Design Patterns

Yonglei Tao

Design Patterns A design pattern describes a recurring

design problem, a solution, and the context in which that solution works A common way of doing things Specialization of a design principle Tested knowledge A named description of a technique with its

benefits and costs

The Strategy Pattern Problem

how to design for varying, but related, algorithms or policies

how to design for the ability to change them Solution

define a family of algorithms, encapsulate each one in a separate class, and make them interchangeable

A Sorting Example Shellsort

No extra space needed, fast but not optimal Very fast on nearly sorted data Does comparatively well on small lists

Quicksort Average case is O (n log n) Relatively poor performance on short lists Require a stack of log(n) in depth

Mergesort Worst case is O (n log n) Requires O (n) extra space Stable

Sorting (cont.) Have a sorted list container, which one gives a

sort algorithm

SortedList studentRecords = new SortedList( new ShellSort() );

studentRecords.add( “Sam” ); …

pubic class SortedList { Object[ ] elements;

SortStrategy sorter;

void sort( ) { sorter.sort( elements); } }

Sale

+Total() : Money-policy : IDiscountPolicy

«Interface»IDiscountPolity

+getDiscount(sale : Sale) : M oney

DiscountPolicyA

+getDiscount()

DiscountPolicyB

+getDiscount()

DiscountPolicyC

+getDiscount()

Money Total () { get subtotals and calculate sum ; d iscount = policy.getD iscount(this); return sum - discount;}

More on Strategy

:PercentDiscountPricingStrategy

s : Sale

st = getSubtotal

t = getTotal

lineItems[ i ] :SalesLineItem

t = getTotal( s )

pdt = getPreDiscountTotal

{ t = pdt * percentage }

note that the Sale s is passed to the Strategy so that it has parameter visibility to it for further collaboration

loop

t = getDiscount(s)

The Factory Pattern Problem

a class cannot anticipate the class of objects it must create

how to handle object creation with complex creation logic or other special considerations

Solution encapsulate the knowledge of object creation

and move it out of the class that uses the object

define an interface for creating an object, but let subclasses decide which class to instantiate

Factory Method

Document

+open()+close()+save()

Application

+createDocum ent()+newDocum ent()+openDocum ent()

MyDoc

MyApp

+createDocum ent()

// new Document ()Docum ent doc = createDocument();docList.Add(doc);doc.open();

// createDocument() - Template M ethodreturn new M yDoc();

// the m ain methodM yApp app;app.New Document ();Application-Sepcific

Fram ework

The Singleton Pattern Problem

objects need a global and single point of access Solution

ensure a class has exactly one instance define a static method that returns the singleton

Consequences facilitates communication between objects not

directly coupled reduce coupling between objects in a system

Singleton

instance: S ingleton

Singleton();getInstance():S ingleton

{ // static m ethod if (instance == null) instance = new Singleton(); return instance;}

// get a reference to the singleton object

theSingleton = Singleton.getInstance();

The Singleton Pattern

Implementationclass Singleton {

private static Singleton _instance = null; private Singleton() { … } public static Singleton getInstance() {

if ( _instance == null ) _instance = new Singleton();

return _instance; } public void otherOperations() { … }

} class Program {

public void aMethod() { X = Singleton.getInstance();

} }

Factory

1PricingStrategyFactory

instance : PricingStrategyFactory

getInstance() : PricingStrategyFactory

getSalePricingStrategy() : ISalePricingStrategygetSeniorPricingStrategy() : ISalePricingStrategy...

{ String className = System.getProperty( "salepricingstrategy.class.name" ); strategy = (ISalePricingStrategy) Class.forName( className ).newInstance(); return strategy; }

Factory

:Sale

1:PricingStrategyFactory

ps =getSale

PricingStrategy

:Register

makeNewSale create

create ps :CompositeBestForCustomerPricingStrategy

create( percent ) s : PercentageDiscountPricingStrategy

add( s )

Navigational Model for Visual C++

:M yApp

:M yView

:CM ainFram e

:M yDoc

m_pM ainW nd

G etDocument()

G etActiveDoc()

G etActiveView ()

A S ingleton objectaccessible throughthe global functionAfxG etApp()

Example: the Null Node in Binary Trees

public class BinaryNode extends Node { Node left , right; int key;

public boolean includes( int value ) { if (key == value)

return true; else if (value < key )

return left.includes( value ); else

return right.includes(value); }

… }

public class NullNode extends Node { public boolean includes( int value ) {

return false; }

… }

The Composite Pattern Problem

How to allow clients to treat individual objects and compositions of objects uniformly

Solution Organize objects into a tree structure that

represents an aggregation hierarchy

Client

«Interface»Component

+operation() : void+addCom ponent() : void+rem oveCom ponent() : void+getChild(n : int) : Com ponent

Composite

+operation() : void+addCom ponent() : void+rem oveCom ponent() : void+getChild(n : int) : Com ponent

Leaf

+operation() : void

Structure

Use of Composite

PercentageDiscountPricingStrategy

percentage : float

getTotal( Sale ) : Money

AbsoluteDiscountOverThresholdPricingStrategy

discount : Moneythreshold : Money

getTotal( Sale ) : Money

«interface»ISalePricingStrategy

getTotal( Sale ) : Money

{ return sale.getPreDiscountTotal() * percentage }

CompositePricingStrategy

add( ISalePricingStrategy )getTotal( Sale ) : Money

{lowestTotal = INTEGER.MAXfor each ISalePricingStrategy strat in pricingStrategies { total := strat.getTotal( sale ) lowestTotal = min( total, lowestTotal ) }return lowestTotal }

1..*

CompositeBestForCustomerPricingStrategy

getTotal( Sale ) : Money

CompositeBestForStorePricingStrategy

getTotal( Sale ) : Money

strategies

All composites maintain a list of contained strategies. Therefore, define a common superclass CompositePricingStrategy that defines this list (named strategies).

Sale

date...

getTotal()...

1

pricingStrategy

{...return pricingStrategy.getTotal( this )}

More on Composite

:CompositeBestForCustomerPricingStrategy

s : Sale

st = getSubtotal

t = getTotal

lineItems[ i ] :SalesLineItem

t = getTotal( s )

the Sale object treats a Composite Strategy that contains other strategies just like any other ISalePricingStrategy

x = getTotal( s )

strategies[ j ] :: ISalePricingStrategy

UML: ISalePricingStrategy is an interface, not a class; this is the way in UML 2 to indicate an object of an unknown class, but that implements this interface

{ t = min(set of all x) }

loop

loop