Head First Design Patterns ch03.pdf

3 the DecoratorPattern

Decorating Objects

I used to think real men

subclassed everything. That was

until I learned the power of

extension at runtime, rather than

at compile time. Now look at me!

Just call this chapter “Design Eye for the Inheritance Guy.”

We’ll re-examine the typical overuse of inheritance and you’ll learn how to decorate

your classes at runtime using a form of object composition. Why? Once you know the

techniques of decorating, you’ll be able to give your (or someone else’s) objects new

responsibilities without making any code changes to the underlying classes.

this is a new chapter 79

the starbuzz story

Welcome to Starbuzz Coffee

Starbuzz Coffee has made a name for itself as the

fastest growing coffee shop around. If you’ve seen one

on your local corner, look across the street; you’ll see

another one.

Because they’ve grown so quickly, they’re scrambling

to update their ordering systems to match their

beverage offerings.

When they ﬁrst went into business they designed their

classes like this...

Beverage

is s et in e ach su bclass and ho lds a

description

des criptio n of t he bev erage, like

The c ost() meth od is

abstr act; s ubclas sses

need to de fine t heir

own i mplem entat ion.

“Mo st Ex cellent Dark Roast ”.

The getDes criptio n() me thod

retur ns the descr iption.

getDescription()

cost()

// Other useful methods...

HouseBlend

DarkRoast

Decaf

Espresso

cost()

Each subclas s implements cost() to return th e cost of the beverage.

80 Chapter 3

The descr iption instan ce var iable

the decorator pattern

In addition to your coffee, you can also ask for several condiments like

steamed milk, soy, and mocha (otherwise known as chocolate), and have

it all topped off with whipped milk. Starbuzz charges a bit for each of

these, so they really need to get them built into their order system.

Here’s their ﬁrst attempt...

Beverage

description

getDescription()

cost()

// Other useful methods...

HouseBlendWithSteamedMilk

andMocha

DarkRoastWithSteamedMilk

andMocha

DecafWithSteamedMilk

andMocha

EspressoWithSteamedMilk

andMocha

HouseBlendWithSteamedMilk

andCaramel

cost()

HouseBlendWithWhipandMocha

EspressoWithSteamedMilk

andCaramel

cost()

DecafWithSteamedMilk

andCaramel

DarkRoastWithSteamedMilk

andCaramel

cost()

EspressoWithWhipandMocha

HouseBlendWithMocha

cost()

DarkRoastWithWhipandMocha

cost()

DecafWithWhipandMocha

HouseBlendWithSoyandMocha

cost()

EspressoWithMocha

HouseBlendWithSteamedMilk

andSoy

cost()

DarkRoastWithMocha

cost()

DecafWithMocha

cost()

EspressoWithSteamedMilk

andSoy

DecafWithSoy

HouseBlendWithSoy

cost()

DecafWithSteamedMilk

andSoy

HouseBlendWithSteamedMilk

cost()

DarkRoastWithSteamedMilk

andSoy

cost()

EspressoWithSteamedMilk

cost()

HouseBlendWithWhip

cost()

DarkRoastWithSoy

cost()

DecafWithSteamedMilk

cost()

DarkRoastWithSteamedMilk

DecafWithSoyandMocha

DarkRoastWithSoyandMocha

DarkRoastWithSoy

cost()

DecafWithSoy

cost()

HouseBlendWithSteamedMilk

andWhip

cost()

DecafWithSoyandMocha

EspressoWhip

cost()

HouseBlendWithWhipandSoy

cost()

DarkRoastWithWhip

DecafWithWhip

cost()

EspressoWithSteamedMilk

andWhip

cost()

DecafWithSteamedMilk

andWhip

DarkRoastWithSteamedMilk

andWhip

EspressoWithWhipandSoy

cost()

DarkRoastWithWhipandSoy

cost()

DecafWithWhipandSoy

cost()

Whoa!

Can you say

“class explosion?”

you are here 4 81

cost()

violating design principles

brain

power

It’s pretty obvious that Starbuzz has created a maintenance nightmare for

themselves. What happens when the price of milk goes up? What do they do

when they add a new caramel topping?

Thinking beyond the maintenance problem, which of the design principles that

we’ve covered so far are they violating?

This is stupid; why do we need

all these classes? Can’t we just use

instance variables and inheritance in

the superclass to keep track of the

condiments?

Well, let’s give it a try. Let’s start with the Beverage base

class and add instance variables to represent whether or

not each beverage has milk, soy, mocha and whip...

Beverage

description

milk

soy

mocha

whip

New boolean va lues for

each condiment .

Now we’ll implem ent cost() in Beverage (instead of

keeping it abstr act), so that it can calculate the

costs associated with the condiments for a partic ular

beverage instanc e. Subclasses will still override

cost(), but they will also invoke the super version so

that they can c alculate the total cost of the ba sic

beverage plus th e costs of the added condiments.

getDescription()

cost()

hasMilk()

setMilk()

hasSoy()

setSoy()

hasMocha()

setMocha()

hasWhip()

setWhip()

// Other useful methods..

82 Chapter 3

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