Decorator Pattern

Decorator PatternSo many options!

Starbuzz Coffee Want to offer a variety of

combinations of coffee and condiments

Cost of a cup depends on the combination that was ordered

First Design Make a beverage class and a

subclass for each legal combination

HouseBlendcost()

DarkRoastcost()

Decafcost()

Espressocost()

BeverageString descriptiongetDescription()

<<abstract>> cost()…

<<abstract>>

But where are the fancy coffees?I could make this at home!

HouseBlendcost()

DarkRoastcost()

Decafcost()

Espressocost()HouseBlendWith

SteamedMilkcost()

DarkRoastWithSteamedMilk

cost()

DecafWithSteamedMilk

cost()

EspressoWithSteamedMilk

cost()

BeverageString descriptiongetDescription()

<<abstract>> cost()…

<<abstract>>

HouseBlendcost()

DarkRoastcost()

Decafcost()

Espressocost()HouseBlendWith

SteamedMilkcost()

DarkRoastWithSteamedMilk

cost()

DecafWithSteamedMilk

cost()

EspressoWithSteamedMilk

cost()HouseBlendMochacost()

DarkRoastMochacost()

DecafMochacost()

EspressoMochacost()

BeverageString descriptiongetDescription()

<<abstract>> cost()…

<<abstract>>

HouseBlendcost()

DarkRoastcost()

Decafcost()

Espressocost()HouseBlendWith

SteamedMilkcost()

DarkRoastWithSteamedMilk

cost()

DecafWithSteamedMilk

cost()

EspressoWithSteamedMilk

cost()HouseBlendMochacost()

DarkRoastMochacost()

DecafMochacost()

EspressoMochacost()HouseBlend

WithWhipcost()

DarkRoastWithWhip

cost()

DecafWithWhip

cost()

EspressoWithWhip

cost()

BeverageString descriptiongetDescription()

<<abstract>> cost()…

<<abstract>>

Second Design Make the superclass contain

booleans to specify which condiments are included and subclasses for each type of coffee

How do we compute cost? What does it take to add a new

condiment?

BeverageString description

milksoy

mochawhip

getDescription()<<abstract>>cost()

hasMilk()setMilk()hasSoy()setSoy()

hasMocha()setMocha()hasWhip()setWhip()

…

HouseBlendcost()

DarkRoastcost()

Decafcost()

Espressocost()

<<abstract>>

Design Principle Classes should be open for

extension, but closed for modification “extension” is NOT subclassing It means the addition of new behavior

(without modifying the code!)

Decorator Pattern Start with an instance of the

“basic”classes and then decorate it with new capabilities

Dark Roast

Mocha

cost()cost()

Whip

cost() 0.990.99+0.200.99+0.20+0.10

Key Points Decorators have the same supertypes

as the objects they decorate This lets us pass around the decorated

object instead of the original (unwrapped) object

Decorator add behavior by delegating to the object it decorates and then adding its own behavior

Can add decorations at any time

Decorator Pattern

<<interface/abstract>>ComponentmethodA()

ConcreteComponentmethodA()

ConcreteDecoratorAComponent wrappedObj

methodA()newBehavior()

<<abstract>>DecoratormethodA()

ConcreteDecoratorBComponent wrappedObj

Object newState()methodA()

HAS-A

Let’s Look at the Code

public abstract class Beverage{ String description = “Unknown”;

public String getDescription() { return description; }

public abstract double cost();}

public abstract class CondimentDecorator extends Beverage { public abstract String getDescription(); public abstract double cost();}

public class Espresso extends Beverage { public Espresso() { description = “Espresso”; }

public double cost() { return 1.99; }}

public class Mocha extends CondimentDecorator { Beverage bev;

public Mocha(Beverage bev) { this.bev = bev; }

public String getDescription { return bev.getDescription() + “, Mocha”; } public double cost() { return .20 + bev.cost(); }}

public class StarBuzz { public static void main(String args[]) { Beverage bev = new Espresso); bev = new Mocha(bev); bev = new Mocha(bev); bev = new Whip(bev); bev = new Soy(bev);

System.out.println(bev.getDescription() + “ $”+ bev.getCost()); }

Decorators in Java File I/O Example

FileInputStream

BufferedInputStreamLineNumberInputStream

Component

Concrete decorators

InputStream

FileInputStream StringBufferInputStream FilterInputStream

BufferedInputStream LineNumberInputStream

concrete components

abstract decorator

concrete decorators

abstract component

Lab Set Up Using the Decorator Pattern to

customize weapons