Understanding and Implementing Design Patterns: Singleton, Decorator, Facade, Flyweight, Lecture notes of Design Patterns

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Design Patterns

CSE 403, Spring 2008, Alverson With material from Marty Stepp 403 lectures.

Design challenges

g

g

y

Designing software is hard! One must find:o

a good problem decomposition o^

a design with flexibility, modularity and elegance o^

a design with flexibility, modularity and elegance

y

Designs often emerge from trial and error

y

Successful designs do exist

y

Successful designs do existo

two designs they are almost never identical o^

they exhibit some recurring characteristics

CSE 403, Spring 2008, Alverson

Online Readings

g

My latest favorite survey of common patterns:

y

My

latest favorite survey of common patterns:

http://sourcemaking.com/design_patterns

y

[Optional] See the “References” link on the classweb page for a number of others

CSE 403, Spring 2008, Alverson

Gang of Four (GoF) patterns

g

) p

y

Creational Patterns

(abstract object instantiation)

Ab t

t F

t^

F

t^

B ild

P

t t

Abstract Factory, Factory, Builder, PrototypeSingleton

y

Structural Patterns

(combine objects)

Adapter, Bridge, Composite, Decorator, Façade,Flyweight ProxyFlyweight, Proxy

y

Behavioral Patterns

(communication btwn objects)

Chain of responsibility, Command, Interpreter,Iterator, Mediator, Memento, Observer, State,Strategy Template Method Visitor

CSE 403, Spring 2008, Alverson

Strategy

, Template Method, Visitor

Restricting object creation

g

j

y

Problem: Sometimes we will really only ever needone instance of a particular classone instance of a particular class.o

We'd like to make it illegal to have more than one o^

Examples: keyboard reader, printer spooler, gradebook

y

Why we care:

Creating lots of objects can take a lot of time o^

Creating lots of objects can take a lot of time o^

Extra objects take up memory o^

It is a maintenance headache to deal with differentobjects floating around if they are the same

CSE 403, Spring 2008, Alverson

Singleton pattern

g

p

y

singleton: an object that is the only object of itstypetypeo

Ensures that a class has at most one instance o^

Provides a global access point to that instance o^

Takes responsibility of managing that instance awayfrom the programmer (illegal to construct moreinstances)instances) o^

Provide accessor method that allows users to see the(one and only) instance(one and only) instance o^

Possibly the most known / popular design pattern!

CSE 403, Spring 2008, Alverson

Implementing singleton (oneinstantiation of the pattern…) y

Make constructor(s)

private

so that they can

not be called from outside

instantiation

of the pattern…)

not

be called from outside

y

Declare a single

s

tatic private

instance of

g

p

the classWrite a public

tI

t

or similar

y

Write

a public

g

etInstance()

or

similar

method that allows access to the single instanceo

possibly protect / synchronize this method to ensure o^

possibly protect / synchronize this method to ensurethat it will work in a multi-threaded program

CSE 403, Spring 2008, Alverson

Singleton sequence diagram

g

q

g

CSE 403, Spring 2008, Alverson

Singleton example 2 y

Variation: don't create the instance until needed

g

p

public class RandomGenerator {

private static RandomGenerator gen = null;public static RandomGenerator getInstance() {

if (gen == null) {

gen = new RandomGenerator(); }} return gen; } ... } y

What could go wrong with this version?

y CSE 403, Spring 2008, Alverson

What

could go wrong with this version?

Singleton example 3 y

Variation: solve concurrency issue by locking

g

p

public class RandomGenerator {

private static RandomGenerator gen = null;public static synchronized RandomGenerator

getInstance() {

if (gen == null) {

gen = new RandomGenerator();gen = new RandomGenerator(); } return gen; } ... } y CSE 403, Spring 2008, Alverson

Is anything wrong with this version?

Singleton exercise

g

y

Consider your projects. What classes could bea singleton in this system?a singleton in this system?

CSE 403, Spring 2008, Alverson

Pattern: Factory

(a variation of Factory Method Abstract(a variation of Factory Method, Abstract

Factory)

a class or method used to

create objects easily

CSE 403, Spring 2008, Alverson

Separate creation from use

p

Client

Factory

Createme an

I’llbaseit on

Client

Factory

InputReader

thetypeof input

newobject 1

Cre

Us

input

newobject 2

eates

es

newobject 3

CSE 403, Spring 2008, Alverson

Separate creation from use

p

Client

Factory

Createme an

I’llbaseit on

Client

Factory

inputReader

thetypeof input

newobject 1

Cre

Us

input

newobject 2

eates

es

newobject 3

Objects should either make

CSE 403, Spring 2008, Alverson

Objects

should either make

other objects or use otherobjects but never both.