Object Basics Ppt

7/28/2019 Object Basics Ppt

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Object BasicsObject BasicsChapter 2


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IntroductionIntroduction If there is a motivation behind object-

oriented system development, it is thedesire to make the software development

easier and more natural by rising the

level of abstraction to the point whereapplications can be implemented in the

same terms in which they are described

by users.


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IntroductionIntroduction Let us develop an notion of an object

through an example. A car is an object ,a real world entity,

identifiably separate to with the

surroun ngs. A car has a well-defined set ob attributes

in relation to other objects such as

color, manufacturer, cost, owner a welldefined set of thing you would usually dowith it drive, lock, carry passengers, etc.


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IntroductionIntroduction In object orientation we call the

attributes ad properties and actions asmethods.

Pro erties describe the state of an

object.

Methods define the behavior.


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An ObjectAn Object-- Oriented PhilosophyOriented Philosophy Most of the programming languages

provide programmers with the way ofdescribing processes.

Althou h most ro rammin lan ua es

are computationally equivalent, the ease

of description, reusability, extensibility,

readability can vary depending upon thelanguage used.


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An ObjectAn Object-- Oriented PhilosophyOriented Philosophy A natural language or programming,

provides its users a base set ofconstructs.

Man ro rammin derive their basic

ideas from the underlying machine.

The machine may understand types such

as integers, floating point numbers andthe programming language will represent

them as structures.


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An ObjectAn Object-- Oriented PhilosophyOriented Philosophy A fundamental characteristics of object

oriented programming is that it allows thebase concepts of the language to extendto include ideas and terms closer to

. New data types can be defined in terms

of existing data types.

Using data abstraction mechanism , it ispossible to create new higher-level andmore specialized types.


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An ObjectAn Object-- Oriented PhilosophyOriented Philosophy The fundamental difference between the

object-oriented system and their traditionalcounterparts is the way in which youapproach problems.

are either algorithm-centric or data centric.

In algorithm-centric methodology, you thinkof an algorithm that can accomplish the task,then build data structures for that algorithmto use.


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An ObjectAn Object-- Oriented PhilosophyOriented Philosophy In data-centric methodology, you think

how to structure the data, then build thealgorithm around that structure.

In Ob ect-oriented s stem however the

algorithm and the data structure arepackaged together as an object, which has

a set of attributes and properties.

The state of these attributes are reflected

in values stored in its data structures.


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An ObjectAn Object-- Oriented PhilosophyOriented Philosophy The object has collection of methods or

procedures things it can do. The methods and attributes are equal

and inse arable arts of the ob ect one

cannot ignore one for the sake of other.

Traditional programming tends to write

lots of code to do all the things that haveto be done.


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An ObjectAn Object-- Oriented PhilosophyOriented Philosophy In summary , object oriented

programming language bridge thesemantic gap between the ideas of the

application and those of the underlying

machine, and objects represent theapplication data in a way that is not forced

by hardware architecture.


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ObjectsObjects The term Object was first formally

utilized in Simula language. The term object means a combination of

data and lo ic that re resents some real

entity.

In object-oriented system, everything is an

object. Each of us deal with object daily.


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ObjectsObjects When developing object-oriented

applications, two basic questions alwaysarise:

What ob ects does the a lication need?

What functionality should those objects

have?


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Objects are grouped into classesObjects are grouped into classes It is fairly natural to partition the world

into objects, properties and procedures. This is useful for classification

have no trouble in identifying it, as eagle

and not airplane, even if we have not seen

that bird before. Classes are used to distinguish one type

of object from another.


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Objects are grouped into classesObjects are grouped into classes In the context of object-oriented systems,

a class is a set of objects that share acommon structure and a common

behavior.

A single object is simply instance of aclass.

The chief role of a class is to define theproperties and procedures.


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Attributes : Object State andAttributes : Object State and

PropertiesProperties Properties represent the state of an

object. It is basically used to refer the description

of an ob ect.

For Examples:

Car has properties such as color,

manufacturer, and cost. Properties could be represented in

several ways in programming langugage


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Attributes : Object State andAttributes : Object State and

PropertiesProperties For Color , we could choose either String

RED or an integer representing RED. Manufacturer could be represented by a

name which is a reference in

Manufacturer object.

Cost could be a floating point number.

The importance of distinction is that anobjects abstract state can be

independent of its physical state.


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Object Behavior and MethodsObject Behavior and Methods When we talk about an elephant or a car

we can describe the set of things we cando with it or that it can do in its own.

This refers to descri tion of ob ects

behavior.

In OO object behavior is described in

methods or procedures. A method implements a behavior of an

object.


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Object Behavior and MethodsObject Behavior and Methods The object, called the receiver, is that on

which the method operates. Methods encapsulates the behavior of the

object.

Methods are used to exclusively accessand update the properties of the object.

Objects can take responsibility of their

own behavior. For ex: the Employee object know how to

compute its own salary.


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Objects Respond to MessagesObjects Respond to Messages An objects capabilities are determined by

the methods defined for it. Methods conceptually are equivalent to

the function definitions used in

proce ura anguage. Objects perform operations in response

to messages.

For Ex: when you press the brake a pedalof the car, you send a stop message to thecar object.


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Objects Respond to MessagesObjects Respond to Messages The car object knows how to respond to

the stop message. Sending the stop message to other

ob ects like tree will be meanin less and

could lead to unanticipated response.

Messages essentially are non specific

function calls: We could send a drawmessage to a chart when we want to

chart object to draw itself.


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Objects Respond to MessagesObjects Respond to Messages A message is different from a subroutine

call , since different objects can respondto the same message in different ways.

For exam le: car motorc cle and bic cle

respond to a stop message, but actualoperations performed are object specific.

It is important to understand thedifference between methods and

messages.


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Objects Respond to MessagesObjects Respond to Messages Example:

Say you .want to tell someone to makeyou a French Onion Soup, your

instruction is the messa e and the wa

the soup is prepared is the method.

In other words , the message is a

instruction and the method is theimplementation.


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Encapsulation and InformationEncapsulation and Information

HidingHiding Information hiding is the principle of

concealing the internal data andprocedures of an object and providing aninterface to each object in such a way as

inner workings.

In C++ there are very general

encapsulation protection mechanismwith public , private and protectedmembers.


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HidingHiding Public members (member data and

member functions) may be accessed fromanywhere.

Private members are accessible onl from

within the class.

Protected members are only accessed

from sub classes.


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HidingHiding Often an object is said to encapsulate the

data and a program.

This means that the user cannot see theinside of the object capsule, but can use

.

Encapsulation or information hiding is adesign goal of an object-oriented system.

Rather than allowing an object direct accessto another objects data , a message is sentto the target object requesting information.


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HidingHiding Another issue is per-object or per-class

protection. In per-class protection, the most common

form e. . ADA C++ Eiffel class

methods can access any object of thatclass and not just the receiver.

In per-object protection, methods canaccess only the receiver.


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HidingHiding An important factor in achieving

encapsulation is the design of differentclasses of objects that operate using a

common protocol, or objects user

interface. This means a program can send a generic

message and leave the implementation

up to the receiving object.


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Class HierarchyClass Hierarchy An object-oriented system organizes

classes into a subclass-superclasshierarchy.

At the to of the class hierarch are the

most general classes and at the bottomare the most specific.

The sub class inherits all the propertiesand methods defined in its superclass.


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Class HierarchyClass Hierarchy


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InheritanceInheritance Inheritance is a property of object-

oriented systems that allows objects tobe built from other objects.

It takes the advanta e from the

commonality of objects whenconstructing new classes.

Inheritance is the relationship betweenthe classes where one is parent class of

another (derived) class.


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InheritanceInheritance Parent class is also known as super class

or base class. The main advantage of this technique is

that we can build on what we alread

have and more, important, reuse what wealready have.

Inheritance allows us to share and reuse

the behaviors and attributes.


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InheritanceInheritance

Ford

Mustang Taurus Thunderbird


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Dynamic InheritanceDynamic Inheritance Dynamic inheritance allows objects to

change and evolve overtime.

Since base classes provide properties andattributes for objects, changing the base

c asses c anges t e propert es anattributes of a class.

More specifically, dynamic inheritance

refers to the ability to add, delete, orchange parents from objects(or classes )at runtime.


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Multiple InheritanceMultiple Inheritance Some object-oriented systems permit a

class to inherit its state(attributes) andbehaviors from more than one superclass.

This kind of inheritance is referred to as

multiple inheritance. Multiple inheritance can pose some

difficulties


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Multiple InheritanceMultiple Inheritance Example: Several distinct parents can

declare a member within a multipleinheritance hierarchy.

One wa of achievin benefits of multi le

inheritance is to inherit appropriate classand then add an object of another class as

an attribute.


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PolymorphismPolymorphism Poly means many and morph means

forms.

It means objects that can take and assumemany forms.

Pol mor hism means that the same

operation may behave differently ondifferent classes.

For example, consider how driving anautomobile with a manual transmission isdifferent from driving a car with anautomatic transmission.


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PolymorphismPolymorphism It is the fundamental concept in OOS.

Polymorphism allows us to write generic,reusable code more easily.


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Object Relationships andObject Relationships and

AssociationsAssociations Association represents the relationship

between objects and classes. For examples: A pilot can flyplanes

pilot and planes entities.

Associations are bidirectional; that means

they can be traversed in both directions,perhaps with different connotations.


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AssociationsAssociations A direction implied by a name is the

forward direction ; the opposite directionis the inverse direction.

Can fl connects a ilot to certain

airplanes. The inverse of can fly could be called

flown by.

Ob R l h dOb R l h d


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AssociationsAssociations Another important issue in association is

cardinality, which specifies how manyinstances of one class may relate to a

single instance of an associated class.

Cardinality constraints the number ofrelated objects and often is described as

being one or many.


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ConsumerConsumer--Producer AssociationProducer Association A special form of association is a

consumer producer relationship, alsoknown as a client server association or a

use relationship.

The consumer-producer relationship canbe viewed as one way interaction: One

object requests the service of another

object.


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ConsumerConsumer--Producer AssociationProducer Association The object that makes request is the

client or consumer, and the object thatreceive request or provide service is the

producer or server

Print Server Item

Request for service

A i d bjA i d bj


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Aggregation and objectAggregation and object

ContainmentContainment All objects, except most basic ones, are

composed of and may contain otherobjects.

For exam le Car ob ect is com osed of

engine and engine is still composed ofshafts and valves.

Breaking down the objects into the object

from which they are composed is called

decomposition.

A ti d bj tA ti d bj t


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Aggregation and objectAggregation and object

ContainmentContainment Since each object has identity, one object

can refer to other objects. This is known as aggregation, where

attribute can be ob ect itself.


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Object and IdentityObject and Identity A special feature of object-oriented

systems is that every object has its ownunique and immutable identity.

An ob ect identit comes into bein

when the object is created and continuesto represent that object then on.

The identity is never confused with

another objects identity even if the object

is deleted.


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Object and IdentityObject and Identity The identity name never changes even if

the properties of the object change- itsindependent of the objects state.

In ob ect oriented s stems ob ect

identity is implemented through somekind of object identifier(OID) or unique

identifier(UID).

An OID guarantees uniqueness of every

identifier.


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Static and Dynamic bindingStatic and Dynamic binding The process of determining (dynamically)

at runtime which function to invoke istermed dynamic binding.

Makin this determination earlier at

compile time, is called static binding. Static binding optimizes the calls.

Dynamic binding occurs whenpolymorphism calls are issued.


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Object persistenceObject persistence Object have lifetime.

They are explicitly created and can existfor a period of time that , traditionally, has

been the duration of the rocess in

which they were created. From language perspective this

characteristics is called persistence.


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Meta ClassesMeta Classes Information about classes is called meta

class.

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