Object-Oriented Programming (OOP) Interview Questions

Embark on a journey into the world of Object-Oriented Programming (OOP) with 'OOP Interview Questions.' This blog is your essential guide for preparing for interviews related to OOP, featuring a comprehensive collection of questions and detailed answers. Whether you're a software developer, a programmer, or an OOP enthusiast, our resource covers OOP principles, inheritance, polymorphism, and best practices. Prepare with confidence and explore the foundational paradigm of designing modular and maintainable software using OOP.

1. What is Object-Oriented Programming?

Object-Oriented Programming (OOP) is a programming paradigm that uses objects – instances of classes – to organize code. It promotes the concept of encapsulation, inheritance, and polymorphism.

2. Explain Encapsulation.

Encapsulation is the bundling of data (attributes) and methods (functions) that operate on the data into a single unit known as a class. It restricts access to some of the object's components and prevents the accidental modification of data.

3. What is meant by the term OOPs?

OOPs refers to Object-Oriented Programming. It is the programming paradigm that is defined using objects. Objects can be considered as real-world instances of entities like class, that have some characteristics and behaviors.

4. What is the need for OOPs?

There are many reasons why OOPs is mostly preferred, but the most important among them are: 

  • OOPs helps users to understand the software easily, although they don’t know the actual implementation.
  • With OOPs, the readability, understandability, and maintainability of the code increase multifold.
  • Even very big software can be easily written and managed easily using OOPs.

5. What is meant by Structured Programming?

Structured Programming refers to the method of programming which consists of a completely structured control flow. Here structure refers to a block, which contains a set of rules, and has a definitive control flow, such as (if/then/else), (while and for), block structures, and subroutines.

Nearly all programming paradigms include Structured programming, including the OOPs model.

6. What are some advantages of using OOPs?

  • OOPs is very helpful in solving very complex level of problems.
  • Highly complex programs can be created, handled, and maintained easily using object-oriented programming.
  • OOPs, promote code reuse, thereby reducing redundancy.
  • OOPs also helps to hide the unnecessary details with the help of Data Abstraction.
  • OOPs, are based on a bottom-up approach, unlike the Structural programming paradigm, which uses a top-down approach.
  • Polymorphism offers a lot of flexibility in OOPs.

8. What is a class?

A class can be understood as a template or a blueprint, which contains some values, known as member data or member, and some set of rules, known as behaviors or functions. So when an object is created, it automatically takes the data and functions that are defined in the class.
Therefore the class is basically a template or blueprint for objects. Also one can create as many objects as they want based on a class.

For example, first, a car’s template is created. Then multiple units of car are created based on that template.

9. What is an object?

An object refers to the instance of the class, which contains the instance of the members and behaviors defined in the class template. In the real world, an object is an actual entity to which a user interacts, whereas class is just the blueprint for that object. So the objects consume space and have some characteristic behavior.
For example, a specific car.

10. How does C++ support Polymorphism?

C++ is an Object-oriented programming language and it supports Polymorphism as well:

  • Compile Time Polymorphism: C++ supports compile-time polymorphism with the help of features like templates, function overloading, and default arguments.
  • Runtime Polymorphism: C++ supports Runtime polymorphism with the help of features like virtual functions. Virtual functions take the shape of the functions based on the type of object in reference and are resolved at runtime.

11. What is meant by Inheritance?

The term “inheritance” means “receiving some quality or behavior from a parent to an offspring.” In object-oriented programming, inheritance is the mechanism by which an object or class (referred to as a child) is created using the definition of another object or class (referred to as a parent). Inheritance not only helps to keep the implementation simpler but also helps to facilitate code reuse.

12. What is Abstraction?

If you are a user, and you have a problem statement, you don't want to know how the components of the software work, or how it's made. You only want to know how the software solves your problem. Abstraction is the method of hiding unnecessary details from the necessary ones. It is one of the main features of OOPs. 
For example, consider a car. You only need to know how to run a car, and not how the wires are connected inside it. This is obtained using Abstraction.

13. How much memory does a class occupy?

Classes do not consume any memory. They are just a blueprint based on which objects are created. Now when objects are created, they actually initialize the class members and methods and therefore consume memory.

14. Is it always necessary to create objects from class?

No. An object is necessary to be created if the base class has non-static methods. But if the class has static methods, then objects don’t need to be created. You can call the class method directly in this case, using the class name.

15. What are the various types of constructors in C++?

The most common classification of constructors includes:

Default constructor: The default constructor is the constructor which doesn’t take any argument. It has no parameters.

class ABC{   int x;         ABC()   {       x = 0;   }}

Parameterized constructor: The constructors that take some arguments are known as parameterized constructors.

 class ABC{   int x;         ABC(int y)   {       x = y;   }}

Copy constructor: A copy constructor is a member function that initializes an object using another object of the same class.

class ABC{   int x;         ABC(int y)   {       x = y;   }   // Copy constructor   ABC(ABC abc)   {       x = abc.x;   }}

16. What is a copy constructor?

Copy Constructor is a type of constructor, whose purpose is to copy an object to another. What it means is that a copy constructor will clone an object and its values, into another object, is provided that both the objects are of the same class.

17. What is a destructor?

Contrary to constructors, which initialize objects and specify space for them, Destructors are also special methods. But destructors free up the resources and memory occupied by an object. Destructors are automatically called when an object is being destroyed. 

18. Are class and structure the same? If not, what's the difference between a class and a structure?

No, class and structure are not the same. Though they appear to be similar, they have differences that make them apart. For example, the structure is saved in the stack memory, whereas the class is saved in the heap memory. Also, Data Abstraction cannot be achieved with the help of structure, but with class, Abstraction is majorly used.

19. Are there any limitations of Inheritance?

Yes, with more powers comes more complications. Inheritance is a very powerful feature in OOPs, but it has some limitations too. Inheritance needs more time to process, as it needs to navigate through multiple classes for its implementation. Also, the classes involved in Inheritance - the base class and the child class, are very tightly coupled together. So if one needs to make some changes, they might need to do nested changes in both classes. Inheritance might be complex for implementation, as well. So if not correctly implemented, this might lead to unexpected errors or incorrect outputs.

20. What is a subclass?

The subclass is a part of Inheritance. The subclass is an entity, which inherits from another class. It is also known as the child class.

21. What is an interface?

An interface refers to a special type of class, which contains methods, but not their definition. Only the declaration of methods is allowed inside an interface. To use an interface, you cannot create objects. Instead, you need to implement that interface and define the methods for their implementation. 

22. What is meant by static polymorphism?

Static Polymorphism is commonly known as the Compile time polymorphism. Static polymorphism is the feature by which an object is linked with the respective function or operator based on the values during the compile time. Static or Compile time Polymorphism can be achieved through Method overloading or operator overloading.

23. What is meant by dynamic polymorphism?

Dynamic Polymorphism or Runtime polymorphism refers to the type of Polymorphism in OOPs, by which the actual implementation of the function is decided during the runtime or execution. The dynamic or runtime polymorphism can be achieved with the help of method overriding.

24. What is the difference between overloading and overriding?

Overloading is a compile-time polymorphism feature in which an entity has multiple implementations with the same name. For example, Method overloading and Operator overloading.

Whereas Overriding is a runtime polymorphism feature in which an entity has the same name, but its implementation changes during execution. For example, Method overriding.

25. How is data abstraction accomplished?

Data abstraction is accomplished with the help of abstract methods or abstract classes.

26. What is an abstract class?

An abstract class is a special class containing abstract methods. The significance of abstract class is that the abstract methods inside it are not implemented and only declared. So as a result, when a subclass inherits the abstract class and needs to use its abstract methods, they need to define and implement them.

27. How is an abstract class different from an interface?

Interface and abstract classes both are special types of classes that contain only the methods declaration and not their implementation. But the interface is entirely different from an abstract class. The main difference between the two is that when an interface is implemented, the subclass must define all its methods and provide its implementation. Whereas in object-oriented programming, when a subclass inherits from an abstract class with abstract methods, the subclass is generally required to provide concrete implementations for all of those abstract methods in the abstract class unless the subclass itself is declared as abstract.

Also, an abstract class can contain abstract methods as well as non-abstract methods.

28. What are access specifiers and what is their significance?

Access specifiers, as the name suggests, are a special type of keywords, which are used to control or specify the accessibility of entities like classes, methods, etc. Some of the access specifiers or access modifiers include “private”, “public”, etc. These access specifiers also play a very vital role in achieving Encapsulation - one of the major features of OOPs.

29. What is an exception?

An exception can be considered as a special event, which is raised during the execution of a program at runtime, that brings the execution to a halt. The reason for the exception is mainly due to a position in the program, where the user wants to do something for which the program is not specified, like undesirable input.

30. What is meant by exception handling?

No one wants its software to fail or crash. Exceptions are the major reason for software failure. The exceptions can be handled in the program beforehand and prevent the execution from stopping. This is known as exception handling.
So exception handling is the mechanism for identifying the undesirable states that the program can reach and specifying the desirable outcomes of such states.
Try-catch is the most common method used for handling exceptions in the program.

31. What is meant by Garbage Collection in OOPs world?

Object-oriented programming revolves around entities like objects. Each object consumes memory and there can be multiple objects of a class. So if these objects and their memories are not handled properly, then it might lead to certain memory-related errors and the system might fail.

Garbage collection refers to this mechanism of handling the memory in the program. Through garbage collection, the unwanted memory is freed up by removing the objects that are no longer needed.

32. Can we run a Java application without implementing the OOPs concept?

No. Java applications are based on Object-oriented programming models or OOPs concept, and hence they cannot be implemented without it.

However, on the other hand, C++ can be implemented without OOPs, as it also supports the C-like structural programming model.

33. What is the output of the below code?

#include<iostream>  using namespace std; class BaseClass1 { public:     BaseClass1()     { cout << " BaseClass1 constructor called" << endl;  } };  class BaseClass2 { public:     BaseClass2()     { cout << "BaseClass2 constructor called" << endl;  } };  class DerivedClass: public BaseClass1, public BaseClass2 {   public:    DerivedClass()     {  cout << "DerivedClass constructor called" << endl;  } };  int main() {   DerivedClass derived_class;   return 0; }


BaseClass1 constructor calledBaseClass2 constructor calledDerivedClass constructor called

The above program demonstrates Multiple inheritances. So when the Derived class’s constructor is called, it automatically calls the Base class's constructors from left to right order of inheritance.

34. What will be the output of the below code?

class Scaler{   static int i;   static   {       System.out.println(“a”);       i = 100;   }}public class StaticBlock{   static   {       System.out.println(“b”);   }   public static void main(String[] args)   {       System.out.println(“c”);       System.out.println(Scaler.i);   }}



Firstly the static block inside the main-method calling class will be implemented. Hence ‘b’ will be printed first. Then the main method is called, and now the sequence is kept as expected.

35. Predict the output?

#include<iostream> using namespace std; class ClassA {  public:    ClassA(int ii = 0) : i(ii) {}    void show() { cout << "i = " << i << endl;} private:    int i; }; class ClassB { public:    ClassB(int xx) : x(xx) {}    operator ClassA() const { return ClassA(x); } private:    int x; }; void g(ClassA a) {; } int main() {  ClassB b(10);  g(b);  g(20);  getchar();  return 0; }  


i = 10i = 20

ClassA contains a conversion constructor. Due to this, the objects of ClassA can have integer values. So the statement g(20) works. Also, ClassB has a conversion operator overloaded. So the statement g(b) also works.

36. What will be the output in below code?

public class Demo{    public static void main(String[] arr){          System.out.println(“Main1”);   }    public static void main(String arr){           System.out.println(“Main2”);   } }



Here the main() method is overloaded. But JVM only understands the main method which has a String[] argument in its definition. Hence Main1 is printed and the overloaded main method is ignored. 

37. Code Snippet: Creating a Class in Java

// Java Class Example
public class Car {
    // Attributes
    String model;
    int year;

    // Constructor
    public Car(String model, int year) {
        this.model = model;
        this.year = year;

    // Method
    public void displayInfo() {
        System.out.println("Model: " + model + ", Year: " + year);

38. Online Resources:

Published On: 2024-01-31