the strategy pattern csc 211: design patterns. reading quiz

The Strategy Pattern

CSC 211: Design Patterns

Reading Quiz

Video Game Example• Aliens vs. Humans

• Both have adjustable life points

• Both take damage

• Both can recover life points

• Let’s design a class…

• Some functionality is shared, so inheriting from a common superclass makes sense

• We’ll differentiate between the two classes in their subclasses

LifeForm

currentLifePoints : int

getLifePoints() : inttakeHit(i : int): void

Class name

Class variables

Classmethods

LifeForm


getLifePoints() : int

takeHit(i : int): void

Human

armorPoints : int

<<create>> Human(life : int,armor : int)setArmorPoints(armor : int) : void

getArmorPoints() : int

<<create>> = constructor

What does Human inherit from LifeForm?

What does Human add to extend its superclass?

extends

LifeForm



Human

armorPoints : int



Alien

<<create>> Alien()

So we’re all set … at least for handling life points, right?

LifeForm



Human

armorPoints : int



Alien

<<create>> Alien()

Design change!Characters allowed to recover life points over time

LifeForm



recover(i : int) : void

Human

armorPoints : int



Alien

<<create>> Alien()

New Problem:Now Humans have a double advantage (armor + recovery)

LifeForm



Human

armorPoints : int



Alien

<<create>> Alien()recover(i : int) : void

Another Design Change:Some aliens should be harder than othersThey should recover at different rates, not all at once

LifeForm



Human

armorPoints : int



AlienmaxLifePoints : int<<create>> Alien()

recoverNone(i : int) : voidrecoverFraction(percent : double) void

recoverLinear(i : int) : void

Possible Solution:Multiple methods

Problems with this?

Design Principle #1

You will spend more time maintaining your code than you will on the initial design.

Plan for change!

Identify the aspects of your application that vary and separate

them from what stays the same

LifeForm



Human

armorPoints : int




recoverNone(i : int) : voidrecoverFraction(percent : double) void

recoverLinear(i : int) : void

Create an interface for the things that vary

LifeForm



Human

armorPoints : int




Create an interface for the things that vary

RecoveryBehavior

calculateRecovery(current : int,max : int) : int

<<interface>>

LifeForm



Human

armorPoints : int




Create an interface for the things that varyCreate behavior classes that implement the interface

RecoveryBehavior


<<interface>>

LifeForm



Human

armorPoints : int




<<interface>>RecoveryBehavior


RecoveryLinear

step : int<<create>> RecoveryLinear(rStep : int)

calculateRecovery(current : int, max : int) : int

implements

RecoveryNone

<<create>> RecoveryNone()calculateRecovery(current : int, max : int) : int

RecoveryFractional

percent : double<<create>> RecoveryFractional(rPercent : double)

calculateRecovery(current : int, max : int) : int

Design Principle #2

Exploit polymorphism in useful ways.

You can only inherit from one superclass

Program to an interface, not an implementation

LifeForm



Human

armorPoints : int



AlienrecoveryBehavior : RecoveryBehavior

maxLifePoints : int<<create>> Alien()

recover() : voidsetCurrentLifePoints(i : int) : void

<<interface>>RecoveryBehavior


RecoveryLinear

step : int

<<create>> RecoveryLinear(rStep : int)calculateRecovery(current : int, max : int) : int

RecoveryFractional

percent : double

<<create>> RecoveryFractional(rPercent : double)calculateRecovery(current : int, max : int) : int

RecoveryNone

<<create>> RecoveryNone()calculateRecovery(current : int, max : int) : int

Has-A

Design Principle #3

Avoid inheriting behavior

Favors flexibility

Favor composition over inheritance


SuperClass

Behavior myBehavior

methodA()methodB()

SubClassA

methodA() <<override>>

<<interface>>Behavior

behaviorMethod()

Behavior_v1behaviorMethod()

SubClassB Behavior_v2behaviorMethod()

IS-A

HAS-A

Implements(also IS-A)


Definition

The Strategy Pattern defines a family of algorithms, encapsulates each one, and makes

them interchangeable. Strategy lets the algorithm vary independently from the clients

that use it.

Lab 2• Due Monday

• Submit your code and tests electronically

• Header the at top of every file you write/*** Description of this class* @author Your Full Name* Your section number*/

• ProjectName_LastName• Zip you eclipse project, name your zip file

» LastName_Lab#

the strategy pattern csc 211: design patterns. reading quiz

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