09 grasp

GRASP: Designing ObjectsGRASP: Designing Objects
with Responsibilitieswith Responsibilities
Applying UML and PatternsApplying UML and Patterns
Craig LarmanCraig Larman
Chapter 17Chapter 17
Glenn D. Blank CSE432, Lehigh UniversityGlenn D. Blank CSE432, Lehigh University

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
 You’ve learned about static class diagramsYou’ve learned about static class diagrams
and dynamic interaction diagramsand dynamic interaction diagrams
 UML is just notation; now you need to learnUML is just notation; now you need to learn
how 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

Design patterns in architectureDesign patterns in architecture
 AA patternpattern is ais a recurringrecurring solutionsolution to a standardto a standard problemproblem,,
in ain a contextcontext..
 Christopher Alexander, professor of architecture…Christopher Alexander, professor of architecture…
– Why is what a prof of architectureWhy is what a prof of architecture
says relevant to software?says relevant to software?
– ““A pattern describes a problemA pattern describes a problem
which occurs over and over againwhich occurs over and over again
in our environment, and thenin our environment, and then
describes the core of the solutiondescribes the core of the solution
to that problem, in such a way thatto that problem, in such a way that
you can use this solution a millionyou can use this solution a million
times over, without ever doing ittimes over, without ever doing it
the same way twice.”the same way twice.”

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 to
make a dress by specifyingmake a dress by specifying
the route of a scissorsthe route of a scissors
through a piece of cloththrough a piece of cloth
in terms of angles andin terms of angles and
lengths of cut. Or, I couldlengths of cut. Or, I could
give you a pattern.give you a pattern.
Reading the specification,Reading the specification,
you would have no ideayou would have no idea
what was being built or if you had built the right thing whenwhat was being built or if you had built the right thing when
you 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.”

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, they reusereuse standard designs with successfulstandard designs with successful
track records, learning from experiencetrack records, learning from experience
– Should software engineers make use of patterns?Should software engineers make use of patterns?
Why?Why?
– ““Be sure that you make everything according to the patternBe sure that you make everything according to the pattern
I 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 support reusereuse of software architecture designof software architecture design

Definitions and namesDefinitions and names
 Alexander: “AAlexander: “A patternpattern is ais a recurringrecurring solutionsolution
to a standardto a standard problemproblem, in a, in a contextcontext.”.”
 Larman: “In OO design, aLarman: “In OO design, a patternpattern is a namedis a named
description of a problem and solution that can bedescription of a problem and solution that can be
applied 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 solution
in varying circumstances and considersin varying circumstances and considers
the forces and trade-offs.”the forces and trade-offs.”
 How is Larman’s definition similar to Alexander’s?How is Larman’s definition similar to Alexander’s?
 How are these definitions significantly different?How are these definitions significantly different?

 Patterns have suggestive names:Patterns have suggestive names:
– Arched Columns Pattern, Easy Toddler Dress Pattern,Arched Columns Pattern, Easy Toddler Dress Pattern,
etc.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 Patterns—important!Naming Patterns—important!
Star and Plume Quilt

GRASPGRASP
 Name chosen to suggest the importance ofName chosen to suggest the importance of
graspgrasping fundamental principles to successfullying fundamental principles to successfully
design object-oriented softwaredesign object-oriented software
 Acronym forAcronym for GGeneraleneral RResponsibilityesponsibility
AAssignmentssignment SSoftwareoftware PPatternsatterns
 Describe fundamental principles ofDescribe fundamental principles of
object design and responsibilityobject design and responsibility
 Expressed as patternsExpressed as patterns

Five GRASP patterns:Five GRASP patterns:
 CreatorCreator
 Information ExpertInformation Expert
 Low CouplingLow Coupling
 ControllerController
 High CohesionHigh Cohesion

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 responsibility
to create an instance of class A if one ofto create an instance of class A if one of
these 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

Who creates the Squares?Who creates the Squares?
Figure 17.3, page 283Figure 17.3, page 283

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

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

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

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 family
of similar classesof similar classes
 In these instances, other patterns areIn these instances, other patterns are
available…available…
– We’ll learn about Factory and other patterns later…We’ll learn about Factory and other patterns later…

Information ExpertInformation Expert
pattern 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 get a SquareE.g., Board information needed to get a Square

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 and High CohesionLow Coupling and High Cohesion
 Remember separation of concerns principleRemember separation of concerns principle
for large sub-systemsfor large sub-systems
 I.e., keep “business” or application logicI.e., keep “business” or application 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.

 Name:Name: Low CouplingLow Coupling
 Problem: How to reduce the impact ofProblem: How to reduce the impact of
change and encourage reuse?change and encourage reuse?
 Solution: Assign a responsibility so thatSolution: Assign a responsibility so that
coupling (linking classes) remains low.coupling (linking classes) remains low.
Low Coupling PatternLow Coupling Pattern

Why does the following designWhy does the following design
violate 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?

Benefits & ContraindicationsBenefits & Contraindications
 Understandability: Classes are easierUnderstandability: Classes are easier
to understand in isolationto understand in isolation
 Maintainability: Classes aren’t affected byMaintainability: Classes aren’t affected by
changes in other componentschanges in other components
 Reusability: easier to grab hold of classesReusability: easier to grab hold of classes
But:But:
 Don’t sweat coupling to stable classes (inDon’t sweat coupling to stable classes (in
libraries or pervasive, well-tested classes)libraries or pervasive, well-tested classes)

Controller patternController pattern
 Name:Name: ControllerController
(see Model-View-Controller architecture)(see Model-View-Controller architecture)
 Problem: Who should be responsible for UIProblem: Who should be responsible for UI
events?events?
 Solution: Assign responsibility for receiving orSolution: Assign responsibility for receiving or
handling 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 (façadefaçade pattern)pattern)
– Represent a use case scenario within which theRepresent a use case scenario within which the
system event occurs (asystem event occurs (a sessionsession controller)controller)

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 system
or relevant use case scenario?or relevant use case scenario?

 Cohesion measures how strongly related andCohesion measures how strongly related and
focused are the responsibilities of an elementfocused are the responsibilities of an element
 Name:Name: High CohesionHigh Cohesion
 Problem: How to keep classes focused andProblem: How to keep classes focused and
manageable?manageable?
 Solution: Assign responsibility so that cohesionSolution: Assign responsibility so that cohesion
remains high.remains high.
High Cohesion patternHigh Cohesion pattern

How does the design on right promoteHow does the design on right promote
high cohesion?high cohesion?
Delegate responsibility & coordinate workDelegate responsibility & coordinate work

Benefits & ContraindicationsBenefits & Contraindications
 Understandability, maintainabilityUnderstandability, maintainability
 Complements Low CouplingComplements Low Coupling
But:But:
 Avoid grouping of responsibilities or code into oneAvoid grouping of responsibilities or code into one
class or component to simplify maintenance byclass or component to simplify maintenance by
one person.one person. Why?Why?
 Sometimes desirable to create less cohesive serverSometimes desirable to create less cohesive server
objects that provide an interface for manyobjects that provide an interface for many
operations, due to performance needs associatedoperations, due to performance needs associated
with remote objects and remote communicationwith remote objects and remote communication

SummarySummary
 Skillful assignment of responsibilities isSkillful assignment of responsibilities is
extremely important in object-oriented designextremely important in object-oriented design
(CRC cards are one technique)(CRC cards are one technique)
 Patterns are named problem/solution pairs thatPatterns are named problem/solution pairs that
codify good advice and principles related tocodify good advice and principles related to
assignment of responsibilitiesassignment of responsibilities
 GRASP identifies five patterns or principles:GRASP identifies five patterns or principles:
– Creator, Information Expert, Controller,Creator, Information Expert, Controller,
Low Coupling and High CohesionLow Coupling and High Cohesion

09 grasp

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