4050_Design_Pattern.ppt

8/14/2019 4050_Design_Pattern.ppt

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List of Design Patterns

Creational Patterns

Abstract Factory

Builder

Facto ry Method

Prototype Singleton

Structural Patterns Adapter

Bridge

Composite (Sammys Slides)

Decorator (Sammys Slides)

Facade

Flyweight Proxy

Behavioral Patterns

Chain of Responsibility

Command

Interpreter

Iterator

Mediator

Memento

Observer State

Strategy

Template Method

Visitor


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FACTORY METHOD

(Class Creational) Intent:

Define an interface for creating an object, but

let subclasses decide which class to

instantiate.

Factory Method lets a class defer instantiation

to subclasses.


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FACTORY METHOD

(Class Creational) Motivation:

Framework use abstract classes to define and

maintain relationships between objects

Framework has to create objects as well -

must instantiate classes but only knows about

abstract classes - which it cannot instantiate

Factory method encapsulates knowledge ofwhich subclass to create - moves this

knowledge out of the framework


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docsDocument

Open()

Close()

Save()

Revert()

Application

MyDocument

CreateDocument()

NewDocument()

OpenDocument()

MyApplication

CreateDocument()

Document* doc=CreateDocument();

docs.Add(doc);

doc->Open();

return new MyDocument

FACTORY METHOD

Motivation


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Applicability

Use the Factory Method pattern when

a class cant anticipate the class of objects it

must create.

a class wants its subclasses to specify the

objects it creates.

classes delegate responsibility to one of

several helper subclasses, and you want tolocalize the knowledge of which helper

subclass is the delegate.


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Product

Creator

ConcreteProduct

FactoryMethod()

AnOperation()

ConcreteCreator

FactoryMethod()

...

product = FactoryMethod()

...

return new ConcreteProduct

FACTORY METHOD

Structure


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Participants

Product

Defines the interface of objects the factory method creates

ConcreteProduct

Implements the product interface

Creator

Declares the factory method which returns object of type product

May contain a default implementation of the factory method

Creator relies on its subclasses to define the factory method so

that it returns an instance of the appropriate Concrete Product.

ConcreteCreator

Overrides factory method to return instance of ConcreteProduct


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Factory Exampleclass 350Z implements Car; // fast car

class Ram implements Car; // truck

class Accord implements Car; // family car

Car fast = new 350Z(); // returns fast car

public class carFactory {public static Car create(String type) {

if (type.equals("fast")) return new 350Z();

if (type.equals("truck")) return new Ram();

else if (type.equals(family) return new Accord();}

}

Car fast = carFactory.create("fast"); // returns fast car


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SINGELTON

(Object Creational) Intent:

Ensure a class only has one instance, and provide a

global point of access to it.

Motivation: Some classes should have exactly one instance

(one print spooler, one file system, one window

manager)

A global variable makes an object accessible butdoesnt prohibit instantiation of multiple objects

Class should be responsible for keeping track of its

sole interface


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Applicability

Use the Singleton pattern when

there must be exactly one instance of a class,

and it must be accessible to clients from a

well-known access point.

when the sole instance should be extensible

by subclassing, and clients should be able to

use an extended instance without modifyingtheir code.


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Singleton

return uniquelnstancestatic Instance()

SingletonOperation()

GetSingletonData()

Static uniquelnstance

singletonData

SINGLETON

Structure


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Participants and Collaborations

Singleton:

Defines an instance operation that letsclients access its unique interface

Instance is a class operation (static inJava)

May be responsible for creating its own

unique instance Collaborations:

Clients access a Singleton instance solely

through Singletons Instance operation.


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Singleton Example

public class Employee {

public static final int ID = 1234; // ID is a singleton

}

public final class MySingleton {

// declare the unique instance of the classprivate static MySingleton uniq = new MySingleton();

// private constructor only accessed from this class

private MySingleton() { }

// return reference to unique instance of classpublic static MySingleton getInstance() {

return uniq;

}

}


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Adapter Pattern

Definition

Convert existing interfaces to new interface

Where to use & benefits

Help match an interface

Make unrelated classes work together

Increase transparency of classes


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Adapter Pattern

Example

Adapter from integer Set to integer Priority

Queue

Original

Integer set does not support Priority Queue

Using pattern

Adapter provides interface for using Set as PriorityQueue

Add needed functionality in Adapter methods


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Adapter Example

public interface PriorityQueue { // Priority

Queue

void add(Object o);

int size();Object removeSmallest();

}


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Adapter Example

public class PriorityQueueAdapter implements PriorityQueue {Set s;

PriorityQueueAdapter(Set s) { this.s = s; }

public void add(Object o) { s.add(o); }

int size() { return s.size(); }

public Integer removeSmallest() {

Integer smallest = Integer.MAX_VALUE;Iterator it = s.iterator();

while ( it.hasNext() ) {

Integer i = it.next();

if (i.compareTo(smallest) < 0)

smallest = i;}

s.remove(smallest);

return smallest;

}

}


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Observer Pattern

Classification & Applicability A behavioral (object) pattern:

Concerns objects and their behavior.

Applicability

Vary and reuse 2 different abstractions

independently.

Change to one object requires change in (one

or more) other objectswhose identity is notnecessarily known


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Observer PatternStructure

Subject

attach (Observer)

detach (Observer)

Notify ()

Observer

Update()

Concrete Observer

Update()

observerState

Concrete Subject

GetState()

SetState()

subjectState

observers

subject

For all x in observers{

x.Update(); }

observerState=

subject.getState();


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Observer Pattern - Participants

Subject Has a list of observers;

interfaces for attaching/detaching an observer

Observer An updating interface for objects that gets notified ofchanges in a subject.

ConcreteSubject

Stores state of interest to observers Sends notification when state changes.

ConcreteObserver Implements updating interface.


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Observer Pattern - Collaborations

:ConcreteSubject :ConcreteObserver-1 :ConcreteObserver-2

GetState()

Notify()

Update()

SetState()

GetState()

Update()

Ob P tt


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Observer Pattern -

Implementationinterface Observer

{void update (Observable sub, Object arg)

}

Java terminology for Subject.

public void addObserver(Observero) {}

public void deleteObserver (Observer o) {}

public void notifyObservers(Object arg){}

class Observable {

}

public boolean hasChanged() {}


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Observer Pattern -

Implementationpublic PiChartView implements Observer {

void update(Observable sub, Object arg) {

// repaint the pi-chart

}}

A Concrete Observer.

class StatsTable extends Observable{

public boolean hasChanged() {

// override to decide when it is considered changed

}

}


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Date post:	04-Jun-2018
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4050_Design_Pattern.ppt

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