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Design Patterns

Software Design and Development

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Outline

Definition and Description of a Design Pattern

Discussion of Selected PatternsKinds of Patterns

Reference: Gamma et al (“Gang-of-4”), Design Patterns

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Pattern

Describes a problem that has occurred over and over in our environment, and then describes the core of the solution of that problem in a way that the solution can be used a million times over, without ever doing it in the same way twice.

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Design Pattern

Solution to a particular kind of problemHow to combine classes and methodsNot solve every problem from first

principlesBased on design experienceUse requires understanding of the

appropriate problem and being able to recognize when such problems occur

Reuse solutions from the past

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Describing a PatternName Intent/Problem

Situation (problem) and contextWhen to apply the pattern; conditions

SolutionElements that make up the design, relationships,

collaboration; more a template rather than a concrete solution

How the general arrangement of elements (classes and objects) solves it

UML diagrams (class relationships and responsibilities) and code implications

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Describing a Pattern

ConsequencesResults, variations, and tradeoffsCritical in understanding cost/benefit

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How Design Patterns Solve Design Problems

Finding appropriate objectsDetermining object granularitySpecifying object interfacesSpecifying object implementationsPutting reuse mechanisms to workDesigning for change

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Finding Appropriate ObjectsFactors involving decomposition of system

into objects (at times in conflicting ways)encapsulation, granularity, flexibility, performance,

evolution, reusability, etcDifferent approaches

Problem statement: nouns and verbs translate to classes and operations

Focus on collaborations and responsibilitiesModel real world (analysis) and translate to design

Many objects come from analysis model, but often have additional classes in design that have no counterpart in real world

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Finding Appropriate Objects

Strict modeling of real world tends leads to system that reflect today’s realities, but not necessarily tomorrow’s

Abstractions that emerge during design are key to making design flexible

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Determine Object Granularity

How to decide what should be an object?Different patterns have different approaches

represent complete subsystems as objectssupport huge numbers of objects at finest

granularitiescreate objects whose only responsibility is

creating other objectscreate objects whose responsibilities are to

implement a request on another object or group of objects

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Specifying Object Interfaces

Interface: set of operation signaturescomplete set of requests that can be made to an

objectDoes not commit to implementation until run-

timeDefine interface by identifying key elements

and kinds of data that get sent across an interface

Specify relationships between interfaces require classes with similar interfacesplace constraints on interfaces

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Specifying Object ImplementationsDefining internal state and implementation of

the operationClass vs Interface inheritance

Class: mechanism for code and representation sharing

Interface: describes when an object can be used in place of another

Distinction important for diff design patternsCommon theme: Variables not instances of

concrete classes; commit only to interface defined by an abstract class; use creational patterns to create instances

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Reuse mechanisms

Inheritance vs compositionWhite box reuse: by subclassing

internals of parent visible to subclassesBlack box reuse: assembling or

composing objects to get more functionality

Both go together, but there is a tendency to overuse inheritance. Favor composition where possible

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Designing for Change

Create objects indirectly instead of using class names (factory, abstract factory, prototype)

Avoid hard coding requests (chain of responsibility, command)

Limit platform dependencies (abstract factory, bridge)

Isolate algorithms that are likely to change (Builder, Iterator, Visitor, …)

Etc

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How to select design patterns

Consider how the design patterns solve design problems

Scan intent sectionConsider how patterns interrelateStudy patterns of like purposeExamine cause of redesignConsider what should be variable in design

(what you might want to change without redesign): Encapsulate the concept that varies

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How to use a design pattern

Read up on the patternStudy structure, collaboration, participantsLook at sample codeChoose names of participants meaningful

in the application contextDefine classesDefine application specific names for

operations in the processImplement the operations

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Selected Patterns for Discussion

SingletonFactory MethodCompositeIterator

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Singleton

Intent ensure a class has only one instance, and

provide a global point of access to itMotivation

Important for some classes to have exactly one instance. E.g., although there are many printers, should just have one print spooler

Ensure only one instance available and easily accessible

global variables gives access, but doesn’t keep you from instantiating many objects

Give class responsibility for keeping track of its sole instance

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Design Solution

Defines a getInstance() operation that lets clients access its unique instance

May be responsible for creating its own unique instance Singleton

static uniqueinstanceSingleton data

static getInstance()Singleton methods…

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Singleton Example (Java)

Database

Database

static Database* DBinstance attributes…

static Database* getDB()instance methods…

public class Database {private static Database DB; ...private Database() { ... }public static Database getDB() { if (DB == null) DB = new Database(); return DB;} ...}

In application code…Database db = Database.getDB();db.someMethod();

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Singleton Example (C++)

class Database{private: static Database *DB; ... private Database() { ... }public: static Database *getDB() { if (DB == NULL) DB = new Database()); return DB; } ...}Database *Database::DB=NULL;

In application code…Database *db = Database.getDB();Db->someMethod();

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ImplementationDeclare all of class’s constructors private

prevent other classes from directly creating an instance of this class

Hide the operation that creates the instance behind a class operation (getInstance)

Variation: Since creation policy is encapsulated in getInstance, possible to vary the creation policy

Subclassing: Variable that refers to singleton instance must get initialized with instance of the subclassuse registration

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Singleton Consequences

Ensures only one (e.g., Database) instance exists in the system

Can maintain a pointer (need to create object on first get call) or an actual object

Can also use this pattern to control fixed multiple instances

Much better than the alternative: global variables

Permits refinement of operations and representation by subclassing

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Abstract Factory

Intent: provide an interface for creating objects without specifying their concrete classes

Example: Stacks, Queues, and other data structuresWant users to not know or care how these

structures are implemented (separation)Example: UI toolkit to support multiple look-

and-feel standards, e.g., Motif, PMAbstract class for widget, supporting class for

specific platform widget

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Solutions in C++

Use of header file (class declarations) and implementation file (method definitions) ok but limitedHeader file usually contains private declarations

which are technically part of the implementationChange in implementation requires that the

application using the data structure be recompiled

Alternative: create an abstract superclass with pure virtual data structure methods

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Design Solution for Abstract Factory

Factory

createProduct()

Product

virtual methods

ConcreteProdA

methods

ConcreteProdB

methods

Client

Note: this is anabbreviated design

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ParticipantsAbstract Factory

declares an interface for operations that create abstract product objects

Concrete Factory implements the operations to create concrete

product objectsAbstract Product

declares an interface for a type of product objectConcrete Product

defines a product object to be created by the corresponding concrete factory

implements the abstract product interface

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Participants

Clientuses only interfaces declared by

AbstractFactory and AbstractProduct classes

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Stack Example (C++)

Stack class defines virtual methodspush(), pop(), etc.

ArrayStack and LinkedStack are derived classes of Stack and contain concrete implementations

StackFactory class defines a createStack() method that returns a ptr to a concrete stackStack *createStack() { return new ArrayStack(); }

Client programs need to be aware of Stack and StackFactory classes only No need to know about ArrayStack()

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Factories in Java

Stack is an InterfaceArrayStack and LinkedStack implement StackStackFactory returns objects of type Stack

through its factory methodsSelect class of the concrete factory it supplies to

client objectsIf using info from requesting client, can hardcode

selection logic and choice of factory objectsUse Hashed Adapter Pattern to separate selection logic

for concrete factories from the data it uses to make the selection

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Abstract FactoryConsequences

Factory class or method can be altered without affecting the applicationConcrete classes are isolated

Factory class can be responsible for creating different types of objectse.g., DataStructure factory that returns

stacks, queues, lists, etc. “product families”

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Abstract Factory Consequences

Promotes consistency among productsWhen products in a family are designed to work

together, it is important that an application use objects from only one family at a time. This pattern enforces this.

Supporting new kinds of products is difficult requires extending the factory interface (fixed

set)change AbstractFactory class and all the

subclasses

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Composite Pattern

Intent: compose objects into tree structures to represent (nested) part-whole hierarchiesClients treat individual objects and

composition of objects uniformlyExample: GUIs (e.g., java.awt.*)

Buttons, labels, text fields, and panels are VisualComponents but panels can also contain VisualComponent objects

Calling show() on a panel will call show() on the objects contained in it

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Structure

+Operation()+Add(Component)()+Remove(Component)()+GetChild(int)()

Component

+Operation()()

Leaf +Operation()()+Add(Component)()+Remove(Component)()+GetChild(int)()

Composite

-End11

-End2*

-End31

-End4*

-End5

1

-End6

*

Client-End7

*

-End8

*

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Participants

Componentdeclares the interface for objects in the composition implements default behavior for interface common

to all classesdeclares interface for managing and accessing child

components (optional) defines interface for accessing a

component’s parent in the recursive structure Leaf

Leaf objects; primitives; has no childrendefine behavior for primitive objects

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Participants

CompositeDefines behavior of components having

childrenStores child componentsImplements child-related operations in

the compositionClient

manipulates objects in the composition through the component interface

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Consequences

Defines class hierarchies consisting of primitive objects and composite objects

Easy to add new kinds of components

Simple client – can treat primitives and composites uniformly

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Iterator Pattern

Intent: provide a way to access the elements of an aggregate object sequentially without expressing its underlying representation

Example: iterators of C++ STL containersNote that you can have several iterator

objects for a container and that the iterators are separate classes

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Motivation

Take responsibility for access and traversal out of the container object and into an iterator or Cursor object

Example

List

Count()Append(Element)Remove(Element)…

ListIterator

First()Next()IsDone()CurrentItem()

index

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Consequences

Supports variations in the traversal of an aggregate

Simplifies the aggregate interfaceMore than one traversal can be

pending on one aggregate

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Kinds of Patterns

CreationalObject creation; e.g., Factory and

SingletonStructural

Object structure; e.g., CompositeBehavioral

Object interaction and distribution of responsibilities; e.g., Iterator

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Creational Patterns

Abstract FactoryBuilderFactory MethodPrototypeSingleton

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Structural Patterns

AdapterBridgeCompositeDecoratorFaçadeFlyweightProxy

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Behavioral Patterns

Chain Of ResponsibilityCommandInterpreterIteratorMediatorMementoAnd a few more …

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Summary

Main point: to recognize that there are proven solutions to problems that a designer/ programmer may encounterSolutions are results of others’ experiencesTowards “standard approaches”

Search for such solutions firstAlthough there is some merit attempting to

create the solution yourself

Becoming a design architect