Lecture 8: Objects/Classes Class Diagram

Lecture 8:Objects/ClassesClass Diagram

MIS 210Information Systems I

• The ERD was the first systems analysis tool to focus on data and how it is Linked and Organized.

• It was from this that objects then evolved.• An Entity is a thing the users need to know (i.e, record)

something about.• An Entity Type is a group or class of entities that are all

the same kind of thing.• An Occurrence of an Entity Type is a specific individual

thing of the kind the Entity Type describes.

The Entity Relationship Diagram

The Entity Relationship Diagram

• Attributes are the things we need to know about an Entity.

• An Association is the interaction of one or more Entities and is represented by a verb.

• These interactions among the Entities (i.e., these associations) show us the paths we need to follow through the database to access the data.

Objects

• It is “Something in the user’s world that has a separate and distinct existence, and is of interest in that they need to keep data about it.”

• Some private data and a set of operations that can access that data– These operations exclusively access the data it carries -

no other code can touch it.

• Contains both data (what it knows) and behavior (what it can do)

• An Object is an entity with behavior.

THINGS (Objects), etc.• Things…

– Car, Train, Elephant, Sale, Invoice, Division, Account, etc.– Physical or Conceptual

• Attributes…– Make, Model, Year, – Color, Weight, – Serial Number, License Number

• Relationships…– e.g., to a Person object, i.e., the Owner.

• Behaviors…– Create– Change Color– Change Owner– Destroy Itself (Suicide?)

Sylnovie Merchant, Ph.D. MIS 210 Fall 2004

Object

• Since an object carries only– The data we need for the job at hand, and– The behavior (program code) for the job at hand

• This makes it an abstraction of the real-world object.• Attributes: what an object knows (about itself)• Methods: what an object can do• Identity: each instance is unique, even if very similar

• Each object instance carries a set of attribute values

• These represent the data items that a user might need to know about the object at any time

• Data types and sizes are defined for the class apply to every instance.

• The domain of an attribute is its set of all permissible values.

Attributes:


Methods and Messages

• A method is what an object can do; a specific implementation of an operation by a particular object– how it does an operation

• An operation (behavior) is an action that an object performs or is subject to – something the object can perform (walk; run)

• A message is an operation that one object performs upon another– An operation (behavior) is initiated by sending a

messages

• Defined: The condition or fact that something is itself and not something else.

• Two red apples. . .– Each is itself, even though identical to the other.

• Identity is inherent in an object.– An object has identity just because it is that

object.

Identity

• A child has identity from the moment of birth, or perhaps before.– It doesn’t get a name until after its birth.– So an identifier does not confer identity– But simply documents or recognizes identity that

already existed.– So do identifiers such as customer numbers, social

security numbers, etc.

• Remember, only objects have identity, attribute values don’t.

Identity


Encapsulation• Encapsulation means an object’s data (attributes)

is hidden within, and protected by, a shell of procedures (methods)

• Also known as information hiding• Methods allow for hiding of information by

concealing the way in which an object satisfies a request

• When we send a message, we do not know how or where it will be satisfied

• The interface is public; the internals are private • Leading to stability and portability

• Do you know how to drive a car?

• Do you know how everything works under the hood?– The correct answer is. . .

Encapsulation

• What does matter is consistency in the interface.

• Every car made these days has the standard pedal layout.

• Now we have standard user interfaces (or close) for all kinds of products: Touch phones, keyboards, cars, motorcycles, microwave ovens, thermostats, TV remote controls.

Encapsulation

• What all of these do, is HIDE THE DETAILS • The complexity is enclosed within the product• Leaving a small, easily-understood set of commands to

operate it.• We hide both data and program code within the object• Giving a level of encapsulation better than ever before,• Leading to stability and portability

Encapsulation

• Stability is improved since once a class is built, it will survive the years with very few changes.– It can be reused in new projects with few changes

• Portability is enhanced because only the narrow published interface needs to be learned by the new developers, i.e., the reusers.

Encapsulation

• All the attributes and their values should be hidden within the object and accessible only via the methods.

• There is a boundary between what is inside the object and what is outside it

• Crossed only by the interface.

Encapsulation

• Since we have encapsulated all the data and behavior of our system within the classes, we reduce the “ripple effect” of change.

• Even though the reduction may be small for each class, remember. . .– The number of possible interactions goes up

exponentially with the number of objects.– In a large system this makes a huge difference in

maintenance.

Encapsulation


Polymorphism

• Polymorphism: the same message can elicit a different response depending on the receiver

A class is a group of objects with: similar properties (attributes), common behavior (operations), common associations to other objects, and common semantics (i.e., meaning.)

Classes

• A class is a set of objects that share a common structure (attributes) and a common behavior (methods)

• We group objects together on the basis of:– Shared Attributes– Shared behavior

• Class of:– All Employees– All Products– All Transactions

• An individual object is then a INSTANCE

Classes

Classes perform such things as:– Creating Instances– Holding the program code– Holding the attribute definitions

• Data type and size, etc.– Point of contact for associations.

Classes

Many people use “object” to refer to a class.

Strictly, as used in UML, an “object” is an

Instance.

Classes or Instances?

Responsibilities and Collaborations

• Responsibilities– Knowing– Doing

• Collaboration– Objects working together to service a request

Class Symbol

• Name– should be unambiguous– should be a noun

• Attributes (what the class/object knows about itself)attribute name : data type = default value {property string}

• Methodsmethod name (parameter-list) : return-type

where parameter-list and return-type are specified as attributes

Visibility: + public - private # protected

Class Components

• Name

• Attributes

• Services

CustomerCustIdNumFirstNameLastName

etc...

AccessAccountDebitAccount

GetAccountBaletc...

What it knows about itself

What it can do

Class Diagrams

• Finding the classes is the core activity of OOSD.

• The class diagram describes the classes in the system and the static relationships among them

• Similar to ERD -- with methods added• Will be directly implemented in an OOPL• Most important model

• Some instances of a class may be grouped together on the basis of features not shared by the rest of the class– Attributes– Behavior – Associations

• Key verb is:

““ISAKINDA”ISAKINDA”““ISAKINDA”ISAKINDA”

Subclasses

• Yes, of course!• But Why?

• Because a Customer is a person, and people have names.

• Does a Customer have a Wage Rate?

• No, only Employee-type people have one of those!

Subclasses

In our company, does a Customer have a name?

Balance Owing Amount Overdue 30 days Amount Overdue 60 days Amount Overdue 90 days

Credit Rating Date of Last Payment

Balance Owing Amount Overdue 30 days Amount Overdue 60 days Amount Overdue 90 days

Credit Rating Date of Last Payment

Name NameAddress AddressPhone Phone

Date of Birth Date of BirthSex Sex

Name NameAddress AddressPhone Phone

Date of Birth Date of BirthSex Sex

SIN Marital Status

Number of DependantsDate HiredWage Rate

SIN Marital Status

Number of DependantsDate HiredWage Rate

Overlapping Sets of AttributesCustomerAttributes

CustomerAttributes

Person Attributes

Person Attributes

EmployeeAttributes

EmployeeAttributes


Inheritance

• A (top-down) relationship among classes wherein one class shares the structure or behavior defined in one (single inheritance) or more (multiple inheritance) other classes.

• Based on a hierarchical (IS-A) relationship

• Inheritance is in the “eye of the beholder”

Single Inheritance

• Subclasses inherit from no more than one superclass

Student

Undergrad Graduate

ssn, name

Grad advisorYear (F,So,J, S)

Compute gpa Compute gpa

Multiple Inheritance

• Subclass inherits from more than one superclass

DVD Player

Audio Video

Repeated Inheritance

• Superclasses inherit from a higher level superclass

ElectronicEquipment

DVDPlayer

VideoAudio

Generalization

• The convention for defining subclasses

• Think substitution - anywhere you have the class you can freely use the sub class– For Example, Wherever You Use Customer

you can use “Corporate Customer”

Generalization/Specialization

Customer

NameAddressPhone

DiveCustomer

CertificateCertificate Date

BoatCustomer

Safety CourseCompletion Date

MultipleInheritance andGeneralization

CustomerNameAddressPhone

DiveCustomer


BoatCustomerSafety Course

Completion Date

Dive&BoatCustomer


Navigation

• Defines the direction of an association

• Line Options– Uni-Directional - “Order Comes From Customer”– Bi-Directional - Two Roles are Inverses of Each Other– No Arrows - Assumed “Unknown” or “Bi-Directional”

Role

• A Label for that direction of the association

• Each Association has a Role for Each Direction of the Association

• When no Role is Identified, the Role Takes the Name of the Target Class

• Draw a line joining two class boxes to show a relationship exists

• Write the verb above the line.• Add a solid arrowhead so that it makes a sentence

when you read it in the direction of the arrow:• “Student enrolls in course

Associations

Student

*Student No.NameAgeSex

CourseOffering

*Course No.*DateRoom No.Max Enrol

Enrolls In

Associations

CourseOffering


Student


Instructor

*Employee No.NameAgeSexSalary

teaches

Enrolls In

Associations

CourseOffering


Student


Instructor


teaches

College

*NameAddressPhoneRating

Employs

Enrolls In

teaches

• We diagram this by adding a star (asterisk) below the relationship line whenever it should show “many.”

• Read this one as :“Instructor teaches many courses”• This is a “One-to-Many” Association, or 1:M

Multiplicity

CourseOffering


Instructor


teaches

*

• And, since we wish many Customers to buy many Products, this one is a Many-to-Many association, or M:M

Multiplicity

Customer

*A/c No.NameAddressPhone No.Balance

Product

*Prod No.DescriptionUnit PriceQty in Stock

buys

**

Multiplicity

• How many objects may participate in a given relationship• One Customer can make “one to many” Orders

– Multiplicity Notation• * = one to many• 1 = one• 0 = zero• 0..* = zero to many• 1..* = one to many• 2,4 = two or four• 2..4 = two to four

Multiplicity

Aggregation

• Part-whole relationship

• A whole is equal to the sum of its parts

• Classes are grouped to form an aggregate class

• Aggregation is a relationship between two classes where the instances of one class are in some way components, members or contents of the instances of the other class.

• These are relationships that appear on the class diagram along with all the subclass hierarchies and the regular 1:M and M:M associations

Aggregation

Aggregation versus Association

Association: refers to the relationship between dissimilar classes of objects

Book

Customer

PageChapter

association

aggregation

Aggregation versus Generalization

Car

ToyotaFord

DoorWheels

Example Class Diagram

Documents

Lecture 8: Objects/Classes Class Diagram