OO DesignOO Design

Roshan ChitrakarRoshan Chitrakar

Analysis to DesignAnalysis to Design

Do the RIGHT thingDo the RIGHT thing Requirement AnalysisRequirement Analysis Domain Modeling with Domain Modeling with

addition of attributes addition of attributes and associationsand associations

Use Case Modeling Use Case Modeling (Behavioral modeling)(Behavioral modeling)

Requisite Pro and Use Requisite Pro and Use case diagramscase diagrams

Do the thing RIGHTDo the thing RIGHT Responsibility Responsibility

assignmentassignment Object DesignObject Design Design Class diagrams Design Class diagrams Determining visibilityDetermining visibility Interaction diagrams Interaction diagrams

(Sequence diagrams & (Sequence diagrams & Collaboration Collaboration diagrams, Class diagrams, Class diagram)diagram)

OO Design: IntroductionOO Design: Introduction

A logical solution based on OO A logical solution based on OO paradigm is developedparadigm is developed

Constitutes a design modelConstitutes a design model Creation of Interaction diagrams Creation of Interaction diagrams

(collaboration among objects and/or (collaboration among objects and/or sequence of messages)sequence of messages)

Class diagrams are drawn in parallel Class diagrams are drawn in parallel with or after the interaction diagramswith or after the interaction diagrams

GRASPGRASP

Fundamental principles of Object Fundamental principles of Object design are: -design are: - Responsibility assignment and Responsibility assignment and Design patternsDesign patterns

GRASP (General Responsibilities GRASP (General Responsibilities Assignment Software Patterns) Assignment Software Patterns) describe the principles of object describe the principles of object design and responsibility design and responsibility assignment, expressed as patterns.assignment, expressed as patterns.

ResponsibilityResponsibility

It is a contract or obligation of a classifierIt is a contract or obligation of a classifier It is assigned to classes during object designIt is assigned to classes during object design Type of ResponsibilitiesType of Responsibilities

Knowing Knowing (e.g. “a Sale is responsible for knowing its total”)(e.g. “a Sale is responsible for knowing its total”) aboutabout private encapsulated data private encapsulated data aboutabout related objects related objects aboutabout things it can derive or calculate things it can derive or calculate

Doing Doing (e.g. “a (e.g. “a SaleSale is responsible for creating is responsible for creating SalesLineItemsSalesLineItems”)”) something itselfsomething itself initiating action in other objectsinitiating action in other objects controlling and coordinating activities in other objectscontrolling and coordinating activities in other objects

MethodsMethods

Responsibilities are translated into Responsibilities are translated into methods methods

Depending upon the granularity, Depending upon the granularity, several methods are implemented to several methods are implemented to fulfill the responsibilitiesfulfill the responsibilities

Methods either act alone or Methods either act alone or collaborativelycollaboratively

Both responsibilities and methods are Both responsibilities and methods are reflected in interaction diagramsreflected in interaction diagrams

Responsibility & Methods Responsibility & Methods in Interaction Diagramin Interaction Diagram

makePayment(cashTendered)create(cashTendered)

: Sale

: Payment

Responsibility: To create Payments

PatternsPatterns A pattern is a A pattern is a namednamed problem/solution problem/solution

pair that can be applied in new context, pair that can be applied in new context, with advice on how to apply it in novel with advice on how to apply it in novel situation and discussion of its trade-offs.situation and discussion of its trade-offs.e.g.e.g.Pattern NamePattern Name: Information Expert: Information ExpertSolutionSolution: Assign a responsibility to the : Assign a responsibility to the class that has the information needed to class that has the information needed to fulfill it.fulfill it.Problem It SolvesProblem It Solves: What is a basic : What is a basic principle by which to assign principle by which to assign responsibilities to objects.responsibilities to objects.

The First Five GRAS The First Five GRAS PatternsPatterns

They are: -They are: -1.1. Information Expert (or Expert)Information Expert (or Expert)

2.2. CreatorCreator

3.3. High CohesionHigh Cohesion

4.4. Low CouplingLow Coupling

5.5. ControllerController They cover very basic, common and They cover very basic, common and

fundamental design issuesfundamental design issues

Information ExpertInformation Expert It guides that objects do things related to It guides that objects do things related to

the information they have.the information they have. It is a class that has information necessary It is a class that has information necessary

to fulfill the responsibilityto fulfill the responsibility There may have partial information experts There may have partial information experts

who will collaborate for the responsibilitywho will collaborate for the responsibility

Design ClassDesign Class ResponsibilityResponsibility

SaleSale Knows sale totalKnows sale total

SalesLineItemSalesLineItem Knows line item subtotalKnows line item subtotal

ProductSpecificationProductSpecification Knows product priceKnows product price

Expert IllustrationExpert Illustration:: To find out the To find out the grand totalgrand total

Domain model showing association of Sale

Expert IllustrationExpert Illustration:: To find out the To find out the grand totalgrand total

•Knowing Line Item sub total and knowing product price in order to get the grand total.

•Responsibilities are being added to both interaction and Class diagrams also.

•More than one class can act as information expert.

•They collaborate by passing messages.

Expert: Pro and ConsExpert: Pro and Cons

Advantages: -Advantages: - Encapsulation is maintainedEncapsulation is maintained Behavior is distributed across the Behavior is distributed across the

classesclasses Disadvantages: -Disadvantages: -

Leads to problems of coupling, cohesion Leads to problems of coupling, cohesion and duplicationand duplication

CreatorCreator

It guides assigning responsibilities It guides assigning responsibilities related to the creation of objectsrelated to the creation of objects

The concept of aggregation is the The concept of aggregation is the basic guiding principle of the basic guiding principle of the Creator.Creator.

It suggests that the enclosing It suggests that the enclosing container or recorder class is a good container or recorder class is a good candidate for the responsibilitycandidate for the responsibility

Creator IllustrationCreator Illustration:: Creating a Sales Creating a Sales Line ItemLine Item

Sale Domain

Creator IllustrationCreator Illustration:: Creating a Sales Creating a Sales Line ItemLine Item

Creator: Pro and ConsCreator: Pro and Cons

Benefits: -Benefits: - Low coupling is supported i.e lower Low coupling is supported i.e lower

maintenance dependence and higher maintenance dependence and higher possibility of reuse.possibility of reuse.

Contraindications: -Contraindications: - In special cases, all creations are In special cases, all creations are

delegated to a helper class called delegated to a helper class called Factory. (Which is against the principle Factory. (Which is against the principle of Creator)of Creator)

Low CouplingLow Coupling

It suggests assigning a responsibilities It suggests assigning a responsibilities to classes such that their placement to classes such that their placement does not produce negative results.does not produce negative results.

It recommends designs of classes to be It recommends designs of classes to be more independent and hence reduce more independent and hence reduce the impacts of change.the impacts of change.

No absolute measure of low/high No absolute measure of low/high coupling, designer has to assess it so coupling, designer has to assess it so that negative results would not occur.that negative results would not occur.

Low Coupling IllustrationLow Coupling Illustration:: Creating a Creating a PaymentPayment

Payment Register Sale

Creator pattern suggests Register as a candidate for creating the Payment.

Coupling of Register class to the knowledge of

Payment class

Low Coupling IllustrationLow Coupling Illustration:: Creating a Creating a PaymentPayment

An alternate solution: Coupling of Sale class with the Payment class

Sale creates the Payment; does not

increase the coupling

High CohesionHigh Cohesion ““High functional cohesion exists when High functional cohesion exists when

the elements of a component (e.g. a the elements of a component (e.g. a class) all work together to provide some class) all work together to provide some well-bounded behavior”well-bounded behavior”

- Booch- Booch High Cohesion has fewer methods, with High Cohesion has fewer methods, with

highly related functionality, collaborates highly related functionality, collaborates with other objects to share the effort.with other objects to share the effort.

High Cohesion pattern has real world High Cohesion pattern has real world analogy or delegating the too much analogy or delegating the too much work.work.

High Cohesion IllustrationHigh Cohesion Illustration:: Creating a Creating a PaymentPayment

Register creates the Payment by taking the work by itself: Low cohesion

High Cohesion IllustrationHigh Cohesion Illustration:: Creating a Creating a PaymentPayment

Register delegates the work to Sale : Low cohesion

High Cohesion: Pro and High Cohesion: Pro and ConsCons

Advantages: -Advantages: - Maintenance and enhancement is Maintenance and enhancement is

simplifiedsimplified Low coupling is often supportedLow coupling is often supported Reuse is increased.Reuse is increased.

Disadvantages: -Disadvantages: - Distributed servers take more overhead Distributed servers take more overhead

and pay reduced performanceand pay reduced performance For only one person to maintain and For only one person to maintain and

support the component, it is difficult.support the component, it is difficult.

ControllerController

It provides guidance for handlers for It provides guidance for handlers for external inputs / events messages or external inputs / events messages or signals that system receives.signals that system receives.

It delegates, coordinates and controls It delegates, coordinates and controls the work to other objects: does not the work to other objects: does not do the actual work.do the actual work.

The System class made during the The System class made during the analysis phase, is now taken care of analysis phase, is now taken care of by Controller class.by Controller class.

System Class to System Class to Controller ClassController Class

Register acts as the single Controller

Class

Two handlers act like

Controllers

Controller IllustrationController Illustration:: enterItem() enterItem()

Controller IllustrationController Illustration:: enterItem() enterItem()

Possible handlers are: -Possible handlers are: -

: Registe

r

enterItem(id,quantity)

: ProcessSaleHa

ndler

enterItem(id,quantity)

Controller: Pro and ConsController: Pro and Cons

Increased potential for reuseIncreased potential for reuse Pluggable interface (i.e. application Pluggable interface (i.e. application

logic is not handled in the interface logic is not handled in the interface layer)layer)

It guarantees that system operations It guarantees that system operations within a use case occur in a legal within a use case occur in a legal sequence (It is possible if the same sequence (It is possible if the same controller is used throughout the use controller is used throughout the use case)case)