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OBJECT ORIENTED PROGRAMMING

THE BASICS

What is Object-Oriented Design?

It promotes thinking about software in a way that models how we think about the real world

It organises program code into classes of objects

What is a Class?

A class is a collection of things (objects) with similar attributes and behaviours.

Attributes: What is looks like

Behaviours: What it does

Classes and ObjectsClass Object A class is a

template or blueprint that defines an object’s attributes and operations and created at design time

An object is a running instance of a class that consumes memory and has a finite lifespan

What is an Object?

Class: Dog

Object:Red Setter

Object:Labrador

Object:Terrier

Object:Bulldog

Every object is an instance of a class Every object has attributes and

behaviours

Class Examples

Dogs

Attributes: Four legs, a tail Behaviours: Barking

Cars

Attributes: Four wheels, engine, 3 or 5 doors Behaviours: Acceleration, braking, turning

In Code

The programming constructs of attributes and behaviours are implemented as: Attributes: Properties Behaviours: Methods

Public Class DogPublic Name As String

Public Sub Sleep()MessageBox.Show(“ZZzz”)

End SubEnd Class

Dim GoldenRetriever as New DogGoldenRetriever.Name = “Rex”GoldenRetriever.Sleep()

Object Class

Property

Method

Another Example Ferrari is an instance of the Car class

Attributes (Use properties or fields): Red Rear wheel drive Max speed 330 km/h.

Behaviours (Methods): Accelerate Turn Stop

VB.NET & OOP

VB .NET is an object-oriented language.

VB.NET Supports:

Encapsulation Abstraction

Inheritance Polymorphism

Encapsulation

How an object performs its duties is hidden from the outside world, simplifying client development Clients can call a method of an object

without understanding the inner workings or complexity

Any changes made to the inner workings are hidden from clients

Example

Car Stereo Standard case size and fittings, regardless of

features Can be upgraded without affecting rest of car Functionality is wrapped in a self-

contained manner

Encapsulation – In Practice Declare internal details of a class as

Private to prevent them from being used outside your class This technique is called data hiding.

This is achieved by using property procedures.

Abstraction

Abstraction is selective ignorance Decide what is important and what is not Focus on and depend on what is

important Ignore and do not depend on what is

unimportant Use encapsulation to enforce an

abstraction

Inheritance

Inheritance specifies an “is-a-kind-of” relationship

Multiple classes share the same attributes and behaviours, allowing efficient code reuse

Examples: A customer “is a kind of” person An employee “is a kind of” person

Customer Employee

Person

Base Class

Derived classes

Inheritance We can create new classes of objects

by inheriting attributes and behaviours from existing classes and then extending themWe can build hierarchies (family trees)

of classes Person

Employee

Part Time Full Time

Inheritance Cont’d

The existing class is called the base class, and the new class derived from the base class is called the derived class.

The derived class inherits all the properties, methods, and events of the base class and can be customized with additional properties and methods.

Inheritance

Example Rally Car

Inherits properties of class Car … … and extends class Car by adding a

rollcage, racing brakes, fire extinguisher, etc.

Polymorphism

The ability for objects from different classes to respond appropriately to identical method names or operators.

Allows you to use shared names, and the system will apply the appropriate code for the particular object.

Different code will execute depending on the context!

DOING IT IN CODEFROM THEORY TO PRACTICE

Creating Classes in Code

Add a class to the project

Provide appropriate name for the class

Create constructors as needed

Create a destructor, if appropriate

Declare properties

Declare methods

Creating Classes in Code

Add a class to the project

Provide appropriate name for the class

Create constructors as needed

Create a destructor, if appropriate

Declare properties

Declare methods

1. Add Class to the Project

Creating Classes in Code

Add a class to the project

Provide appropriate name for the class

Create constructors as needed

Create a destructor, if appropriate

Declare properties

Declare methods

2. Provide Appropriate Name

Creating Classes in Code

Add a class to the project

Provide appropriate name for the class

Create constructors as needed

Create a destructor, if appropriate

Declare properties

Declare methods

3. Create Constructors

Sub New replaces Class_Initialize Executes code when object is

instantiated

Can overload, but does not use Overloads keyword

Public Sub New( ) 'Perform simple initialization Course = “BIS”End Sub

Public Sub New( ) 'Perform simple initialization Course = “BIS”End Sub

Public Sub New(ByVal i As Integer) 'Overloaded without Overloads 'Perform more complex initialization intValue = iEnd Sub

Public Sub New(ByVal i As Integer) 'Overloaded without Overloads 'Perform more complex initialization intValue = iEnd Sub

Creating Classes in Code

Add a class to the project

Provide appropriate name for the class

Create constructors as needed

Create a destructor, if appropriate

Declare properties

Declare methods

4. Create Destructor

Sub Finalize replaces Class_Terminate event

Use to clean up resources Code executed when destroyed by

garbage collection Important: destruction may not happen

immediatelyProtected Overrides Sub Finalize( )

'Can close connections or other resources

conn.Close

End Sub

Protected Overrides Sub Finalize( )

'Can close connections or other resources

conn.Close

End Sub

Creating Classes in Code

Add a class to the project

Provide appropriate name for the class

Create constructors as needed

Create a destructor, if appropriate

Declare properties

Declare methods

5. Declare Properties

Specify accessibility of variables and procedures

Keyword DefinitionPublic Accessible everywhere.

Private Accessible only within the type itself.

Friend Accessible within the type itself and all namespaces and code within the same assembly.

Protected Only for use on class members. Accessible within the class itself and any derived classes.

Protected Friend

The union of Protected and Friend.

5. Cont’d

Properties represent a classes attributes

Student First Name Last Name StudentID Age Course

5. Properties (Property Procedures)

To store values for a property you use the SET property procedure

To retrieve values from a property you use the GET property procedure

You must specify whether the value stored in the property can obtained and changed

If a procedure can only obtain a property it is Read Only

If it can be obtained and changed it is Read-Write

Creating Classes in Code

Add a class to the project

Provide appropriate name for the class

Create constructors as needed

Create a destructor, if appropriate

Declare properties

Declare methods

6. Declare Methods

Methods represent a classes behaviours

Student Eat Sleep Drink Study Pass Fail Graduate

USING OUR CLASSFROM THEORY TO PRACTICE

Instantiating our Class

Create the Object

Write object attributes

Read object attributes

Use object behaviours

Instantiating our Class

Create the Object

Write object attributes

Read object attributes

Use object behaviours

Create the Object

Dim myStudent As New Student

Instantiating our Class

Create the Object

Write object attributes

Read object attributes

Use object behaviours

Write Object Attributes

myStudent.FirstName = _ txtFirstName.Text

myStudent.LastName = txtLastName.Text

myStudent.Age = Val(txtAge.Text)

myStudent.StudentID = _ txtStudentID.Text

Write Object Attributes

myStudent.FirstName = _ txtFirstName.Text

myStudent.LastName = txtLastName.Text

myStudent.Age = Val(txtAge.Text)

myStudent.StudentID = _ txtStudentID.Text

Write Object Attributes

myStudent.FirstName = _ txtFirstName.Text

myStudent.LastName = txtLastName.Text

myStudent.Age = Val(txtAge.Text)

myStudent.StudentID = _ txtStudentID.Text

Write Object Attributes

myStudent.FirstName = _ txtFirstName.Text

myStudent.LastName = txtLastName.Text

myStudent.Age = Val(txtAge.Text)

myStudent.StudentID = _ txtStudentID.Text

EXTENDING OUR CLASSFROM THEORY TO PRACTICE