Advanced Object Oriented Concepts

Tonga Institute of Higher Education

Inheritance Similarities

“You look just like your father.”People inherit features from their parents:

Eye color Hair color Height

You are similar to your parents

Inheritance in Visual Basic .Net Inheritance – When a class inherits

members from another class Objects are like people. They can have

children!Children can inherit variables from a parentChildren can inherit methods from a parent

Parent / Base Class

Child / Sub Class

History of Inheritance

In the sixties, a programmers created a program to simulate traffic They used objects for their vehicles

Cars Trucks Vans

They noticed that all vehicles did the same things Turn left Turn right Brake Go

Plan #1 - Van, Car and Truck Objects

Create one class for each vehicle Van Car Truck

VanTurnLeft()

TurnRight()

Brake()

Go()

CarTurnLeft()

TurnRight()

Brake()

Go()

TruckTurnLeft()

TurnRight()

Brake()

Go()

Plan #1 - Advantages

It’s quick and easy to understand

VanTurnLeft()

TurnRight()

Brake()

Go()

CarTurnLeft()

TurnRight()

Brake()

Go()

TruckTurnLeft()

TurnRight()

Brake()

Go()

Plan #1 - Disadvantages

Code is repeated in each object Changing the code in Brake() requires 3 changes to 3

different objects

Method names can be changed. After a while, the objects are not similar

VanTurnLeft() -> Left()

TurnRight() -> Right()

Brake()

Go()

CarTurnLeft()

TurnRight()

Brake()

Go() -> Move()

TruckTurnLeft()

TurnRight()

Brake()

Go() -> Start()

Plan #2 - Inheritance Make one object with common methods. The code in the parent object is used in the child

objects. VehicleTurnLeft()

TurnRight()

Brake()

Go()

CarTurnLeft()TurnRight()Brake()Go()

TruckTurnLeft()TurnRight()Brake()Go()

Methods in theparent comedown to thechildren!

Plan #2 – Advantages A change in the method code in the parent automatically changes the children

classes Method code is consistent and easy to maintain

A change in the method name in the parent automatically changes the children. Names are consistent and easy to maintain

We can change a class that someone else created It is difficult to write your own button class. But we can add changes to the button class

using inheritance

VehicleTurnLeft() -> Left()

TurnRight() -> Right()

Brake()

Go()

CarTurnLeft() -> Left()TurnRight() -> Right()Brake()Go()

TruckTurnLeft() -> Left()TurnRight() -> Right()Brake()Go()

Plan #2 – Disadvantages Inheritance requires special code Inheritance requires more understanding

VehicleTurnLeft()

TurnRight()

Brake()

Go()

CarTurnLeft()TurnRight()Brake()Go()

TruckTurnLeft()TurnRight()Brake()Go()

Inheritance Differences

“You look just like your father.”People inherit features from their parents:

Eye color Hair color Height

You are similar to your parents. “But you are much taller”

You are also different from them.

Inheritance Differences Each child object can have additional different

members.VehicleTurnLeft()

TurnRight()

Brake()

Go()

CarTurnLeft()TurnRight()Brake()Go()ConservePetrol()

TruckTurnLeft()TurnRight()Brake()Go()CarryLoad()

Example of Parent / Base Class

All non-private members are shared with child classes

Example of Child / Sub Class

Specialkeywordthat bringsmembersfromparent

Child specific classes

Example of Driver for Inheritance

Inherited classes work exactly like a normal class

There is no difference when using an inherited method and a non-inherited method

Demonstration

Inheritance Code

Inheritance in VB.Net Classes

Look in the class definitions.

Almost every class inherits from another class!

Demonstration

Inheritance in VB .Net Classes

Access Specifiers

Public Can be used by

everything

Private Can only be used

by code inside the same class

Dim Same as Private

Friend Can be used by

code inside the same project

Dim FirstName as String

Access SpecifierName

Type

Protected Can be used by code that inherits

from this class Protected Friend

Combination of Protected and Friend

Demonstration

Access Specifiers

Multi-Level Inheritance

When a subclass is also a base class.

VehicleTurnLeft()

TurnRight()

Brake()

Go()

CarTurnLeft()TurnRight()Brake()Go()ConservePetrol()

RentalCarTurnLeft()TurnRight()Brake()Go()ConservePetrol()RentalPrice()

Demonstration

Multi-Level Inheritance

Overriding Base Methods Overriding – When a child

class replaces the behavior of a method defined in a base class.

To override (replace) a method: Define a method in the

parent class to be overridable

Define a method in the child class to override

Parent Class

Child Class

Demonstration

Overriding Base Methods

MyBase

Use in a child class

Use the MyBase keyword to call methods in a base class.

Demonstration

MyBase

MyClass and Me 1 If inheritance is not

used, they work the same way.

MyClass and Me 2 If inheritance is used, use

these in parent classes Use the MyClass

keyword to call methods in the class this keyword is used.

Use the Me keyword to call methods in the current instance.

Demonstration

MyClass and Me

Abstract Classes and Methods Abstract class – A class that must be inherited from

An abstract class cannot be instantiated MustInherit – Used to make a class abstract

Abstract method – A method that must be overridden Method code does not exist in the base class because it will be

overridden MustOverride – Used to make a method abstract

Demonstration

Abstract Class or Method

Preventing Inheritance

NotInheritable – Used to make a class uninheritable.

Demonstration

Preventing Inheritance

Introduction to Interfaces 1

All DVD players have the same controls, even if they are made by different companies (Sony, Panasonic, Toshiba, etc.)

The buttons are a contract for how to operate the DVD player

An interface is a contract

detailing how an object is used Using this interface, I know how to

use the DVDPlayer Object

DVD PlayerPlay()Pause()Stop()Rewind()FastForward()

Introduction to Interfaces 2

All DVD players have the same controls, even if they are made by different companies (Sony, Panasonic, Toshiba, etc.)

But these different companies are not related to each other

This is the difference between interfaces and inheritance

Defining an Interface Defining an interface tells the computer that any class

that uses this interface is guaranteed to have the elements described in the interface

Example: Any class that implements the ISummary will have these functions and properties GetShortSummary GetFullSummary HasFullSummary

Demonstration

Defining an Interface

Implementing an Interface

Demonstration

Implementing an Interface

Primary/Native Interfaces vs. Secondary Interfaces

Primary/Native Interface of a class is composed of all the members defined in a class

Secondary Interfaces involves the implementation of other interfaces

SecondaryInterface

PrimaryInterface

Abstract Classes vs. Interfaces

It is sometimes difficult to tell the difference between an abstract class and an interfaceAn abstract class is a class that cannot be

instantiated, but must be inherited fromAn interface, by contrast, is a set of members

that defines a contract for conduct

Abstract Classes vs. Interfaces 2

It is sometimes also difficult to decide whether to use an abstract class or an interface A consideration is that a class may implement more

than one interface. A class may only inherit from only 1 class.

Abstract classes are best for objects that are closely related. Interfaces are best for providing common functionality to unrelated classes.

Use an abstract class to provide members that use the same code among many objects. Interfaces only force classes to have the same member names.