29/09/2012

OO - Basics

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Introduction to Object Orientation Topics

Object technologies

Basic Principles of Object Orientation

Basic Concepts of Object Orientation

Strengths of Object Orientation

Object-oriented programming

OOP means developing software based on objects

In OOP, the data and the programming logic (the

code/functions), are both encapsulated in the object

Data and code encapsulated (combined) in an object

Procedural or structural programming focused

mainly on functions

In procedural programming, the data and the operations

that manipulate the data (the code) are separate

Data is separate from the programming logic (the

code/functions)

Object-orientation puts the focus on the

combination of function and data 3

Evolution of general-purpose programming languages

Machine code (1940s and 1950s)

Dependent on hardware architecture

No distinction between of executable instructions and data

Assembler code (1950s1970s)

Relocatable code - no fixed addresses in code but still

hardware-specific

Branch or link instructions led to spaghetti code

Executable instructions and data still

Programmable in modules with link-load to create

executable whole

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Evolution of general-purpose programming languages

Third generation (1960s to 1990)

Compiler translate high level languages such as Fortran,

PL1, COBOL, RPC, C and many more to assembled code

Source is platform dependent, executable code is hardware

specific

Separation of code and data

Subprograms/subroutines/functions/methods

Interpreted languages (1980 to 1990)

Just-in-time translation from source to executable code

BASIC, LISP, APL, APL and many more

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Evolution of programming styles

Third generation languages enabled coding

practices aimed at improving code quality,

reliability, readability and maintainability

Procedural:

Sequential execution

Conditional execution, loops, go to or branch to alter flow of

control

Subroutine call and return

Structured:

Decompose functionality into callable subprograms

Replace go to with nested blocks of code

Extensive use of subroutine call and return

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Evolution of programming styles

Modular programming:

Group subroutines with related functionality in to modules

Object oriented

Paradigm shift to reusable objects with data and behavior

Service oriented

Paradigm shift to reusable objects with data and behavior

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Some object-oriented programming languages

Simula and Simula 67 (1960s)

Early precursor to OO

Smalltalk (1970s to 1990s)

Pure OO language in part derived in part from Simula

Introduced message passing metaphor, class hierarchies

Distinctive integrated development environment with

visual coding

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Some object-oriented programming languages

C++ (1980s to 1990s)

A hybrid of C grammar with object constructs added to the

language

Difficult to master: often used like C

Very difficult to implement an efficient compiler

Java (1990s to present day)

Architectural neutral interpreted OO language from Sun

Microsystems

Syntax similar to C++, but greatly simplified

Evolved into dominant programming platform

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Why object-oriented programming?

Object-oriented programming enables:

A natural way of thinking

About the problem domain, not about processes and procedures.

Ability to support change:

Adapt to changing requirements

Adopt new, better, or cheaper technology with minimal impact

Manage the extensibility of a system

Code reuse

Not just copy, paste and then edit

Use existing as is and build upon it

Creation of code that can be maintained and updated

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Considering data and function

Procedural approach:

Data and function are handled separately.

No common way to access data can be enforced easily.

Object-oriented approach:

Data and function are combined in objects.

Function becomes the behavior of objects.

The use of an object only requires knowledge of the

function names.

This hiding the data and methods (functions) inside objects

is called encapsulation.

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The procedural approach to data and function

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The OO approach to data and function

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Approaches to modeling and coding

Procedural approach

Model only the business processes or the data or each separately.

The two are not modeled together.

The concept of the business and the pieces of the business are lost.

The solution becomes just a collection of functions.

OO approach

Model the business: Both structure and process.

Allows for better communication from user to analyst to designer to

programmer because they are all speaking of the business.

Allows the ability to more easily capture changes to the structure and to

the process.

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Modeling and coding example

Requirement:

Model withdrawing money from a savings account.

Procedural approach

Identify where the data is stored.

List the algorithmic steps necessary.

Object approach

Identify what objects are involved.

Example: Bank, savings account, teller and transaction

Show how these objects interact to:

Enforce business rules for withdrawals.

Decrement the balance. 15

Code reuse is a key benefit of OOP

Procedural

Hard to identify opportunities to reuse code because of the lack of

encapsulation

Reusability happens at the procedural level

Objects

Reuse of code, writing functions once and using them again in other

objects (similar to procedural).

Reuse of objects or a set of objects, since these are business objects

they can be reused throughout the enterprise.

Reuse of function names or sets of function names, in the procedural

approach multiple functions cannot have the same name, however with

objects, a function name can be reused for different objects with

different implementations.

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Code reuse is limited in procedural code

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OOP facilitates reuse of code

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OOP facilitates reuse of objects

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OOP facilitates reuse of names

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Communication in the development team

Procedural approach

Differences documents and styles of working are used during system

analysis, design, and programming.

Each stage ends with a milestone or sign off.

The resulting throw it over the wall approach can lead to

miscommunication.

Object-oriented approach

The same language is used for analysis, design, implementation

The language is objects

It is often written visually in the Unified Modeling Language (UML)

This leads to better communication between analyst, designer and

programmer.

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Benefits provided by objects

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Object Orientation

En

cap

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tio

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Ab

stra

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Hie

rarc

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Mo

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lari

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Basic Principles of Object Orientation

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What Is Abstraction?

The essential characteristics of an entity that

distinguishes it from all other kinds of entities.

Defines a boundary relative to the perspective of

the viewer.

Is not a concrete manifestation, denotes the ideal

essence of something.

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Salesperson

Not saying

Which

salesperson

just a salesperson

in general!!!

Customer Product

Manages Complexity

What is Abstraction?

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What Is Encapsulation?

Improves Resiliency

Hides implementation from clients.

Clients depend on interface.

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Encapsulation

"Encapsulation is a mechanism used to hide the data, internal structure, and implementation details of an object. All interaction with the object is through a public interface of operations." (Craig Larman)

A boundary exists around each object; the boundary encapsulates the objects characteristics (data elements) and behaviors (functionality).

The reason for hiding features is to:

(1) keep users from touching parts of the object they shouldnt touch;

(2) allows creator of the object to change the objects internal working without affecting the users of the object.

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Order Processing

System

Billing

Order

Entry

Order

Fulfillment

Manages Complexity

What is Modularity?

The breaking up of something complex into

manageable pieces

Helps people understand complex systems.

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Decreasing

abstraction

Increasing

abstraction

Asset

RealEstate

Savings

BankAccount

Checking Stock

Security

Bond

Elements at the same level of the hierarchy

should be at the same level of abstraction

What is Hierarchy?

Levels of abstraction

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Basic Concepts of Object Orientation

Object

Class

Attribute

Operation

Interface (Polymorphism)

Component

Package

Subsystem

Relationships

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Informally, an object represents an entity, either

physical, conceptual, or software

Physical entity

Conceptual entity

Software entity

Truck

Chemical Process

Linked List

What is an Object?

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A More Formal Definition

An object is a concept, abstraction, or thing with

sharp boundaries and meaning for an application

An object is something that has:

State

Behavior

Identity

A real live object

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A person object

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OO Principle: Abstraction

What is a Class?

A class is a description of a group of objects with

common properties (attributes), behavior

(operations), relationships, and semantics

An object is an instance of a class

A class is an abstraction in that it:

Emphasizes relevant characteristics

Suppresses other characteristics

What is necessary to define a class

Name

The class name needs to specify what the abstraction is

capturing. The name of the class should:

Be singular Clearly identify the concept

Not be too vague or general Be short and concise

List of data elements

The pieces of data that need to be captured for the

abstraction.

List of messages

The messages (and their implementation, methods) that the

abstraction can receive.

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a + b = 10

Class

Course

Properties

Name

Location

Days offered

Credit hours

Start time

End time

Behavior

Add a student

Delete a student

Get course roster

Determine if it is full

Sample Class

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Representing Classes in the UML

A class is represented using a rectangle with three

compartments:

The class name

The structure (attributes)

The behavior (operations)

Professor

- name

- employeeID : UniqueId

- hireDate

- status

- discipline

- maxLoad

+ submitFinalGrade()

+ acceptCourseOffering()

+ setMaxLoad()

+ takeSabbatical()

+ teachClass()

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Classes of Objects

How many classes do you see?

The difference between a class and a table

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Object

s Clas

s

Professor Smith

Professor Jones

Professor

The Relationship Between Classes and Objects

A class is an abstract definition of an object

It defines the structure and behavior of each object in

the class

It serves as a template for creating objects

Objects are grouped into classes

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What Is an Attribute?

An attribute is a named property of a class that describes

the range of values that instances of the property may

hold.

A class may have any number of attributes or no attributes

at all.

Attributes

Student

- name

- address

- studentID

- dateOfBirth

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What Is an Operation?

A service that can be requested from an object to effect

behavior. An operation has a signature, which may restrict

the actual parameters that are possible.

A class may have any number of operations or none at all.

Operations

Student

+ get tuition() + add schedule() + get schedule() + delete schedule() + has prerequisites()

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What Is Polymorphism?

Manufacturer A Manufacturer B

Manufacturer C

OO Principle:

Encapsulation Remote Control

The ability to hide many different implementations behind

a single interface.

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Polymorphism

Occurs with inheritance.

Different subclasses may have different implementations of the identical operations

Allows you to treat an object as the base class, rather than as a specific inherited type.

Programmer doesnt have to keep track of the specific subclasses, the system selects the correct operation based on the object type.

Accomplished at run time using dynamic binding.

lets you substitute objects that have identical interfaces for each other at run-time.

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Tube

Pyramid

Cube

Shape

Draw

Move

Scale

Rotate

Realization relationship (stay tuned for realization relationships)

What is an Interface?

Interfaces formalize polymorphism

Interfaces support plug-and-play architectures

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Interface

How do you get an object to do useful work for you?

Objects are known only through their interfaces

Every object presents an interface to the world.

Interface determines what you can get an object to

do.

Represent a "contract" with other objects.

Communicate with objects by sending messages to

them; an objects interface is the set of messages

an object will respond to.

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A package is a general purpose mechanism for

organizing elements into groups

A model element which can contain other model

elements

Uses

Organize the model under development

A unit of configuration management

Package Name OO Principle:

Modularity

What is a Package?

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A combination of a package (can contain other

model elements) and a class (has behavior)

Realizes one or more interfaces which define its

behavior

OO Principles: Encapsulation and Modularity

Subsystem Name

Interface Interface

Realization Subsystem

What is a Subsystem?

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A non-trivial, nearly independent, and replaceable

part of a system that fulfills a clear function in the

context of a well-defined architecture

A component may be

A source code component

A run time components or

An executable component

Source File

Name

OO Principle:

Encapsulation

What is a Component?

Component

Name Component

Interface

Executable

Name

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Component

Name

Design Model Implementation Model

Component Name Component

Interface

Component

Interface

OO Principles: Encapsulation and Modularity

Subsystems and Components

Components are the physical realization of an

abstraction in the design

Subsystems can be used to represent the

component in the design

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Relationships

Association

Aggregation

Dependency

Generalization

Realization

ConsoleWindow

Window

open() close() move() display() handleEvent()

Event

DialogBox Control

association generalization

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Professor University Works for

Class

Association

Association

Name

Relationships: Association

Models a semantic connection among classes

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Relationships: Association Aggregation

To model a whole/part relationship, use

aggregation.

Aggregation represents a has-a or is a part-

of relationship, meaning that an object of the

whole has objects of the part. [Martin Fowler]

Company Department * 1

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Association: Multiplicity and Navigation

Student Schedule 1 0..*

Multiplicity

Navigation

Multiplicity defines how many objects participate in a relationships

Specified for each end of the association

Associations are bi-directional by default, but it is often desirable to restrict navigation to one direction

If navigation is restricted, an arrowhead is added to indicate the direction of the navigation

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Packag

e

ClientPackage SupplierPackage

Client Supplier Class

Dependency

relationship Client Supplier

Dependency

relationship

Componen

t

Relationships: Dependency

A relationship between two model elements where a change

in one may cause a change in the other

E.g. Class A depends on class B: method of A manipulates objects of B

in the parameters, local variables, or return types

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Tube

Pyramid

Cube

Shape

Draw

Move

Scale

Rotate

Realization relationship (stay tuned for realization relationships)

Relationships: Realization

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Inheritance

[Martin Fowler]

Shape origin

draw() erase() move() setColor() getColor()

Circle

draw() erase()

Square

draw() erase() move()

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Polymorphism

[Martin Fowler]

Goose

move()

Penguin

move()

FlockManager

reLocate()

Bird

move()

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Airplane Helicopter Wolf Horse

FlyingThing Animal

Bird

multiple

inheritance

Use multiple inheritance only when needed, and

always with caution !

Example: Multiple Inheritance

A class can inherit from several other classes

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Inheritance leverages the similarities among classes

What Gets Inherited?

A subclass inherits its parents attributes,

operations, and relationships

A subclass may:

Add additional attributes, operations, relationships

Redefine inherited operations (use caution!)

Common attributes, operations, and/or

relationships are shown at the highest applicable

level in the hierarchy

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Truck

tonnage

GroundVehicle

weight

licenseNumber

Car

owner

register( )

getTax( )

Person

0..*

Trailer

1 Superclass

(parent)

Subclass

generalization

size

Example: What Gets Inherited

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The Strengths of Object Technology

Provides a single paradigm

A single language used by users, analysts, designers, and implementers

Facilitates architectural and code reuse

Models more closely reflect the real world

More accurately describes corporate entities

Decomposed based on natural partitioning

Easier to understand and maintain

Provides stability

A small change in requirements does not mean massive changes in the

system under development

Is adaptive to change