4 major principles of Object-Oriented Programming, Raymond Lewallen

4 programming

4 major principles of Object-Oriented Programming

For you new programmers or programmers new to OOP, this

article will briefly explain the 4 major principles that make a

language object-oriented: Encapsulation, Data Abstraction,

Polymorphism and Inheritence. All examples will be in VB.Net,

because in my opinion its easier for a new OOP programmer to read and

understand at first. Certainly don’t think I’m saying you should use

one .Net based language over another, as they all are based on the

CLR/CLS and all end up as the same assembly language when compiled. Its

your preference that determines what language you use. Of course,

there are other OOP languages out there, such as Ruby, a pure OOP

language, and hybrid languages such as Python, C++ and Java to mention

What is encapsulation? Well, in a nutshell, encapsulation is the

hiding of data implementation by restricting access to accessors and

mutators. First, lets define accessors and mutators:

An accessor is a method that is used to ask an object about itself. In

OOP, these are usually in the form of properties, which have, under

normal conditions, a get method, which is an accessor method.

However, accessor methods are not restricted to properties and can be

any public method that gives information about the state of the object.

Public Class Person

‘ We use Private here to hide the implementation of the objects

‘ fullName, which is used for the internal implementation of Person.

Private _fullName As String = “Raymond Lewallen”

‘ This property acts as an accessor. To the caller, it hides the

‘ implementation of fullName and where it is set and what is

‘ setting its value. It only returns the fullname state of the

‘ Person object, and nothing more. From another class, calling

‘ Person.FullName() will return “Raymond Lewallen”.

‘ There are other things, such as we need to instantiate the

‘ Person class first, but thats a different discussion.

Public ReadOnly Property FullName() As String

Mutator Mutators are public methods that are used to modify

the state of an object, while hiding the implementation of exactly how

the data gets modified. Mutators are commonly another portion of the

property discussed above, except this time its the set method that lets the caller modify the member data behind the scenes.

Public Class Person

‘ We use Private here to hide the implementation of the objects

‘ fullName, which is used for the internal implementation of Person.

Private _fullName As String = “Raymond Lewallen”

‘ This property now acts as an accessor and mutator. We still

‘ have hidden the implementation of fullName.

Public Property FullName() As String

Set ( ByVal value As String )

Ok, now lets look at a different example that contains an accessor and a mutator:

Public Class Person

Private _fullName As String = “Raymond Lewallen”

‘ Here is another example of an accessor method,

‘ except this time we use a function.

Public Function GetFullName() As String

‘ Here is another example of a mutator method,

‘ except this time we use a subroutine.

Public Sub SetFullName( ByVal newName As String )

So, the use of mutators and accessors provides many advantages. By

hiding the implementation of our Person class, we can make changes to

the Person class without the worry that we are going to break other

code that is using and calling the Person class for information. If we

wanted, we could change the fullName from a String to an array of

single characters (FYI, this is what a string object actually is behind

the scenes) but they callers would never have to know because we would

still return them a single FullName string, but behind the scenes we

are dealing with a character array instead of a string object. Its

transparent to the rest of the program. This type of data protection

and implementation protection is called Encapsulation. Think of accessors and mutators as the pieces that surround the data that forms the class.

Data abstraction is the simplest of principles to understand. Data

abstraction and encapuslation are closely tied together, because a

simple definition of data abstraction is the development of classes,

objects, types in terms of their interfaces and functionality, instead

of their implementation details. Abstraction denotes a model, a view,

or some other focused representation for an actual item. Its the

development of a software object to represent an object we can find in

the real world. Encapsulation hides the details of that implementation.

is used to manage complexity. Software developers use abstraction to

decompose complex systems into smaller components. As development

progresss, programmers know the functionality they can expect from as

yet undeveloped subsystems. Thus, programmers are not burdened by

considering the waysin which the implementation of later subsystesm

will affect the design of earlier development.

definition of abstraction I’ve ever read is: “An abstraction denotes

the essential characteristics of an object that distinguish it from all

other kinds of object and thus provide crisply defined conceptual

boundaries, relative to the perspective of the viewer.” — G. Booch,

Object-Oriented Design With Applications, Benjamin/Cummings, Menlo

Park, California, 1991.

Lets look at this code for a person

object. What are some things that a person can do? Those things must be

represented here in our software model of a person. Things such as how

tall the person is, and the age of the person; we need to be able to

see those. We need the ability for the person to do things, such as

run. We need to be able to ask the person if they can read.

Public Class Person

Private _height As Int16

Public Property Height() As Int16

Set ( ByVal Value As Int16)

Private _weight As Int16

Public Property Weight() As Int16

Set ( ByVal Value As Int16)

Private _age As Int16

Public Property Age() As Int16

Set ( ByVal Value As Int16)

‘ Code that makes the person sit

‘ Code that makes the person run

‘ Code that make the person cry

Public Function CanRead() As Boolean

‘ Code that determines if the person can read

‘ and returns a true or false

So, there we have started to create a software model of a person

object; we have created an abstract type of what a person object is to

us outside of the software world. The abstract person is defined by the

operations that can be performed on it, and the information we can get

from it and give to it. What does the abstracted person object look

like to the software world that doesn’t have access to its inner

workings? It looks like this:

You can’t really see what the code is that makes the person run. This is encapsulation that we discuseed.

in short, data abstraction is nothing more than the implementation of

an object that contains the same essential properties and actions we

can find in the original object we are representing.

Now lets discuss inheritance. Objects can relate to eachother

with either a “has a”, “uses a” or an “is a” relationship. “Is a”

is the inheritance way of object relationship. The example of

this that has always stuck with me over the years is a library (I think

I may have read it in something Grady Booch wrote). So, take a

library, for example. A library lends more than just books, it

also lends magazines, audiocassettes and microfilm. On some

level, all of these items can be treated the same: All four types

represent assets of the library that can be loaned out to people.

However, even though the 4 types can be viewed as the same, they are

not identical. A book has an ISBN and a magazine does not.

And audiocassette has a play length and microfilm cannot be checked out

Each of these library’s assets should be represented by its own

class definition. Without inheritance though, each class must

independently implement the characteristics that are common to all

loanable assets. All assets are either checked out or available

for checkout. All assets have a title, a date of acquisition and

a replacement cost. Rather than duplicate functionality,

inheritance allows you to inherit functionality from another class,

called a superclass or base class.

Let us look at loanable assets base class. This will be used

as the base for assets classes such as book and audiocassette:

Public Class LibraryAsset

Private _title As String

Public Property Title() As String

Set ( ByVal Value As String )

Private _checkedOut As Boolean

Public Property CheckedOut() As Boolean

Set ( ByVal Value As Boolean )

Private _dateOfAcquisition As DateTime

Public Property DateOfAcquisition() As DateTime

Set ( ByVal Value As DateTime)

Private _replacementCost As Double

Public Property ReplacementCost() As Double

Set ( ByVal Value As Double )

This LibraryAsset is a superclass, or base class, that maintains

only the data and methods that are common to all loanable assets.

Book, magazine, audiocassette and microfilm will all be subclasses or derived classes

or the LibraryAsset class, and so they inherit these

characteristics. The inheritance relationship is called the “is

a” relationship. A book “is a” LibraryAsset, as are the other 3

Let’s look at book and audiocassette classes that inherit from out LibraryAsset class:

Public Class Book

Private _author As String

Public Property Author() As String

Set ( ByVal Value As String )

Private _isbn As String

Public Property Isbn() As String

Set ( ByVal Value As String )

Public Class AudioCassette

Private _playLength As Int16

Public Property PlayLength() As Int16

Set ( ByVal Value As Int16)

Now, lets create an instance of the book class so we can record a new book into the library inventory:

Dim myBook As Book = New Book

myBook.Author = “Sahil Malik”

myBook.Title = “The Best Ado.Net Book You’ll Ever Buy”

You see, when we create a new book, we have all the properties of

the LibraryAsset class available to us as well, because we inherited

the class. Methods can be inherited as well. Let’s add a

few methods to our LibraryAsset class:

Public Class LibraryAsset

‘ Pretend the properties listed above are right here

Public Sub CheckOut()

If Not _checkedOut Then _checkedOut = True

Public Sub CheckIn()

If _checkedOut Then _checkedOut = False

Now, our “myBook” we created above automatically inherited these

methods, and we didn’t even have to touch the Book class in order for

it to happen. The book and audiocassette classes above

automatically inherited the abilities to be checked out and checked

in. In our “myBook” above, now we can check the book out by

calling “myBook.CheckOut()”. Simple! One of the most

powerful features of inheritance is the ability to extend components

without any knowledge of the way in which a class was implemented.

Declaration options, such as Public and Private, dictate which

members of a superclass can be inherited. For more information on

this, see the Declaration Option section of Eric’s post.

Polymorphism means one name, many forms. Polymorphism

manifests itself by having multiple methods all with the same name, but

slighty different functionality. Many VB6ers are familiar

with interface polymorphism. I’m only going to discuss

polymorphism from the point of view of inheritance because this is the

part that is new to many people. Because of this, it can be

difficult to fully grasp the full potential of polymorphism until you

get some practice with it and see exactly what happens under different

scenarios. We’re only going to talk about polymorphism, like

the other topics, at the basic level.

There are 2 basic types of polymorphism. Overridding, also

called run-time polymorphism, and overloading, which is referred to as

compile-time polymorphism. This difference is, for method

overloading, the compiler determines which method will be

executed, and this decision is made when the code gets compiled.

Which method will be used for method overriding is determined at

runtime based on the dynamic type of an object.

Let’s look at some code:

‘ Base class for library assets

Public MustInherit Class LibraryAsset

‘ Default fine per day for overdue items

Private Const _finePerDay As Double = 1.25

‘ Due date for an item that has been checked out

Private _dueDate As DateTime

Public Property DueDate() As DateTime

Set ( ByVal Value As DateTime)

‘ Calculates the default fine amount for an overdue item

Public Overridable Function CalculateFineTotal() As Double

Dim daysOverdue As Int32 = CalculateDaysOverdue()

If daysOverdue > 0 Then

Return daysOverdue * _finePerDay

‘ Calculates how many days overdue for an item being returned

Protected Function CalculateDaysOverdue() As Int32

Return DateDiff(DateInterval.Day, _dueDate, DateTime.Now())

‘ Magazine class that inherits LibraryAsset

Public NotInheritable Class Magazine

‘ Book class that inherits LibraryAsset

Public NotInheritable Class Book

‘ This is morphing the CalculateFineTotal() function of the base class.

‘ This function overrides the base class function, and any call

‘ to CalculateFineTotal from any instantiated Book class will

‘ use this function, not the base class function.

‘ This type of polymorphism is called overriding.

Public Overrides Function CalculateFineTotal() As Double

Dim daysOverdue As Int32 = CalculateDaysOverdue()

If daysOverdue > 0 Then

Return daysOverdue * 0.75

‘ AudioCassette class that inherits LibraryAsset

Public NotInheritable Class AudioCassette

‘ This is morphing the CalculateFineTotal() function of the base class.

‘ This is morphing the CalculateFineTotal(double) function of the

‘ This function overrides the base class function, and any call

‘ to CalculateFineTotal() from any instantiated AudioCassette

‘ Class will use this function, not the base class function.

‘ This type of polymorphism is called overloading and overriding.

Public Overloads Overrides Function CalculateFineTotal() As Double

Dim daysOverdue As Int32 = CalculateDaysOverdue()

If daysOverdue > 0 Then

Return daysOverdue * 0.25

‘ This is morphing the CalculateFineTotal() function of the

‘ This type of polymorphism is called overloading.

Public Overloads Function CalculateFineTotal( ByVal finePerDay As Double ) As Double

Dim daysOverdue As Int32 = CalculateDaysOverdue()

If daysOverdue > 0 AndAlso finePerDay > 0.0 Then

Return daysOverdue * finePerDay

You see our library asset class. Pay attention to the

overridable function CalculateFineTotal(). In LibraryAsset, we

have defined the default functionality for this method that any derived

classes can use. Any class derived from LibraryAsset can use this

default behavior and calculate fines based on the default

implementation of $1.25 per day late. This is true for our

Magazine class. We didn’t override the function so when late fees

are calculated for late magazine returns, it will use the default

Now look at the book class. We have overridden the

CalculateFineTotal to use a different value when determining late

fees. The overrides keywork in VB tells the caller that any

method call will use the virtual method found in Book, not the default

implementation found in LibraryAsset. We have implemented runtime

polymorphism – method overriding.

Lets move on to AudioCassette. Here we have the same method

overriding we found in the book class. Fines are calculated based

on $0.25 per day. Notice we’ve added something extra. We’ve

added the Overloads keywork to our function and to a new function with

the same name, except the new function now accepts a parameter.

Now the caller can call either method, and depending on whether or not

a parameter is passed, that determines with method will be

executed. Notice we do not include the overrides keywork in the

2nd function with a parameter. This is because not method exists

in LibraryAsset with that same signature (accepting a parameter of type

double). You can only override methods with the same signature in

Now lets look at some code that creates all these library items and

checks them in and cacluates our fines based on returning them 3 days

Public Class Demo

‘ Set the due date to be three days ago

Dim dueDate As DateTime = DateAdd(DateInterval.Day, -3, Now())

Public Sub ReturnMagazine( ByVal dueDate As DateTime)

Dim myMagazine As LibraryAsset = New Magazine

Dim amountDue As Double = myMagazine.CalculateFineTotal()

Public Sub ReturnBook( ByVal dueDate As DateTime)

Dim myBook As LibraryAsset = New Book

Dim amountDue As Double = myBook.CalculateFineTotal()

Public Sub ReturnAudioCassette( ByVal dueDate As DateTime)

Dim myAudioCassette As AudioCassette = New AudioCassette

Dim amountDue As Double

The output will look like the following:

You can see how all of our output was different, based on the method

that was executed. We created a new Magazine, which is a type of

LibraryAsset. That is why the instantiation says “myMagazine As

LibraryAsset”. However, since we actually want a magazine, we

create a “New Magazine”. Same thing with book. For Book,

its a little bit more tricky. Since we created a Book of the type

LibraryAsset, this is where the polymorphism comes into play.

Book overrides the CalculateFineTotal of LibraryAsset.

Audiocassette is a little bit different. It actually extends the

implementation of LibraryAsset by including an overloaded function for

CalculateFineTotal(). If we weren’t going to use the function

that took a parameter, we would create it the same way we created the

Book and Magazine classes. But in order to use the overloaded

function, we have to create a new AudioCassette of the type

AudioCassette, because LibraryAsset doesn’t support the overloaded

Only the Magazine used the default method found in the base

class. Book and AudioCassette used their own implementations of

the method. Also, at compile time, the decision was made which

method would be used when we calculate amountDue for the AudioCassette

class. The first call used the 1st method in AudioCassette

without parameters. The 2nd call used the 2nd method with a

27 Responses to 4 major principles of Object-Oriented Programming

Revising for my exam tomorrow and found this really helpful for the major OOD principles, Thanks.

I don’t quite understand how these four items are principles. What is the principle of Inheritance? Always use inheritence. What is the princple of Encapsulation? Wouldn’t a principle of OO provide guidance on how the concepts of OO are used.

For example, the principles: encapsulate variation, tell don’t ask, favor composition over inheritence.

I don’ t think mutators are a major principle of OO. In fact, from OO’s point of view they violate encapsulation. Mutators are a principle of component based programming and a compromise w/OO principles.

Great article. Loved the polymorphism section, brought a greater understanding of it for me. Thank you.

how i can used the different codes in java

how we can make the the full program in java

Raymond, for your piece of mind, look back at what Fransisco said and I believe you’ll see that he didn’t rip on your examples as being “bad”, he said you lacked showing us some Bad Examples to compare to good ones. That is a great learning tool (seeing why something is bad and how to correct it). So I wouldn’t take that as a bash on what you did do here.

im a bit confused..isnt Encapsulation the ability to hold similat information together (not information hiding) and abstraction is the ability to hide data which is achieved via encapsulation ?


It is really an excellent article that I have ever seen online. Basically the way explained takes to the bottom of understanding thats what made me understand how they can be implemented.

Could you help me with an online source paid or free or any DVD course that I could buy for a complete sample application (with out the front end I can take care of the front end) that takes a small datbase that has few tables in it and then creating the .NET objects using these 4 OOP prinicples based on the database we have in place.

I will look forward to hear from you.

Thank you in advance,

Excellent just enough for refreshing/ brushing up on OOP concepts. I really liked the examples especially on polymorphism

this artical is good. but the example ? . thanks for a good explanation

Raymond, u out did yourself on this one, leave the haters alone, why don’t they post something paramount to programmers like this. in the years i have been programming this is the best article on OO i have come across including detailed examples!

Many thanks for taking time to explane basics so simply, using a libruary.

I have struggled to write an accounts package in Access, now Im going further and want a good grasp of this OOP way of thinking.

I have nothing to say about the above 4 OOP concepts, Its really very nice. In my 4 years exp. in .net I have never learnt this way. I think this is very usefull for every programmer(s) life and may be the easyest way to understand OOP concepts wth ur examples.

Yudhi, I’ve never read that book. I call them principles. They call them basics. Either way, its important to understand what makes up the high level foundation of OOP.

Err.. I’m a bit confused about which is which. O’Reilly’s Head First Design Patterns said that the things you wrote were named OO Basics. Under the OO Principles there are entries “Encapsulate what varies”, “Favor composition over inheritance”, “Program to iterfaces, not implementation”. So, which is which?


nice site. but will be more helpfull if it goes on related deep and deep thro links if needed.

Good site. But we must distinguishwhen a thing exists potentially and when it does not; for it is not atany and every time.

Francisco, would you care to elaborate why I have provided bad design examples? If you’re going to criticize what I publish, you should at least explain why my examples are bad, otherwise your comment is meaningless and without merit.

With all my respect and bad english… this article lacks of really bad design examples.

Sahil, if anybody buy’s your book because they saw it on this post, you owe me money

I really like the inheritance section. Nice nice !!

Good article Ray it was good brush up the basics one more time,especially liked the polymorphism section.

I don’t know who maintains code better but I have a small suggestion: When u click the link post comment it takes the user to top of the page. I found a wonder full control by Steve Stucher

sstchur:SmartScroller. Its really good control for maintaining the page position for the user.

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