ConcreteDecoratorAOperation()AddedState

ConcreteDecoratorBOperation()

Addedbehavior

Decorator Pattern Participants

• Component• Define the interface for objects that can have responsibilities added to them

dinamically

• Concrete Component• Defines an object to which additional responsibilities can be attached

• Decorator • Mantains a reference to a Component object and defines an interface that

conforms to Component’s interface

• ConcreteDecorator• Added responsibilities to the component

Decorator Pattern Consequences

• More flexibility than static inheritance

• Avoids feature-laden classes high up in the hierarchy

• A decorator and its component are not identical

• Lots of little objects

Example of Decorator

Motivation• If you have a TextView object that displays text in a Window

• TextView has no scroll bars by default

• TextView has no bord by default …

Example of Decorator Structure

aBorderDecorator

component

aScrollDecorator

component

aTextView

Example of Decorator StructureVisualComponent

Draw()

TextView

Draw()

Decorator

Draw()

ScrollDecorator BorderDecorator

Draw()ScrollTo()

ScrollPosition

Draw()DrawBorder()

BorderWidth

Iterator Pattern Also know as

• Cursor

Motivation• Provide more way to access to the elements of an aggregate

List

Count()Append(Element)Remove(Element)

…

ListIterator

First()Next()

IsDone()CurrentItem

Index

Iterator Pattern

Applicability• Access an aggregate object’s contents

• Support multiple traversals of aggregate objects

• Provide an uniform interface

Iterator Pattern StructureAggregate

CreateIterator()

ConcreteAggregate

CreateIterator()

Iterator

First()Next()

IsDone()CurrentItem()

ConcreteIterator

Return newConcreteIterator(This)

Client

Iterator Pattern Participants

• Iterator• Defines interfaces for accessing elements

• ConcreteIterator• Implements the Iterator interface

• Keeps track of the current position in the traversal of the aggregate

• Aggregate• Defines an interface for creating an Iterator object

• Concrete Aggregate• Implements the Iterator creation interface

• Return an instance of ConcreteIterator

Iterator Pattern Consequences

• Simplify the Aggregate interface

• More that one traversal can be pending on an aggregate

• Variations in the traversal of an aggregate

Patterns and Frameworks

Design patterns are more abstract than frameworks Design patterns are smaller architectural elements than

frameworks Design patterns are less specialized than frameworks Framework is executable software, design pattern

represent knowledge about software Frameworks are the physical realization of one or more

software pattern solutions,pattern are instructions for how to implement those solutions

Set of cooperating classes that make up reusable design for a specific class of software. Provides

architectural guidance by partitioning the design into abstract classes and defining their responsibilities

and collaborations.A developer customizes a framework to a particular application by subclassing and composing instances

of framework classes.

Conclusions At present, the software community is using

pattern largely for software architecture and design

More recently the software community is using pattern also for software development processes and o organizations

Several object-oriented software design notations/methods have added support for the modeling and representation of design patterns

BibliographyFor the pattern catalogue: Erich Gamma, Richard Helm,Ralph Johnson,John

Vlissides. Design Patterns: Elements of Reusable Object-Oriented Software. ADDISON-WESLEY 1995

For the pattern origin: Cristopher Alexander. The Timeless Way of Building.

Oxford Press 1979For “Night Patterns”: James O.Coplien,Douglas C.Schmidt.Pattern

Languages of Program Design. ADDISON-WESLEY 1995

More InformationHere you can find some example in Italian of design patterns. http://www.ugolandini.net/PatternsHome.htmlHere you can find some general information about patterns http://www.mokabyte.it/1998/10/pattern.htm http://www.c2.comA complete Web site about pattern http://www.enteract.com/~bradapp/docs/patterns-

intro.html