09 grasp-object oriented analysis and design

8/7/2019 09 Grasp-OBJECT ORIENTED ANALYSIS AND DESIGN

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GRASP: Designing ObjectsGRASP: Designing Objects

with Responsibilitieswith ResponsibilitiesApplying UML and PatternsApplying UML and Patterns

Craig LarmanCraig LarmanChapter 17Chapter 17

Glenn D. Blank CSE432, Lehigh UniversityGlenn D. Blank CSE432, Lehigh University


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Chapter Learning ObjectivesChapter Learning Objectives

Learn about design patternsLearn about design patterns

Learn how to apply five GRASP patternsLearn how to apply five GRASP patterns

Youve learned about static class diagramsYouve learned about static class diagramsand dynamic interaction diagramsand dynamic interaction diagrams

UML is just notation; now you need to learnUML is just notation; now you need to learnhow to make effective use of the notationhow to make effective use of the notation

UML modeling is an art, guided by principlesUML modeling is an art, guided by principles


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Design patterns in architectureDesign patterns in architecture

AA patternpattern is ais arecurringrecurring solutionsolution to a standardto a standardproblemproblem,,in ain acontextcontext..

Christopher Alexander, professor of architectureChristopher Alexander, professor of architecture

Why is what a prof of architectureWhy is what a prof of architecturesays relevant to software?says relevant to software?

A pattern describes a problemA pattern describes a problemwhich occurs over and over againwhich occurs over and over againin our environment, and thenin our environment, and then

describes the core of the solutiondescribes the core of the solutionto that problem, in such a way thatto that problem, in such a way thatyou can use this solution a millionyou can use this solution a milliontimes over, without ever doing ittimes over, without ever doing itthe same way twice.the same way twice.


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Design and dress patternsDesign and dress patterns Jim Coplein, a software engineer:Jim Coplein, a software engineer:

I like to relate this definition to dress patterns I like to relate this definition to dress patterns

I could tell you how toI could tell you how tomake a dress by specifyingmake a dress by specifyingthe route of a scissorsthe route of a scissorsthrough a piece of cloththrough a piece of clothin terms of angles andin terms of angles andlengths of cut. Or, I couldlengths of cut. Or, I couldgive you a pattern.give you a pattern.Reading the specification,Reading the specification,

you would have no ideayou would have no ideawhat was being built or if you had built the right thing whenwhat was being built or if you had built the right thing whenyou were finished. The pattern foreshadows the product:you were finished. The pattern foreshadows the product:it is the rule for making the thing, but it is also,it is the rule for making the thing, but it is also,in many respects, the thing itself.in many respects, the thing itself.


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Patterns in engineeringPatterns in engineering

How do other engineers find and use patterns?How do other engineers find and use patterns? Mature engineering disciplines haveMature engineering disciplines have handbookshandbooks

describing successful solutions to known problemsdescribing successful solutions to known problems

Automobile designers don't design cars from scratchAutomobile designers don't design cars from scratch

using the laws of physicsusing the laws of physics

Instead, theyInstead, theyreusereuse standard designs with successfulstandard designs with successfultrack records, learning from experiencetrack records, learning from experience

Should software engineers make use of patterns? Why?Should software engineers make use of patterns? Why?

Be sure that you make everything according to the patternBe sure that you make everything according to the patternI have shown you here on the mountain. Exodus 25:40.I have shown you here on the mountain. Exodus 25:40.

Developing software from scratch is also expensiveDeveloping software from scratch is also expensive

Patterns supportPatterns supportreusereuse of software architecture designof software architecture design


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Definitions and namesDefinitions and names

Alexander: AAlexander: A patternpattern is ais arecurringrecurring solutionsolutionto a standardto a standardproblemproblem, in a, in acontextcontext..

Larman: In OO design, aLarman: In OO design, a patternpattern is a namedis a nameddescription of a problem and solution that can bedescription of a problem and solution that can beapplied in new contexts; ideally,applied in new contexts; ideally,a pattern advises us on how to apply the solutiona pattern advises us on how to apply the solutionin varying circumstances and considersin varying circumstances and considersthe forces and tradethe forces and trade--offs.offs.

How is Larmans definition similar to Alexanders?How is Larmans definition similar to Alexanders?

How are these definitions significantly different?How are these definitions significantly different?


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Patterns have suggestive names:Patterns have suggestive names: Arched Columns Pattern, Easy Toddler Dress Pattern, etc.Arched Columns Pattern, Easy Toddler Dress Pattern, etc.

Why is naming a pattern or principle helpful?Why is naming a pattern or principle helpful? It supports chunking and incorporating that conceptIt supports chunking and incorporating that concept

into our understanding and memoryinto our understanding and memory It facilitates communicationIt facilitates communication

Naming PatternsNaming Patternsimportant!important!

Star and Plume Quilt


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GRASPGRASP

Name chosen to suggest the importance ofName chosen to suggest the importance ofgraspgrasping fundamental principles to successfullying fundamental principles to successfullydesign objectdesign object--oriented softwareoriented software

Acronym forAcronym forGG

eneraleneralRR

esponsibilityesponsibilityAAssignmentssignmentSSoftwareoftware PPatternsatterns

Describe fundamental principles ofDescribe fundamental principles ofobject design and responsibilityobject design and responsibility

Expressed as patternsExpressed as patterns


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Five GRASP patterns:Five GRASP patterns:

CreatorCreator

Information ExpertInformation Expert

Low CouplingLow Coupling ControllerController

High CohesionHigh Cohesion


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Creator patternCreator pattern

Name:Name: CreatorCreator

Problem: Who creates an instance of A?Problem: Who creates an instance of A?

Solution: Assign class B the responsibilitySolution: Assign class B the responsibilityto create an instance of class A if one ofto create an instance of class A if one ofthese is true (the more the better):these is true (the more the better):

B contains or aggregates A (in a collection)B contains or aggregates A (in a collection)

B records AB records A

B closely uses AB closely uses A

B has the initializing data for AB has the initializing data for A


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Who creates the Squares?Who creates the Squares?

Figure 17.3, page 283Figure 17.3, page 283


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How does Create pattern lead toHow does Create pattern lead to

this partial Sequence diagram?this partial Sequence diagram?

Figure 17.4, page 283Figure 17.4, page 283


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How does Create pattern developHow does Create pattern develop

this Design Class Diagram (DCD)?this Design Class Diagram (DCD)?

Figure 17.5 , page 283Figure 17.5 , page 283

Board has a composite aggregation relationship with Square I.e., Board contains a collection of Squares


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Discussion of Creator patternDiscussion of Creator pattern

Responsibilities for object creation are commonResponsibilities for object creation are common

Connect an object to its creator when:Connect an object to its creator when:

Aggregator aggregates PartAggregator aggregates Part Container contains ContentContainer contains Content

Recorder recordsRecorder records

Initializing data passed in during creationInitializing data passed in during creation


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Contraindications or caveatsContraindications or caveats

Creation may require significant complexity:Creation may require significant complexity:

recycling instances for performance reasonsrecycling instances for performance reasons

conditionally creating instances from a familyconditionally creating instances from a familyof similar classesof similar classes

In these instances, other patterns are availableIn these instances, other patterns are available

Well learn aboutFactoryand other patterns laterWell learn aboutFactoryand other patterns later


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Information ExpertInformation Expertpattern or principlepattern or principle

Name: Information ExpertName: Information Expert

Problem: How to assign responsibilities to objects?Problem: How to assign responsibilities to objects?

Solution: Assign responsibility to the class that hasSolution: Assign responsibility to the class that has

the information needed to fulfill it?the information needed to fulfill it? E.g., Board information needed to geta SquareE.g., Board information needed to geta Square


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Benefits and ContraindicationsBenefits and Contraindications

Facilitates information encapsulation:Facilitates information encapsulation: why?why? Classes use their own info to fulfill tasksClasses use their own info to fulfill tasks

Encourages cohesive, lightweight class definitionsEncourages cohesive, lightweight class definitions

But:But: Information expert may contradict patterns ofInformation expert may contradict patterns of

Low Coupling andHigh CohesionLow Coupling andHigh Cohesion

Remember separation of concerns principleRemember separation of concerns principlefor large subfor large sub--systemssystems I.e., keep business orapplication logicI.e., keep business orapplication logic

in one place, user interface in other place,in one place, user interface in other place,database access in another place, etc.database access in another place, etc.


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Name:Name: Low CouplingLow Coupling

Problem: How to reduce the impact ofProblem: How to reduce the impact ofchange and encourage reuse?change and encourage reuse?

Solution: Assign aresponsibility so thatSolution: Assign aresponsibility so thatcoupling (linking classes) remains low.coupling (linking classes) remains low.

Low Coupling PatternLow Coupling Pattern


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Whydoes the following designWhydoes the following designviolate Low Coupling?violate Low Coupling?

Why is a better idea to leaveWhy is a better idea to leave

getSquare responsibility in Board?getSquare responsibility in Board?


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Benefits & ContraindicationsBenefits & Contraindications

Understandability: Classes are easierUnderstandability: Classes are easierto understand in isolationto understand in isolation

Maintainability: Classes arentaffected byMaintainability: Classes arentaffected bychanges in other componentschanges in other components

Reusability: easier to grab hold of classesReusability: easier to grab hold of classes

But:But: Dont sweat coupling to stable classes (inDont sweat coupling to stable classes (in

libraries or pervasive, welllibraries or pervasive, well--tested classes)tested classes)


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Controller patternController pattern

Name:Name: ControllerController(see Model(see Model--ViewView--Controllerarchitecture)Controllerarchitecture)

Problem: Who should be responsible for UIProblem: Who should be responsible for UI

events?events?

Solution: Assign responsibility forreceiving orSolution: Assign responsibility forreceiving orhandling a system event in one of two ways:handling a system event in one of two ways:

Represent the overall system (Represent the overall system (faadefaade pattern)pattern) Representa use case scenario within which theRepresenta use case scenario within which the

system event occurs (asystem event occurs (a sessionsession controller)controller)


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Who is the controller of playGame operation?Who is the controller of playGame operation?

Figure 17.9, p. 288Figure 17.9, p. 288

What class represents the overall systemWhat class represents the overall systemorrelevant use case scenario?orrelevant use case scenario?


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Cohesion measures how stronglyrelatedandCohesion measures how stronglyrelatedandfocusedare the responsibilities ofan elementfocusedare the responsibilities ofan element

Name:Name: High CohesionHigh Cohesion Problem: How to keep classes focusedandProblem: How to keep classes focusedand

manageable?manageable?

Solution: Assign responsibility so that cohesionSolution: Assign responsibility so that cohesion

remains high.remains high.

High Cohesion patternHigh Cohesion pattern


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How does the design on right promoteHow does the design on right promotehigh cohesion?high cohesion?

Delegate responsibility & coordinate workDelegate responsibility & coordinate work


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Benefits & ContraindicationsBenefits & Contraindications

Understandability, maintainabilityUnderstandability, maintainability

Complements Low CouplingComplements Low Coupling

But:But: Avoid grouping ofresponsibilities or code into oneAvoid grouping ofresponsibilities or code into one

class or component to simplify maintenance byclass or component to simplify maintenance byone person.one person. Why?Why?

Sometimes desirable to create less cohesive serverSometimes desirable to create less cohesive serverobjects that provide an interface for manyobjects that provide an interface for manyoperations, due to performance needs associatedoperations, due to performance needs associatedwith remote objects andremote communicationwith remote objects andremote communication


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SummarySummary

Skillful assignment ofresponsibilities isSkillful assignment ofresponsibilities isextremely important in objectextremely important in object--orienteddesignorienteddesign(CRC cards are one technique)(CRC cards are one technique)

Patterns are named problem/solution pairs thatPatterns are named problem/solution pairs thatcodify goodadvice and principles related tocodify goodadvice and principles related toassignment ofresponsibilitiesassignment ofresponsibilities

GRASP identifies five patterns or principles:GRASP identifies five patterns or principles:

Creator, Information Expert, Controller,Creator, Information Expert, Controller,Low Coupling andHigh CohesionLow Coupling andHigh Cohesion

09 grasp-object oriented analysis and design

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