Decorator Pattern

So many options!So many options!

Starbuzz CoffeeStarbuzz Coffee

Want to offer a variety of combinations of coffee and condiments

Cost of a cup depends on the combination that was ordered

Want to offer a variety of combinations of coffee and condiments

Cost of a cup depends on the combination that was ordered

First DesignFirst Design

Make a beverage class and a subclass for each legal combination

Make a beverage class and a subclass for each legal combination

HouseBlendcost()

HouseBlendcost()

DarkRoastcost()

DarkRoastcost()

Decafcost()Decafcost()

Espressocost()

Espressocost()

BeverageString descriptiongetDescription()

<<abstract>> cost()…

<<abstract>>

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

HouseBlendcost()

HouseBlendcost()

DarkRoastcost()

DarkRoastcost()

Decafcost()Decafcost()

Espressocost()

Espressocost()

HouseBlendWithSteamedMilk

cost()

HouseBlendWithSteamedMilk

cost()

DarkRoastWithSteamedMilk

cost()

DarkRoastWithSteamedMilk

cost()

DecafWithSteamedMilk

cost()

DecafWithSteamedMilk

cost()

EspressoWithSteamedMilk

cost()

EspressoWithSteamedMilk

cost()

BeverageString descriptiongetDescription()

<<abstract>> cost()…

<<abstract>>

HouseBlendcost()

HouseBlendcost()

DarkRoastcost()

DarkRoastcost()

Decafcost()Decafcost()

Espressocost()

Espressocost()

HouseBlendWithSteamedMilk

cost()

HouseBlendWithSteamedMilk

cost()

DarkRoastWithSteamedMilk

cost()

DarkRoastWithSteamedMilk

cost()

DecafWithSteamedMilk

cost()

DecafWithSteamedMilk

cost()

EspressoWithSteamedMilk

cost()

EspressoWithSteamedMilk

cost()HouseBlendMochacost()

HouseBlendMochacost()

DarkRoastMochacost()

DarkRoastMochacost()

DecafMochacost()

DecafMochacost()

EspressoMochacost()

EspressoMochacost()

BeverageString descriptiongetDescription()

<<abstract>> cost()…

<<abstract>>

HouseBlendcost()

HouseBlendcost()

DarkRoastcost()

DarkRoastcost()

Decafcost()Decafcost()

Espressocost()

Espressocost()

HouseBlendWithSteamedMilk

cost()

HouseBlendWithSteamedMilk

cost()

DarkRoastWithSteamedMilk

cost()

DarkRoastWithSteamedMilk

cost()

DecafWithSteamedMilk

cost()

DecafWithSteamedMilk

cost()

EspressoWithSteamedMilk

cost()

EspressoWithSteamedMilk

cost()HouseBlendMochacost()

HouseBlendMochacost()

DarkRoastMochacost()

DarkRoastMochacost()

DecafMochacost()

DecafMochacost()

EspressoMochacost()

EspressoMochacost()HouseBlend

WithWhipcost()

HouseBlendWithWhip

cost()

DarkRoastWithWhip

cost()

DarkRoastWithWhip

cost()

DecafWithWhip

cost()

DecafWithWhip

cost()

EspressoWithWhip

cost()

EspressoWithWhip

cost()

BeverageString descriptiongetDescription()

<<abstract>> cost()…

<<abstract>>

Second DesignSecond 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?

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()

HouseBlendcost()

DarkRoastcost()

DarkRoastcost()

Decafcost()Decafcost()

Espressocost()

Espressocost()

<<abstract>>

Design PrincipleDesign 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!)

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 PatternDecorator Pattern

Start with an instance of the “basic”classes and then decorate it with new capabilities

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 PointsKey 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

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 PatternDecorator Pattern

<<interface/abstract>>ComponentmethodA()

ConcreteComponentmethodA()

ConcreteComponentmethodA()

ConcreteDecoratorAComponent wrappedObj

methodA()newBehavior()

ConcreteDecoratorAComponent wrappedObj

methodA()newBehavior()

<<abstract>>DecoratormethodA()

<<abstract>>DecoratormethodA()

ConcreteDecoratorBComponent wrappedObj

Object newState()methodA()

ConcreteDecoratorBComponent wrappedObj

Object newState()methodA()

HAS-A

Let’s Look at the CodeLet’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 JavaDecorators in Java

File I/O Example File I/O Example

FileInputStream

BufferedInputStreamLineNumberInputStream

Component

Concrete decorators

InputStreamInputStream

FileInputStreamFileInputStream StringBufferInputStreamStringBufferInputStream FilterInputStreamFilterInputStream

BufferedInputStreamBufferedInputStream LineNumberInputStreamLineNumberInputStream

concrete componentsconcrete components

abstract decoratorabstract decorator

concrete decoratorsconcrete decorators

abstract componentabstract component

Lab Set UpLab Set Up

Using the Decorator Pattern to customize weapons

Using the Decorator Pattern to customize weapons