OOPS Object Oriented Programming System Simple Notes For Interview

What is OOPs? 

∙ OOPs stands for Object-Oriented Programming. 

It is based on objects 

It follows Bottom-up programming approach. 

It is based on real world. 

It provides data hiding so it is very secure. 

It provides reusability feature.

OOPS (Object Oriented Programming System)
OOPS (Object Oriented Programming System)

What is a class? 

A class is a collection of objects. Classes don’t consume any space in the memory. 

It is a user defined data type that act as a template for creating objects  of the identical type. 

A large number of objects can be created using the same class. Therefore, Class is  considered as the blueprint for the object. 

What is an object? 

An object is a real world entity which have properties and functionalities.  Object is also called an instance of class. Objects take some space in memory. 

For eg . 

Fruit is class and its object s are mango ,apple , banana 

Furniture is class and its objects are table , chair , desk 

What is the difference between a class and an object? 

Class  Object
1. It is a collection of objects.  It is an instance of a class.
2. It doesn’t take up space in memory.  It takes space in memory.
3. Class does not exist physically  Object exist physically.
4. Classes are declared just once  Objects can be declared as and when required

What is the difference between a class and a structure? 

Class  Structure
1.Class is a collection of objects.  Structure is a collection of variables of  different data types under a single unit
2. Class is used to combine data and methods  together. Structure is used to grouping data.
3. Class’s objects are created on the heap  memory. Structure’s objects are created on the  stack memory.
4. A class can inherit another class.  A structure can’t inherit another  structure.
5. A class has all members private by default  A structure has all members public by  default
6. Classes are ideal for larger or complex  objects Structures are ideal for small and  isolated model objects

Following are the basic features of OOPs –

OOPS (Object Oriented Programming System)
OOPS (Object Oriented Programming System)


The main advantage of encapsulation is that data is hidden and protected from randomly access by outside non-member methods of a class. 

Encapsulation is the process of binding data and methods in a single unit. 

In encapsulation, data(variables) are declared as private and methods are declared as public.

OOPS(Object Oriented Programming System)

What are access specifiers ? 

It allows us to restrict the scope or visibility of a package, class, constructor, methods,  variables, or other data members. 

There are three types of most common access specifiers, which are following.  • Private  

  • Public  
  • Protected 

Public Modifiers : 

means that class, variable or method is accessible throughout from within or outside  the class, within or outside the package, etc.  

It provides highest level of accessibility

Private Modifiers : 

means that class, variable or method is not accessible from within or outside the  class, within or 

outside the package, etc.  

Private field or method can’t be inherited to sub class.  

This provides lowest level of accessibility. 

Protected Modifiers : 

means that class, variable or method is accessible from classes in the same package,  sub-classes in 

the same package, subclasses in other packages but not accessible from classes in  other packages.



Accessible by  classes in the  same package Accessible by  classes in  

other packages

Accessible by  subclasses in  the same  


Accessible by  

subclasses in  

other packages

Private  NO  No  No  No
Public  Yes  Yes  Yes  Yes
Protected  Yes  NO  Yes  Yes



Allows to hide unnecessary data from the user. This reduces program complexity  efforts. 

it displays only the necessary information to the user and hides all the internal  background details. 

If we talk about data abstraction in programming language, the code implementation  is hidden from the user and only the necessary functionality is shown or provided to  the user. 

In other words , it deals with the outside view of an object (Interface). 


-All are performing operations on the ATM machine like cash withdrawal etc.  but we can’t know internal details about ATM 

-phone call we don’t know the internal processing  

We can achieve data abstraction by using 

  1. Abstract class  
  2. Interface 

What is an abstract class? 

Abstract class is that class which contains abstract method.  

Abstract methods are those methods which have only declaration not the  implementation.  

An abstract class is declared with abstract keyword.  

An abstract class can also contain non-abstract methods. 


Inheritance is the procedure in which one class inherits the attributes and methods  of another class. 

In other words It is a mechanism of acquiring properties or behaviors of existing class to a new class 

The Base Class, also known as the Parent Class is a class, from which other classes are


The Derived Class, also known as Child Class, is a class that is created from an existing  class 

There are four types of inheritance in OOP:  

∙ Single Level Inheritance  

∙ Hierarchical Inheritance  

∙ Multi-Level Inheritance  

∙ Multiple Inheritance 

∙ Hybrid inheritance 

Single Level Inheritance  

When a class inherits properties and behaviour of only one class. In other words, in single inheritance there is only one base class and only one sub  class

Single Level Inheritance  
Single Level Inheritance

Hierarchical Inheritance 

 When more than class inherit properties and behaviour of only one class  In Hierarchical Inheritance there are only one parent and many child class 

Hierarchical Inheritance
Hierarchical Inheritance

Multi-Level Inheritance  

 In this type of inheritance, a derived class is created from another derived class

Multi-Level Inheritance
Multi-Level Inheritance

Multiple Inheritance 

When a class inherits the properties and the behaviour of more than one class Java, C#, most of high level language don’t support Multiple Inheritance

Multiple Inheritance 
Multiple Inheritance

Hybrid Inheritance 

Hybrid inheritance is a of inheritance is a combination of more than one type of inheritance.

Hybrid Inheritance 
Hybrid Inheritance

Why Java or C# don’t support multiple inheritance? 

because of following reasons – 

Ambiguity Around The Diamond Problem Multiple inheritance does complicate the  design and creates problem during casting, constructor chaining etc.


Polymorphism is the ability of an object to take on many forms. 

we can define polymorphism as the ability of a message to be displayed in  more than one form.  


A real-life example of polymorphism, a man at the same 

time is a father, a husband, an employee. 

Another good real time example of polymorphism is water. Water is a liquid at  normal temperature, but it can be changed to solid when it frozen, or same water  changes to a gas when it is heated at its boiling point .Thus, same water exhibiting  different roles is polymorphism. 


polymorphism is mainly divided into two types:  

  • Compile time Polymorphism (CTP) 

It is also called static polymorphism or early binding. 

  • Runtime Polymorphism (RTP) 

It is also called dynamic polymorphism or late binding. 


Types of Polymorphism:  

  1. Compile time polymorphism:  

This type of polymorphism is achieved by function overloading or operator overloading.  

Function overloading:  

When there are multiple functions with same name but different parameters then these  functions are said to be overloaded. Functions can be overloaded by change in number of  arguments or/and change in type of arguments 

multiple methods of same names performs different tasks within the same class. 

2.Runtime polymorphism:  

Runtime polymorphism refers to the process when a call to an overridden process is  resolved at the run time. 

This type of polymorphism is achieved by Function Overriding.  

Function Overriding:

on the other hand, occurs when a derived class has a definition for one of the member  functions of the base class.  

methods having same name which can have different functionalities.  That base function is said to be overridden.

Overriding & Overloading
Overriding & Overloading

Difference  between Abstract Classes & Interfaces

Features  Abstract Class  Interface
Multiple Inheritance  A class can inherit only one abstract  class A class can inherit multiple  interfaces
Default Implementation  Provide signature ,partial and full  implementation of its methods  ,variables and other members Provide only the signature of  its methods ,variables  

,properties and other member

Access Modifier  It allows to assign access modifier  to its members No access modifier can be  assigned .All the members are  treated as public
Core VS Peripheral  It defines the core identity of the  class and there is used for objects  of same type It identify the peripheral  identity of the class .it means  human and vehicle can inherit  from IMovable interface
Homogeneity  If various implementation of same  nature which requires shared code  that represent same status or  behaviour , then use Abstract class If various implementation of  different nature and requires  the member with same  

signature ,then use interface

Performance  It is faster to access the  

implemented class member

It takes time to find the  

members of the  

corresponding class

Extensibility (Versioning)  If any changes made to the abstract  class ,not necessarily 

We need to change all the  

implementation classes

If any changes made to the  interfaces , changes should be  made in all the implemented  classes
Field and Constants  Fields and constants can be defined  No fields and constants can be  defined 

What is static function? 

Static functions are those functions that can be called without creating an object of the  class. That means, Static methods do not use any instance variables of any object of the  class they are defined in. 

Static methods can not be overridden. They are stored in heap space of the memory. 

What are virtual functions? 

Virtual function is a function or method used to override the behavior of the function in an  inherited class with the same signature to achieve the polymorphism.  

Virtual function defined in the base class and overridden in the inherited class.  

The Virtual function cannot be private, as the private functions cannot be overridden. It is  used to achieve runtime polymorphism. 

What are pure virtual functions? 

A pure virtual function is that function which have no definition. That means a virtual  function that doesn’t need implementation is called pure virtual function. 

A pure virtual function have not definitions but we must override that function in the  derived class, otherwise the derived class will also become abstract class.

What is Constructor? 

Constructor is a special type of member function which is used to initialize an object.  

It is similar as functions but it’s name should be same as its class name and must have no  explicit return type. 

It is called when an object of the class is created. At the time of calling constructor,  memory for the object is allocated in the memory. 

We use constructor to assign values to the class variables at the time of object creation.

What are the types of Constructor? 

Constructor have following types – 

  • Default constructor  
  • Parameterized constructor  
  • Copy constructor  
  • Static constructor  
  • Private constructor 

What is default constructor? 

A constructor with 0 parameters is known as default constructor. 

What is private constructor? 

if a constructor is declared private, we cannot create an object of the class. What is copy constructor? 

A copy constructor is that constructor which use existing object to create a new object.  It copy variables from another object of the same class to create a new object. What is static constructor? 

A static constructor is automatically called when the first instance is generated,  or any static member is referenced.  

The static constructor is explicitly declared by using a static keyword What is destructor? 

Destructor is a type of member function which is used to destroy an object. 

It is called automatically when the object goes out of scope or is explicitly destroyed by a  call to delete.  

It destroy the objects when they are no longer in use.  

A destructor has the same name as the class, preceded by a tilde (~).

Shallow Copy and Deep Copy 

Shallow Copy and Deep Copy play important role in copying the objects in Prototype Design Pattern. Shallow copy 

In the case of Shallow copy, it will create the new object from the existing object and then copying the  value type fields of the current object to the new object.  

But in the case of reference type, it will only copy the reference, not the referred object itself.  

Therefore the original and clone refer to the same object in the case of reference type. In order to  understand this better, please have a look at the following diagram. 

Shallow Copy
Shallow Copy
Shallow Copy
Shallow Copy
Memory Representation
Memory Representation

As shown in the above diagram, first we create an object i.e. emp1, and then initialize the object with  some values. Then we create the second object i.e. emp2 using the GetClone method. As shown in  the memory representation, the value type fields (Name and Department) are copied and stored in a  different memory location while the reference type field i.e. EmpAddress is still pointing to the same  old memory location. That means now, both the object i.e. emp1 and emp2 is now referring to the  same Address object. So, if we do any changes to the employee address then it will affect each other. 

Deep Copy 

In the case of deep copy, it will create the new object from the existing object and then copying the  fields of the current object to the newly created object. If the field is a value type, then a bit-by-bit copy  of the field will be performed. If the field is a reference type, then a new copy of the referred object is  created.

Deep Copy
Deep Copy
Deep Copy & Memory Representation
Deep Copy & Memory Representation

As shown in the above image, the Name and Department properties are value types so it creates a  copy of that and stores it in a different location. The EmpAddress is a Reference type property and in  Deep Copy there is a clone of the reference type field which also will be stored in a different location.  So, the point that you need to keep in mind is, in the case of Deep Copy the field type does not matter  whether it is a value type or reference type. It always makes a copy of the whole data and stores it in a  different memory location. 

In C++ we can pass arguments into a function in different ways. These different ways are  

Call by Value 

Call by Reference 

Call by Address 

Sometimes the call by address is referred to as call by reference, but they are different in C++. In call by  address, we use pointer variables to send the exact memory address, but in call by reference we pass the  reference variable (alias of that variable). This feature is not present in C, there we have to pass the pointer  to get that effect. In this section we will see what are the advantages of call by reference over call by value,  and where to use them

Call by Value 

In call by value, the actual value that is passed as argument is not changed after performing  some operation on it. When call by value is used, it creates a copy of that variable into the  stack section in memory. When the value is changed, it changes the value of that copy, the  actual value remains the same. 

Example Code 


using namespace std

void my_function(int x) { 

x = 50

cout << “Value of x from my_function: ” << x << endl

main() { 

int x = 10


cout << “Value of x from main function: ” << x




Value of x from my_function: 50 

Value of x from main function: 10

Call by Reference 

In call by reference the actual value that is passed as argument is changed after performing  some operation on it. When call by reference is used, it creates a copy of the reference of  that variable into the stack section in memory. Is uses a reference to get the value. So  when the value is changed using the reference it changes the value of the actual variable. 


using namespace std

void my_function(int &x) { 

x = 50

cout << “Value of x from my_function: ” << x << endl

main() { 

int x = 10


cout << “Value of x from main function: ” << x




Value of x from my_function: 50 

Value of x from main function: 50 

Where to use Call by reference? 

The call by reference is mainly used when we want to change the value of the passed  argument into the invoker function. 

One function can return only one value. When we need more than one value from a  function, we can pass them as an output argument in this manner.

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