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Object Modeling Crash Course

Object Analysis

– Classification

– Identifying Object Relationships, Attributes, and Methods

Class Diagrams

Agenda

Object Modeling Crash Course

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Basic Object Concepts

Attributes and Methods

Objects vs. Classes

Encapsulation, Inheritance, Polymorphism

Relationships

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What is an object?

The term object was first formally utilized in the Simula language to simulate some aspect of reality.

An object is an entity.

– It knows things (has attributes)

– It does things (provides services or has methods)

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Attributes

Objects know things

I am an Employee. I know my name, social security number and my address.

I am a Car.I know my color, manufacturer, cost, owner and model.

I am a Fish. I know my date of arrival andexpiration.

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Methods

Objects do things

I know how to computemy payroll.

I know how to stop.

I know how to cook myself.

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What is an object?

Attributes or properties describe object’s state (data) and methods define its behavior.

Object is whatever the information system wants to talk about.

– For example, Parts and assemblies might be objects of bill of material application.

– Stocks and bonds might be objects of financial investment applications.

In an object-oriented system, everything is an object: numbers, arrays, records, fields, files, forms, an invoice, etc.

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Two Basic Questions of OO Analysis and Design

When developing an OO information system, two basic questions always arise.

– What objects does the application need?

– What functionality should those objects have?

Object Analysis

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Objects and Classes

The role of a class is to define the attributes and methods (the state and behavior) of its instances.

The class car, for example, defines the property color.

Each individual car (object) will have a value for this property, such as "maroon," "yellow" or "white."

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A Class is an Object Template, or an Object Factory.

Boeing Factory(Boeing Class)

Boeing Airplane Objects(Boeing Instances)

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Class Hierarchy

An object-oriented system organizes classes into subclass-super hierarchy.

At the top of the hierarchy are the most general classes and at the bottom are the most specific

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Class Hierarchy (Cont’d)

A subclass inherits all of the properties and methods (procedures) defined in its superclass.

Motor Vehicle

Truck CarBus

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Inheritance

Inheritance is a relationship between classes where one class is the parent class of another (derived) class.

Inheritance allows classes to share and reuse behaviors and attributes.

The real advantage of inheritance is that we can build upon what we already have and, reuse what we already have.

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Inheritance (Cont’d)

Ford

Vehicle

Car

Mustang Taurus Thunderbird

stop (myMustang)

I don’t know how to stop

I know how to stopstop method is reusable

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Multiple Inheritance

OO systems permit a class to inherit from more than one superclass.

This kind of inheritance is referred to as multiple inheritance.

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Multiple Inheritance (Cont’d)

For example utility vehicle inherits from Car and Truck classes.

MotorVehicle

Truck Car Bus

UtilityVehicle

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Encapsulation and Information Hiding

Information hiding is a principle of hiding internal data and procedures of an object.

– Reveal as little as possible about its inner workings.

– Encapsulation protects the data from corruption.

Public Protocol

Messages

Data

Permissible operations

Private Protocol

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Polymorphism

Polymorphism means that the same operation may behave differently on different classes.

Example: computePayroll

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Polymorphism – computePayroll

1. Managers: Receive a regular salary

2. Office Workers: Receive an hourly wage and are eligible for overtime after 40 hours

3. Production Workers: Are paid according to a piece rate

Employee

OfficeWorker Manager ProductionWorker

nameaddresssalarySS#

dataEntryComputePayrollprintReport

dataEntryComputePayrollprintReport

dataEntryComputePayrollprintReport

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Associations

The concept of association represents relationships between objects and classes.

For example a pilot can fly planes.

Pilot Planescan fly flown by

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Clients and Servers

A special form of association is a client-server relationship.

This relationship can be viewed as one-way interaction: one object (client) requests the service of another object (server).

PrintServer ItemRequest for printing

Object Analysis

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Object Analysis = Classification / Generalization

Classification and Generalization is Difficult

– God is Love

– Love is blind

– Ray Charles is blind

– ergo...

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Identifying Classes

1. Noun phrase approach.

2. Common class patterns approach.

3. Use Case driven approach.

4. Class Responsibilities Collaboration (CRC) approach.

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1. Noun Phrase Approach

Using this method, read through the Use cases, interviews, and requirements specification carefully, looking for noun phrases.

It is safe to scrap the Irrelevant Classes.

Formulate a statement of purpose for each candidate class; if it’s not possible, simply eliminate it.

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Guidelines For Identifying Classes

Look for nouns and noun phrases in the problem statement.

Some classes are implicit or taken from general knowledge.

All classes must make sense in the application domain.

Avoid computer implementation classes, defer it to the design stage.

Carefully choose and define class names.

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Guidelines For Refining Classes

Redundant Classes:

– Do not keep two classes that express the same information.

– If more than one word is being used to describe the same idea, select the one that is the most meaningful in the context of the system.

Adjective Classes:

– Does the object represented by the noun behave differently when the adjective is applied to it?

– If the use of the adjective signals that the behavior of the object is different, then make a new class.

– For example, If Adult Membership and Youth Membership behave differently, than they should be classified as different classes.

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Guidelines For Refining Classes (Cont’d)

Attribute Classes:

– Tentative objects which are used only as values should be defined or restated as attributes and not as a class.

– For example the demographics of Membership are not classes but attributes of the Membership class.

Irrelevant Classes:

– Each class must have a purpose and every class should be clearly defined and necessary.

– If you cannot come up with a statement of purpose, simply eliminate the candidate class.

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2. Common Class Patterns Approach

This approach is based on the knowledge-base of the common classes that have been proposed by various researchers.

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Candidate Classes

Events

– These are points in time that must be recorded and remembered.

– Things happen, usually to something else, at a given date and time, or as a step in an ordered sequence.

– For example order which is an event that must be remembered.

Organization

– The organizational units that people belong to.

– For example, accounting department might be considered as a potential class.

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Candidate Classes (II)

People and Person (Roles and Roles Played)

– The different roles users play in interacting with the application.

– Two Types of People (Coad and Yourdon):

– Users of the system– E.g., Operator, Clerk

– Those who do not use the system but about whom information is kept.

– E.g., Client, Employee

Places

– Physical locations, such as buildings, stores, sites or offices that the system must keep information about.

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Candidate Classes (III)

Tangible Things and Devices

– Physical objects, or group of objects, that are tangible, and devices with which the application interacts.

– E.g., Cars, ATM

Concepts

– Concepts are principles or ideas not tangible but used to organize or keep track of business activities and/or communications.

– E.g., Order, LineItem

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3. Use Case Driven Approach

Once the system has been described in terms of its scenarios, examine the textual description or steps of each scenario to determine what objects are needed for the scenario to occur.

To identify objects of a system and their behaviors, the lowest level of executable use cases is further analyzed with a sequence and collaboration diagram pair.

By walking through the steps, you can determine what objects are necessary for the scenario to take place.

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Sequence Diagram – Example

Client ATMMachine BankClient

Bad PIN Number

Bad PIN NumberMessage

Eject ATM card

Request take card

Take card

Display main screen

Verify PIN Number

Request PIN number

Request PIN

Insert ATM card

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ATM Machine:Definition

Checking Account

Account Bank Client

5: Process Transaction

8: Transaction succeed

4: Enter Amount

13: Terminate

1: Request Kind

2: Enter Kind

3: Request Amount

9: Dispense Cash

10: Request Take Cash

11: Take Cash12: Request Continuation

14: Print Receipt

7: Withdraw Successful 6: Withdraw Checking Account

Collaboration Diagram – Example

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3. CRC Cards Approach

CRC stands for Class, Responsibilities and Collaborators developed by Cunningham, Wilkerson and Beck.

CRC can be used for identifying classes and their responsibilities.

Process of the CRC Technique

IdentifyClasses

Responsibility

IdentifyClasses

Responsibility

AssignResponsibility

AssignResponsibility

IdentifyCollaboration

IdentifyCollaboration

Iterate

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Collaborations

An object can accomplish either a certain responsibility itself, or it may require the assistance of other objects.

If it requires an assistance of other objects, it must collaborate with those objects to fulfill its responsibility.

CRC Cards:

– CRC cards are 4" x 6" index cards. All the information for an object is written on a card.

ClassName

Responsibilities Collaborators

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CRC Cards – Example

Order

Check if items in stock OrderLine

Determine Price OrderLine

Check for valid payment Customer

Dispatch to delivery address

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CRC Cards Process (Cont’d)

CRC starts with only one or two obvious cards.

If the situation calls for a responsibility not already covered by one of the objects:

– Add, or

– Create a new object to address that responsibility.

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Object Modeling Issues – Naming Classes

Guidelines

– The class should describe a single object, so it should be the singular form of noun.

– Use names that the users are comfortable with.

– The name of a class should reflect its intrinsic nature.

– By the convention, the class name must begin with an upper case letter.

– For compound words, capitalize the first letter of each word

– E.g., LoanWindow.

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Object Modeling – Summary

Finding classes is not easy.

The more practice you have, the better you get at identifying classes.

There is no such thing as the “right set of classes.”

Finding classes is an incremental and iterative process.

Unless you are starting with a lot of domain knowledge, you are probably missing more classes than you will eliminate.

Naming a class is also an important activity.

Class Diagram

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Different Perspectives

Class diagrams can be used in a number of ways:

– Conceptual – You draw a class diagram of elements under study

– Specification – You develop an programming interface

– Implementation – You give guidance to an programmer

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The Class

MyClass

Attribute1Attribute2

Method1Method2(Integer)Method3():String

Class Name

Attributes

Operations

ParameterReturn Value

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Associations

Order

Order Line

*

1

{if customer.creditRating=0 then isPrepaid must = true}

Line items

Multiplicity: many valued

Multiplicity: required Multiplicity: optional

Customer* 0..1

Role Name

Association

Constraint

Navigability

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Associations

Conceptual perspective

– Conceptual relationships between classes

Specification perspective

– Class responsibilities

– Interfaces

Implementation perspective

– Pointers

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Associations

Each Association:

– Involves two classes

– Has two roles

– Has two multiplicities (one for each role)

– 0..1, 0..*, 1, 1..*, *

– Can have zero, one, or two navigability indicators – indicates one-way dependency

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Attributes

Attributes describe a possible state of a class

Similar to Associations

– But,

– One way navigability (from Class to Attribute)

– Usually for single valued simple classes (e.g., strings, dates, integer, double)

Syntax:

– visibility name: type = defaultValue

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Operations

Operations describe possible methods in a class

Syntax:

– visibility name (parameter-list): return-type-expression {property-

string}

Query vs. Modifier operations

– Get Methods

– Set Methods

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Associations: Generalization

Customer

PersonalCustomer

CorporateCustomer

Generalization

Inheritance is also called generalization

Passes the “is a” test

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Constraint Rules

Syntax:

– Not formal

– {constraint-statement}

Assertions

– Pre-condition

– Post-condition

– Invariants

Class DiagramAdvanced Concepts

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Stereotypes

Stereotypes are a high-level classification of an object.

Not an ancestor, but rather a set of attributes that many objects share.

Indicated using the following convention: <<stereotype>>

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Multiple and Dynamic Classification

Multiple classification -- multiple subtypes

Dynamic classification -- allows change

Person{abstract}

Male Female Employee

Sex{complete}

Job

Male Female

Programmer

Sex{complete}

Job <<dynamic>>

Manager

Person{abstract}Abstract Class

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Aggregation and Composition

Aggregation -- when the purpose of a class is to collect instances of other classes.

Composition -- Aggregation with dependency on the whole -- 2 alternative drawings:

Polygon

Point

Style

colorisFilled

Circle

radius

3..*

{ordered}

1

11

* *

Composition

Aggregation

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Composition

Alternative Notation

Style

colorisFilled

Circle

radius 1

Point

Polygon

radius{ordered} 3..*

Point

* *

1 1

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Derived Associations and Attributes

Derived associations and attributes are not explicit but rather derived

Noted by using a “/”

Sale

/tax:Currency

Salesperson

/seller

Seller role is derived using seller.territory

Derived Role

Derived Attribute

Note

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Qualified Associations

Constraints on the association

ProductOrderOrderLine

amount:Number

0..1

LineItem

OrderOrderLine

amount:Number

0..*

LineItem

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Association Classes

Just as with associative tables in databases, associative classes resolve many-to-many relationships

Person Department

Employmentperiod:dateRange

* *

Association Indicator