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Domain, Encumbrance, Domain, Encumbrance, and Cohesionand Cohesion

• Domains of Object ClassesDomains of Object Classes– application domain– business domain– architecture domain– foundation domain

• EncumbranceEncumbrance

• Class CohesionClass Cohesion

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Foundation DomainFoundation Domain• Classes used in different applications Classes used in different applications

from different industries on different from different industries on different computer architectures computer architectures

• Classes of highest reusabilityClasses of highest reusability

• A foundation class may be built upon A foundation class may be built upon other foundation classesother foundation classes

• Fundamental Classes - primitive typesFundamental Classes - primitive types

• Structural Classes - implementation of Structural Classes - implementation of data structuresdata structures

• Semantic Classes - used conventionally Semantic Classes - used conventionally

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Architecture DomainArchitecture Domain

• Usable in different applications from Usable in different applications from different industriesdifferent industries

• Reusability is limited to a single Reusability is limited to a single computer (hardware & software) computer (hardware & software) architecturearchitecture

• Machine-Communication ClassesMachine-Communication Classes

• Database-Manipulation ClassesDatabase-Manipulation Classes

• Human-Interface ClassesHuman-Interface Classes

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Business DomainBusiness Domain

• Useful in different applications within a Useful in different applications within a single industrysingle industry

• Reusability may even limited to one Reusability may even limited to one corporationcorporation

• Attribute Classes - properties of thingsAttribute Classes - properties of things

• Role Classes - roles played in businessRole Classes - roles played in business

• Relationship Classes - associations Relationship Classes - associations among thingsamong things

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Application DomainApplication Domain• Used only within a single applicationUsed only within a single application

• Lowest reusabilityLowest reusability

• Event-Recognizer ClassesEvent-Recognizer Classes– event daemons–monitor input to check for occurrence of

specific events in the environment

• Event-Manager ClassesEvent-Manager Classes– carry out business policy when an event of a

given type occurs

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Domains of Classes and ReusabilityDomains of Classes and Reusability• Application DomainApplication Domain

– event manager classes– event recognizer classes

• Business DomainBusiness Domain– relationship classes– role classes– attribute classes

• Architecture DomainArchitecture Domain– machine communication classes– database manipulation classes– human interface classes

• Foundation DomainFoundation Domain– semantic classes– structural classes– fundamental classes

Low Reusability

Medium Reusability

High Reusability

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EncumbranceEncumbrance

• Quantitative Measure of Class SophistiQuantitative Measure of Class Sophisticationcation

• Definition of Encumbrance in Object-OrDefinition of Encumbrance in Object-Oriented Systemsiented Systems

• Use of EncumbranceUse of Encumbrance

• Law of DemeterLaw of Demeter

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Class ReferenceClass Reference• Direct Class-Reference Set - Direct Class-Reference Set - the set of classes the set of classes

referred directly by a class (class C refer directly to D if:)referred directly by a class (class C refer directly to D if:)

– C inherits from D

– C has an attribute of class D

– C has an operation with an input argument of class D

– C has a method that sends a message with a returned argument of class D

– C has a method containing a local variable of class D

– C supplies D as an actual class parameter to a parameterize class

– C has a friend class D (in C++ only)

• Indirect Class-Reference Set - union of direct Indirect Class-Reference Set - union of direct class-reference setsclass-reference sets

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Definition of EncumbranceDefinition of Encumbrance• Measures the total ancillary machinery of a Measures the total ancillary machinery of a

classclass– comprises all the other classes that the given

class must rely on in order to work– total number of classes referred (both directly

and indirectly) to by the given class

• Direct EncumbranceDirect Encumbrance– size of its direct class-reference set

• Indirect EncumbranceIndirect Encumbrance– size of its indirect class-reference set

• Used to indicate class couplingUsed to indicate class coupling

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Use of EncumbranceUse of Encumbrance• Measure of class sophisticationMeasure of class sophistication

• Higher domains have higher Higher domains have higher encumbranceencumbrance

• Unexpected encumbrance may indicate Unexpected encumbrance may indicate a fault in class designa fault in class design– low domain with high encumbrance =>

may have a problem with class cohesion– high domain with low encumbrance =>

lack of reusability

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Law of DemeterLaw of Demeter• Guiding principle for limiting direct Guiding principle for limiting direct

encumbrance of a classencumbrance of a class

• Limitation of message receivers:Limitation of message receivers:– the object itself– an object referred to by a method signature– an object referred to by a variable (attribute)• current class onlycurrent class only• current class plus inherited classescurrent class plus inherited classes

– an object created by a method– an object referred to by a global variable

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Class CohesionClass Cohesion• Measure of interrelationship of attributes Measure of interrelationship of attributes

and operations located in the external and operations located in the external interface of a classinterface of a class

• Observe from outside instead of inside of Observe from outside instead of inside of the classthe class

• Three major cohesion problemsThree major cohesion problems–mixed instance–mixed domain –mixed role

• A class may have more than one cohesion A class may have more than one cohesion problemproblem

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Mixed-Instance CohesionMixed-Instance Cohesion

• A class has some features (attributes A class has some features (attributes or operations) that are undefined for or operations) that are undefined for some objects of the classsome objects of the class

• Design problem of type definitionDesign problem of type definition

• Usually involve redesign of the Usually involve redesign of the inheritance hierarchyinheritance hierarchy

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Mixed-Domain CohesionMixed-Domain Cohesion• A class contains an element that A class contains an element that

directly encumbers the class with an directly encumbers the class with an extrinsic class of a different domainextrinsic class of a different domain

• Class B is Class B is extrinsicextrinsic to A if A can be to A if A can be fully defined with no notion of Bfully defined with no notion of B

• Class B is Class B is intrinsicintrinsic to A if B captures to A if B captures some characteristic inherent to Asome characteristic inherent to A

• Always include classes from lower Always include classes from lower domains because of intrinsic domains because of intrinsic properties of the classproperties of the class

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Mixed-Role CohesionMixed-Role Cohesion

• A class contains an element that A class contains an element that directly encumbers the class with an directly encumbers the class with an extrinsic class of the same domainextrinsic class of the same domain

• Depend on the basic definition Depend on the basic definition (original representation) of the class(original representation) of the class

• Least serious cohesion problemLeast serious cohesion problem

• may reduce reusabilitymay reduce reusability