351 Class Design - Part 1. 352 Note: there is so much more than below… More facts regarding Class Design Re-look at Persistent Classes Re-look at Class

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Class Design - Part 1

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Note: there is so much more than below…

More facts regarding Class DesignRe-look at Persistent ClassesRe-look at Class Operations

Scope of OperationsMethodsStates – State ChartsAttributes

Defining Dependenciesand Associations

GeneralizationsMultiple InheritancePolymorphism

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We want to: Refine relationships, operations, and attributes

Focus on fleshing out the details of a particular class operations needed and allocated to classes; how operations / methods collaborate to support the

responsibilities allocated to the class. Later:

Address non- functional requirements We will look at Design Patterns in the future.

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Class Design in Context

Architect

Designer

ArchitecturalAnalysis

ArchitectureReviewer

Review theDesign

Review theArchitecture

Use-CaseAnalysis

ArchitecturalDesign

DescribeConcurrency

DescribeDistribution

ClassDesign

Subsystem Design

Use-Case Design

DesignReviewer

This is where we stand. Recall:Architectural Design: decide on the infrastructure;

pieces and parts of the architecture and how they interact). Use Case Design is where the responsibilities of the system are allocated to the pieces/parts; Subsystem and Class design are where we detail the specifics of the pieces/parts.

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A class should have a single well focused purpose.

A class should do one thing and do it well!

How Many Classes Are Needed? Many, simple classes means that each class

Encapsulates less of the overall system intelligence

Is more reusable Is easier to implement (more cohesive…)

Few, complex classes means that each class Encapsulates a large portion of the overall system

intelligence Is less likely to be reusable Is more difficult to implement

Proper size may depend heavily on implementation environment – classes should map directly to some phenomenon in the implementation language in such a way that the mapping results in good code.

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MainForm

SubWindow

DropDownListButton

MainWindow

Recall: Boundary Classes – from Analysis External System Interface:

Note: Usually model as subsystem Oftentimes these interfaces have complex

internal behavior (hence the design modeling as a subsystem)

Analysis

Design (within Subsystem)

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Recall: Entity Classes (1 of 3) Entity objects are often passive and persistent

In Analysis, we identified entity classes. These classes may have been associated with analysis mechanisms for persistence representing manipulated units of information. (recall java.sql classes) May be associated with legacy systems too. May be distributed… Performance concerns may suggest re-factoring of persistent classes, causing changes to the Design Model.

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Analysis Design

FatClass- transientBookeeping+ getCommonlyUsedAtt1()+ getCommonlyUsedAtt2()+ getRarelyUsedAtt3()+ getRarelyUsedAtt4()

FatClassDataHelper

+ commonlyUsedAtt1+ commonlyUsedAtt2

FatClassLazyDataHelper

+ rarelyUsedAtt3+ rarelyUsedAtt4

1 1

FatClass- transientBookeeping+ commonlyUsedAtt1+ commonlyUsedAtt2+ rarelyUsedAtt3+ rarelyUsedAtt4

<< entity >>

Entity Classes - Sample Re-factoring Have persistent class with five attributes. One attribute is not really persistent – used during runtime but Use Cases tell us two attributes used a lot; two others less. In design, we would like to retrieve commonly used attributes

right away but defer others until asked for. But we don’t want a complex design for the client. So:

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AnalysisDesign

FatClass- transientBookeeping

+ getCommonlyUsedAtt1()+ getCommonlyUsedAtt2()+ getRarelyUsedAtt3()+ getRarelyUsedAtt4()

FatClassDataHelper

+ commonlyUsedAtt1+ commonlyUsedAtt2

FatClassLazyDataHelper

+ rarelyUsedAtt3+ rarelyUsedAtt4

1 1

FatClass- transientBookeeping+ commonlyUsedAtt1+ commonlyUsedAtt2+ rarelyUsedAtt3+ rarelyUsedAtt4

<< entity >>

Entity Classes From a data standpoint, will consider the FatClass to be a proxy in front

of the two real persistent data classes.

It will retrieve FatClassDataHelper from database when it is first retrieved. Will retrieve FatClassLazyDataHelper in rare occasion when a client asks for one of these attributes.

This is a view from a data-oriented perspective while retaining a logical object-oriented view for clients to use.

So, which would you ratherSo, which would you rather retrieve?? FatClass or retrieve?? FatClass or FatClassDataHelper?FatClassDataHelper?

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Recall: Control Classes What Happens to Control Classes?

Are they really needed? Split them? If they seem like just ‘pass throughs’ from the boundary

to the entity classes, eliminate them.

Control Classes may become true design classes for any of following reasons: Encapsulate significant control flow behavior Behaviors are to be distributed across multiple

processes and/or processors (often JSP, servlets…) The behavior they encapsulate requires some

transaction management

A single analysis control class can easily become two or more design classes.

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Class Design Steps: Identify Persistent Classes

In Use-Case Analysis, a vague notion that certain classes need to be persistent But this is just the 'tip of the iceberg' of system design.

Now, in Class Design, get really specific about what the classes are, (are there more?) what their behaviors are (parameters, et al), and what attributes these classes really have.

Now, we must be certain which classes will have persistent instances, and that all persistent classes are mapped to a storage

mechanism.

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Persistent class: Any instance of the class that requires its state to be preserved. A persistent class may have both persistent and transient

instances Labeling a class 'persistent' means merely that some

instances of the class may need to be persistent.

ClientClass

Persistency

AnalysisMechanism

(Conceptual)

DesignMechanism(Concrete)

ImplementationMechanism

(Actual)

OODBMS

RDBMS JDBC to Ingres

ObjectStore

Legacy Data

New Data

Course

StudentPersistency

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Important Note. Persistent classes may come from entity classes. May also be needed to handle some non-

functional requirements…

Examples: Persistent objects needed to maintain information

relevant to process control, or Persistent objects needed to maintain state information

between transactions.

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A Closer look at Classes

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Class Operations Purpose

Map responsibilities (analysis) to operations (design) that implement them

Things to consider regarding class operations: Operation name, signature, and description Operation visibility Operation scope

Class operation vs. instance operation Operations: define at most primitive level to

promote reusability and maintainability.

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Class Operations: Name and Describe

Provide appropriate operation names.Indicate the outcome – e.g. getBalance(). Consistent across classes

Define operation signatures operationName(parameter : class,..) : returnType Best to specify operations and their parameters using

implementation language syntax and semantics – if you know it.

Thus the interfaces will already be specified in terms of the implementation language when coding starts.

Helpful to provide short textual description, including meaning of all parameters for an operation.

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Class Operation Signatures: Guidelines In addition to a short description of the parameter, be sure to include:

Parameters passed by-value or by-reference? If Call by Value, parameter cannot be changed in call. If Call by Reference, parameter may be changed in call.

Parameters optional? Default parameter values? Valid parameter ranges?

The fewer the parameters, the better. less coupling; more understandable and maintainable.

Pass objects instead of “data bits” – a rich strength of OO.

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Discovering Additional Classes and Relationships

Additional classes and relationships may be added to support signature

ClassA Class2

op1(var1:Class2): Class3

Class3

• Parameters and return types may lead to discovery of other classes.• Operation parameters and return classes denote a relationship between these classes and the parameter class and/or the return class.

• In many cases, the relationships added to support operation signatures are dependency relationships.

• Dependency relationships are discussed ahead.• Dependency also applies to attributes as well as operations..

What does the class diagram above tell you???

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Class- privateAttribute# protectedAttribute

+publicOp()# protectedOp()- privateOp()

Class Visibility – How Noted? The following symbols are used to specify export control for

attributes and operations: + Public access # Protected access - Private access In Java we also have package visibility.

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Class

- classifierScopeAttribute

classifierScopeOperation()

- instanceScopeAttribute

instanceScopeOperation()

Class Operation and Attribute Scope. Determines number of instances of attribute or

operation Instance scope: one instance for each class instance (object)

(instance variables…) Class scope: one instance for all class instances (objects)

Static variables in Java. Don’t need instantiations… Class scope: underline attribute/operation name

Generally, we have instance scope; but can have class scope for may other practical reasons: counters, etc.

Class scoped operations can only access class-scoped attributes. (in Java, class scope static)

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Example: ScopeStudent

- name- address

- nextAvailID : int

+ addSchedule(theSchedule : Schedule, forSemester : Semester)+ getSchedule(forSemester : Semester) : Schedule+ hasPrerequisites(forCourseOffering : CourseOffering) : boolean# passed(theCourseOffering : CourseOffering) : boolean+ getNextAvailID() : int

<<entity>>

- studentID

• Here, have a single class scoped attribute, nextAvailID; single classifier scoped operation, getNextAvailID().

• Each Student instance has it’s own unique student-ID, name, address, whereas, there is only one nextAvailID for all Student instances.

• The getNextAvailID() classifier scoped operation can only access nextAvailID.• Why??

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Example: See Visibility indicators…

CourseOffering(from University Artifacts)

<<entity>>

Student.

+ getTuition() : double+ addSchedule(theSchedule : Schedule)+ getSchedule(forSemester : Semester) : Schedule+ deleteSchedule(forSemester : Semester)+ hasPrerequisites(forCourseOffering : CourseOffering) : boolean# passed(theCourseOffering : CourseOffering) : boolean<<class>> + getNextAvailID() : int+ getStudentID() : int+ getName() : string+ getAddress() : string

(from University Artifacts)

<<entity>>

RegistrationController

+ submitSchedule()+ saveSchedule()+ getCourseOfferings() : CourseOfferingList+ getCurrentSchedule(forStudent : Student, forSemester : Semester) : Schedule+ deleteCurrentSchedule()<<class>> + new(forStudent : string)+ getStudent(withID : string) : Student

(from Registration)

<<control>>

Schedule(from University Artifacts)

<<entity>>

0..1

0..1+registrant

0..*

1

0..10..1

+currentSchedule

0..*

0..*

+primaryCourses

0..4

+alternateCourses

0..2

ICourseCatalogSystem

+ getCourseOfferings()+ initialize()

(from External System Interfaces)

<<Interface>>

10..*

Note: the <<class>> operations.Note: + classes (invoked by clients); # only invoked by defining class/subclasses,

(usually correspond to reflexive operations on interaction diagrams; (more )

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Review: Package Element VisibilityPackageA

Class A1

Class A3

Class A2A

BPackageB

+Class B1-Class B2Class has Public visibility

Class has Private visibility

Only public classes can be referenced outside of the owning package

Can specify visibility for package elements (i.e., classes….) in same way as class attributes / operations (can protect classes)

Shows how other packages can access the elements owned by the package.

(Have visibility symbols for packages)

OO Principle: Encapsulation

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Defining Dependenciesand Associations

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What Is a Dependency? A relationship between two objects

In Analysis, we assumed relationships were ‘structural’that is,

associations, aggregations, composition.

These refer to parts, numbers, coincident lifetimes, one–to– many, etc.

In Design, we must decide what type of communication pathway is required.

Define Dependency

Dependency - more A dependency relationship denotes a semantic

relationship between model elements, where a change in the supplier may cause a change in the client.

Semantics focuses on the relation between signifiers, such as words, phrases, signs, symbols, ….

What would happen if java.sql changed, where all of our, say, persistency mechanisms use (depend on) the objects in this package?

Need to Determine what causes the supplier to be visible to the client

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Client Supplier

Usually the association will have numbers relating objects of one type to the other; roles too…

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Associations are structural relationships i.e., numbers of one type related to another; parts, lifetimes, ….

Dependencies are non-structural (semantic) relationships. Dependencies are strong relationships!

Association

Client

Supplier1

Supplier2

Dependency

Dependencies vs. Associations*

1

0..1 buyerbroker

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Communication pathways to Suppliersa.k.a. how are suppliers made visible to clients Four communications pathways to supplier; that is, how are

suppliers made ‘visible’ to clients. Local variable reference –

Supplier object is declared locally (created temporarily during execution of an operation / method)

Client object declares a local variable within a specific method within client.

Parameter reference – supplier object is a parameter to, or the return class of an operation

in the client object. A method in client has a formal parameter of type supplier or

the method returns an object of type supplier… Global reference – supplier object is global.

Self-explanatory Field reference – The supplier object is a data member in

the client object. There is an instance variable within object of type supplier.

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Associations and aggregations are structural relationships (Visibility = Field Visibility; four slides up).

We’re talking about ‘Association relationships’ realized by variables that exist in the data member section of the class definition.

Field: ALL objects have own copy of instance variables!

Each class stands on it's own.

Dependency is a type of communication pathway that is a more temporary type of relationship – (Visibility = Global, Parameter, & Local; next three slides)

(Parameter – method may/may not be invoked; Local? Temporary life for execution of method; Global– life apart from object…)

Any relationships not associations are dependency

Dependencies vs. Associations: Look at Relationships: What are they going to be/become?

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ClassA

op1 ()

ClassB

Local Variable Visibility Dependency The op1() operation contains a local

variable (an object) of type ClassB (local variable is not a parameter)

Hence there is a dependency between these two classes.

boolean op1(….) // method in ClassA { ClassB myClassB = new ClassB (…) …

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ClassA

op1 (param1: ClassB)

ClassB

Parameter Visibility Dependency The ClassB instance is passed to the

ClassA instance – hence a dependency.

Format: object name; Class e.g. op1(myClassB: ClassB)

If op1 is invoked, it is passed an object of type ClassB

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ClassUtility

utilityOp ()

ClassA

op1 ()

Global Visibility Dependency The ClassUtility instance is visible

because it is global. Clear dependency. op1() uses something in ClassUtility

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ClassB

utilityOp ()

ClassA

ClassB myClassB; op1 ()

Field Visibility Association The ClassB instance is visible. There is an

‘association.’ Not temporary; Every instance has copy

instance variable

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Example:Define Dependencies (before looking for dependecies) (VOPC Register for Courses Use Case)


+ getCourseOfferings(forSemester : Semester) : CourseOfferingList


<<Interface>>

Student

- name- address- StudentID : int

+ addSchedule(theSchedule : Schedule, forSemester : Semester)+ getSchedule(forSemester : Semester) : Schedule+ hasPrerequisites(forCourseOffering : CourseOffering) : boolean# passed(theCourseOffering : CourseOffering) : boolean


<<entity>>


+ // submit schedule()+ // save schedule()+ // create schedule with offerings()+ // getCourseOfferings(forSemester) : CourseOfferingList

(from Registration)

<<control>>

0..1

0..1registrant

0..*1

courseCatalog

Schedule

- semester

+ submit()+ // save()# any conflicts?()+ // create with offerings()


<<entity>>

0..*

1

0..1 0..1

currentSchedule

CourseOffering

- number : String = "100"- startTime : Time- endTime : Time- days : Enum

+ addStudent(studentSchedule : Schedule)+ removeStudent(studentSchedule : Schedule)+ new()+ setData()


<<entity>>

0..*

0..4

primaryCourses

0..*

0..2alternateCourses

Up to here, most relationships have been associations and aggregations. Now, will see how some of these are refined into dependencies.Too, a lot of this was done during analysis!

The dependency shown (next slide) was previously defined in the Define Ops section to support the Schedule operation signatures.

All associations/aggregations should be examined to see if they are dependencies.

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Example: Define Dependencies (after 1 of 2)

Global visibility

Parameter visibility




<<Interface>>

Student




<<entity>>



(from Registration)

<<control>>

0..1

0..1registrant

Schedule

- semester



<<entity>>

0..*

1

0..1 0..1

currentSchedule

CourseOffering




<<entity>>

0..*

0..4primaryCourses

0..*

0..2alternateCoursesField visibility

Field

Changed one association to a dependency relationship. (This change discussed on ‘next’ slide) Here, during a registration session, the Registration Controller works with a single Student, the registrant, and one Schedule, the current Schedule for the Student.

These instances need to be accessed by more than one of the Registration Controller’s, operations so Field Visibility is chosen from Registration Controller to Student and from Registration Controller to Schedule.Thus relationships remain associations.(more ‘permanent’)

A Student manages his/her own Schedules, so Field visibility is chosen from Student to Schedule – and relation remains aggregation. Again, more‘permanent.’ More see Course Offering in

Student as parameter

see Schedule as parameter below

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Example: Define Dependencies (after; more explanation)

Global visibility

Parameter visibility




<<Interface>>

Student




<<entity>>



(from Registration)

<<control>>

0..1

0..1registrant

Schedule

- semester



<<entity>>

0..*

1

0..1 0..1

currentSchedule

CourseOffering




<<entity>>

0..*

0..4primaryCourses

0..*

0..2alternateCoursesField visibility

Field visibility

Course Offerings are part of semantics of what defines a Schedule (courses Student has selected). Thus Field visibility is chosen from Schedule to CourseOffering; relationships remain associations.

The Student class has several operations where CourseOffering appears in the parameter list. Thus, Parameter visibility is chosen from Student to CourseOffering.

It is envisioned the course Catalog System may need to be accessed by multiple clients in the system, so Global visibility was chosen – and relationship becomes a dependency.

Documents

351 Class Design - Part 1. 352 Note: there is so much more than below… More facts regarding Class Design Re-look at Persistent Classes Re-look at Class