Chapter Eight Expanding Our Horizons Ku-Yaw Chang Assistant Professor, Department of Computer...

Chapter EightChapter Eight

Expanding Our HorizonsExpanding Our HorizonsKu-Yaw ChangKu-Yaw Chang

[email protected]@mail.dyu.edu.twAssistant Professor, Department of Assistant Professor, Department of

Computer Science and Information EngineeringComputer Science and Information EngineeringDa-Yeh UniversityDa-Yeh University

mailto:[email protected]

22Ku-Yaw ChangKu-Yaw Chang Expanding Our HorizonsExpanding Our Horizons

OutlineOutlineOverviewOverviewObjects: the Traditional View and the New ViewObjects: the Traditional View and the New ViewEncapsulation: the Traditional View and the New Encapsulation: the Traditional View and the New ViewViewFind What is Varying and Encapsulate ItFind What is Varying and Encapsulate ItCommonality/Variability and Abstract ClassesCommonality/Variability and Abstract ClassesSummarySummary


OverviewOverview

Fundamental conceptsFundamental concepts ObjectsObjects EncapsulationEncapsulation AbstractAbstract

A new way of seeing object-oriented designA new way of seeing object-oriented design From the perspective that design patterns createFrom the perspective that design patterns create


ObjectsObjects

The traditional viewThe traditional view Data with methodsData with methods

Smart dataSmart dataFrom implementation perspectiveFrom implementation perspective

The new viewThe new view Things with responsibilitiesThings with responsibilities

From conceptual perspectiveFrom conceptual perspectiveFocus on what the objects are supposed to doFocus on what the objects are supposed to do

Make a preliminary design without worrying about all of the Make a preliminary design without worrying about all of the details involveddetails involved

Implement the designImplement the design


ObjectsObjects

Things with responsibilitiesThings with responsibilities Easier to think in terms of responsibilitiesEasier to think in terms of responsibilities Help to define the object’s public interfaceHelp to define the object’s public interface

For exampleFor example A Shape object’s responsibilitiesA Shape object’s responsibilities

To know where it is locatedTo know where it is located getLocation( … )getLocation( … ) drawShape( … )drawShape( … ) unDrawShape( … )unDrawShape( … )

To be able to draw itself on a displayTo be able to draw itself on a displayTo be able to remove itself from a displayTo be able to remove itself from a display


ObjectsObjects

Focus on motivation rather than on Focus on motivation rather than on implementation is a recurring theme in design implementation is a recurring theme in design patterns.patterns. Basic viewpoint for objectsBasic viewpoint for objects Superior designsSuperior designs


EncapsulationEncapsulation

The traditional viewThe traditional view Data hidingData hiding

Too limitedToo limited Umbrella and carUmbrella and car

The new viewThe new view Any kind of hidingAny kind of hiding


EncapsulationEncapsulation

Multiple levels of encapsulationMultiple levels of encapsulation Encapsulation of Encapsulation of datadata Encapsulation of Encapsulation of methodsmethods Encapsulation of Encapsulation of subclassessubclasses Encapsulation of Encapsulation of other objectsother objects


InheritanceInheritance

In object-oriented paradigmIn object-oriented paradigm Reuse of classes was one of its big benefitsReuse of classes was one of its big benefits

Generalized classGeneralized classSpecialized classSpecialized class

InheritanceInheritance as a concept versus for reuseas a concept versus for reuse


ProblemsProblems

Can cause weak cohesionCan cause weak cohesion Different types of borders or other thingsDifferent types of borders or other things Pentagon aren’t just concerned about pentagons Pentagon aren’t just concerned about pentagons

anymoreanymore

Reduces possibility of reuseReduces possibility of reuse Code for different bordersCode for different borders

Does not scale well with variationDoes not scale well with variation Need to specialize the pentagon class repeatedlyNeed to specialize the pentagon class repeatedly


Find What Is Varying and Find What Is Varying and Encapsulate ItEncapsulate It

GoF suggests the followingGoF suggests the following Consider what should be variable in your design.Consider what should be variable in your design. This This

approach is the opposite of focusing on the cause of approach is the opposite of focusing on the cause of redesign. Instead of considering what might force a redesign. Instead of considering what might force a change to a design, consider what you want to be change to a design, consider what you want to be able to change without redesign. The focus here is on able to change without redesign. The focus here is on encapsulating the concept that variesencapsulating the concept that varies, a theme of , a theme of many design patterns.many design patterns.


Find What Is Varying and Find What Is Varying and Encapsulate ItEncapsulate It

Use encapsulation to create layers between Use encapsulation to create layers between objectsobjects Change things on different sides of the layers without Change things on different sides of the layers without

affecting the other sideaffecting the other side Loose-coupling between the sidesLoose-coupling between the sides


Variation in Data VersusVariation in Data Versusin Behaviorin Behavior

RequirementsRequirements Each type of animal can have Each type of animal can have a different number of a different number of

legslegs Animal objects must be able to remember and Animal objects must be able to remember and

retrieve this informationretrieve this information Each type of animal can have Each type of animal can have a different type of a different type of

movementmovementWalking and flyingWalking and flying

Animal objects must be able to return how long it will Animal objects must be able to return how long it will take to move from one place to anothertake to move from one place to another

Given a specified type of terrainGiven a specified type of terrain



SolutionsSolutions A data memberA data member A choice of approachA choice of approach

Having a data memberHaving a data memberHaving two different types of Animals (both derived Having two different types of Animals (both derived from the base Animal class)from the base Animal class)

One for walkingOne for walking One for flyingOne for flying



ProblemsProblems Too many detailsToo many details

A plethora of classesA plethora of classes Eagles : carnivores that flyEagles : carnivores that fly Lions : carnivores that walkLions : carnivores that walk Sparrows : vegetarians that flySparrows : vegetarians that fly Cows: vegetarians that walkCows: vegetarians that walk

Cannot handle Animals that can both walk and flyCannot handle Animals that can both walk and fly Weakly cohesiveWeakly cohesive



Another possibility existsAnother possibility exists Have the Animal class contain an object that has the appropriate Have the Animal class contain an object that has the appropriate

movement behaviormovement behavior


Comparing the twoComparing the two

Is one object containing another object Is one object containing another object inherently different from an object having a mere inherently different from an object having a mere data member ?data member ? Are data members objects?Are data members objects?

In object-oriented programming, In object-oriented programming, everythingeverything is an is an object.object. Using objects to contain variation in attributesUsing objects to contain variation in attributes Using objects to contain variation in behaviorUsing objects to contain variation in behavior


Commonality and Variability Commonality and Variability AnalysisAnalysis

Commonality analysisCommonality analysis Identify Identify wherewhere things vary things vary

Search for common elementsSearch for common elements

Variability analysisVariability analysis Identify Identify howhow they vary they vary


Commonality/Variability and Commonality/Variability and Abstract ClassesAbstract Classes


Two-Step ProcedureTwo-Step Procedure

When DefiningWhen Defining You Must Ask yourselfYou Must Ask yourself

An abstract classAn abstract class(commonality)(commonality)

What interface is needed to handle all What interface is needed to handle all of the responsibilities of this class?of the responsibilities of this class?

Derived classesDerived classesGiven this particular implementation Given this particular implementation (this variation), how can I implement it (this variation), how can I implement it with the given specification?with the given specification?


RelationshipsRelationships

The relationship between the specification The relationship between the specification perspective and the conceptual perspectiveperspective and the conceptual perspective It identifies the interface I need to use to handle all of It identifies the interface I need to use to handle all of

the cases of the concept (that is, the commonality)the cases of the concept (that is, the commonality)

The relationship between the specification The relationship between the specification perspective and the implementation perspectiveperspective and the implementation perspective Given this specification, how can I implement this Given this specification, how can I implement this

particular case (this variation)?particular case (this variation)?


SummarySummary

EncapsulationEncapsulation Any kind of hidingAny kind of hiding

More than simply hiding dataMore than simply hiding data

InheritanceInheritance A method of consistently dealing with different A method of consistently dealing with different

concrete classes that are conceptually the sameconcrete classes that are conceptually the sameNot a means of specializationNot a means of specialization

Using objects to hold variations in behaviorUsing objects to hold variations in behavior Data members to hold variations in dataData members to hold variations in dataCommonality/variability analysisCommonality/variability analysis More effectively than looking for nouns and actionsMore effectively than looking for nouns and actions

The EndThe End

Chapter Eight Expanding Our Horizons Ku-Yaw Chang Assistant Professor, Department of Computer...

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