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Table of contents
1.
Introduction
2.
Basic OOPs Interview Questions
2.1.
1 What do you mean by OOPs?
2.2.
2 Name some of the programming languages which use the concept of OOPs?
2.3.
3 What is the need for Object-oriented programming?
2.4.
4 What are the essential features of OOPs?
2.4.1.
Encapsulation
2.4.2.
Polymorphism
2.4.3.
Inheritance 
2.4.4.
Data abstraction
2.5.
5 Discuss the advantages and disadvantages of OOPs?
2.6.
6 What are classes and objects in OOPs?
2.7.
7 What are the differences between OOPs and structural programming?
2.8.
8. What are the differences between class and object?
2.9.
9. What are the differences between class and structure?
2.10.
10. Do class and object acquire any memory?
2.11.
11. Explain the four pillars of OOP.
2.12.
12. Describe the use cases for inheritance.
2.13.
13. Differentiate between method overloading and overriding.
2.14.
14. Explain the benefits of encapsulation.
2.15.
15. Describe a scenario where polymorphism is useful.
3.
Advanced OOPs Interview Questions
3.1.
16. What is polymorphism in OOP and how is it implemented in programming languages?
3.2.
17. Explain the difference between abstraction and encapsulation with examples.
3.3.
18. What is the Liskov Substitution Principle in OOP?
3.4.
19. How does the Single Responsibility Principle (SRP) benefit software design?
3.5.
20. What is method overriding and when would you use it?
3.6.
21. Can you explain the concept of inheritance and its types?
3.7.
22. What is an abstract class and how is it different from an interface?
3.8.
23. Explain the concept of encapsulation with a real-world example.
3.9.
24. How do constructors work in OOP?
3.10.
25. What is multiple inheritance and why is it not supported in some languages like Java?
3.11.
26. What is a destructor and how is it different from a constructor?
3.12.
27. How does method overloading differ from method overriding?
3.13.
28. What is the Open/Closed Principle in OOP?
3.14.
29. Explain the concept of dynamic dispatch.
3.15.
30. What are design patterns and how do they relate to OOP?
4.
Frequently Asked Questions
4.1.
What are some of the programming paradigms?
4.2.
What are some of the languages that follow procedural programming paradigms?
4.3.
What are the different types of variables in OOPs?
4.4.
What are the 4 pillars of OOP interview questions?
4.5.
How to explain OOPs in an interview?
4.6.
What are the 7 concepts of OOPs?
5.
Conclusion
Last Updated: Jun 3, 2024
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OOPs Interview Questions

Introduction

Object-oriented programming (OOP) is a fundamental paradigm in modern software development, shaping the way developers design and structure their code. Mastering OOP concepts is crucial for anyone looking to excel in the field of programming. 

Object-oriented programming has always been a favourite topic to be asked in interviews, So we have compiled a list of some crucial questions that are asked on the subject of OOPs. After going through these questions, your preparation for placement will reach its zenith.

Let us begin!

OOPs Interview Questions

Basic OOPs Interview Questions

1 What do you mean by OOPs?

Ans - OOPs stand for object-oriented programming. As the name suggests, this is a pattern of programming that emphasizes more on objects and classes rather than functions. It is not a programming language but a paradigm used by almost all new generations of programming languages like Java, C++, etc.

2 Name some of the programming languages which use the concept of OOPs?

Ans - Nowadays, almost all programming languages use object-oriented concepts.

Some of them are mentioned below.

  1. C++
  2. Java
  3. Python
  4. C#
  5. Ruby

3 What is the need for Object-oriented programming?

Ans - As we evolve, our problems also become more complex and the code. To solve this problem of writing complex codes, we need object-oriented programming. It allows reusability of code which shortens the code to a greater extent. The primary importance of OOPs is to bind together the data and the functions that operate on them with the help of its essential features. 

4 What are the essential features of OOPs?

Ans - There are four main pillars of object-oriented programming

  1. Encapsulation
  2. Polymorphism
  3. Inheritance
  4. Data Abstraction

Encapsulation

Encapsulation name suggests, It is the wrapping up of data and methods (functions) that manipulates them in a single unit. It is one of the essential features of OOPs and is achieved with the help of access modifiers.

Polymorphism

Polymorphism in OOPs refers to the objects created that can take many forms in different instances. The Word polymorphism has been made from "poly," which means many, and "morphs," which means forms. It can also be understood as function overloading, as in it, any object or method gets to act differently on different instances.

Inheritance 

The capability of a class to derive properties and characteristics from another class is called Inheritance. It is an actual world entity that gets implemented in OOPs.

There are three different modes of Inheritance.

  1. Public mode
  2. Protected mode
  3. Private mode

These modes of Inheritance are used in OOPs according to the requirements.

Inheritance can be further divided into six types

  1. Single Inheritance
  2. Multiple Inheritance
  3. Multilevel Inheritance
  4. Hierarchical Inheritance
  5. Hybrid Inheritance
  6. Multipath Inheritance

Data abstraction

Data abstraction is the concept of object-oriented programming which shows only the necessary attributes of a code and hides all the unnecessary things. It helps in avoiding code duplication and thus increases reusability.

5 Discuss the advantages and disadvantages of OOPs?

Following are some of the advantages of OOPs

  1. Decreases the complexity of the code
  2. It allows code reusability rather than rebuilding the same thing again and again.
  3. It enhances the security of the code with the help of data abstraction
  4. We can easily extend the code without disturbing other functionalities.
  5. It helps in creating a flexible piece of code
  6. Easy handling and manipulation of code
  7. It follows a bottom-up approach

Following are the disadvantages of OOPs

  1. Requires more time to get executed
  2. Programmers need to be highly proficient in handling objects 
  3. It can get a little tricky sometimes
  4. Not suitable for all types of problems

6 What are classes and objects in OOPs?

Ans - A class in object-oriented programming is a user-defined data type that contains a type's variables and methods(functions). A class object can only access these data variables and member functions.

An object in object-oriented programming is nothing but an instance of a class used in accessing the data members and member functions of a class.

7 What are the differences between OOPs and structural programming?

Ans - 

Object-oriented programmingStructural programming
It is used to solve lengthily and complex problemsIt is used to solve moderate or easy level problems
Follows a bottom-top approachFollows a top-bottom approach
Offers reusability of codeReusability of code is not allowed
It is more secureIt is less secure
It uses the concept of inheritanceIt does not use the concept of inheritance
The code is more flexibleGenerates less flexible code
Uses data abstraction to hide unnecessary informationDoes not hide anything from the user

8. What are the differences between class and object?

Ans - 

                          Class                      Object
Class is a template on which objects are declared.An object is an instance of the class.
When a class is created, no memory is allocatedWhenever an object is created. Memory is allocated.
It is a logical entity.It is an actual entity.
It has to be declared only once.It can be declared multiple times as per the requirements.
A class can exist without an object.An object cannot exist without a class.

Must Read Characteristics of OOPS

9. What are the differences between class and structure?

Ans - 

                            Class                      Structure
We declare a class using the ‘class’ keyword.We declare a structure using the ‘struct’ keyword.
Members of the class are private by default.Members of the structure are public by default.
Inheritance is supported.Inheritance is not supported.
It is of reference type.It is of a value type.
It can deal with both variables and functions.It can only deal with variables.

10. Do class and object acquire any memory?

Ans - 

When we build a class, no memory is allocated to it as it is just a blueprint. But when an object of the class is created, memory is allocated accordingly. 

We saw some of the fundamental questions the Interviewer can ask in an interview. In the next blog, we will look for some advanced questions on OOPs. Let us now summarize what we learned in this article.

11. Explain the four pillars of OOP.

Ans - The four pillars of Object-Oriented Programming (OOP) are:

  1. Encapsulation: Bundling data (attributes) and methods (functions) that operate on that data within a single unit called a class. This promotes data protection and information hiding.
  2. Abstraction: Focusing on the essential details of an object while hiding its underlying implementation. This allows users to interact with objects without worrying about the complexities of their internal workings.
  3. Inheritance: Creating new classes (subclasses) that inherit properties and behaviors from existing classes (superclasses). This promotes code reusability and simplifies the creation of class hierarchies.
  4. Polymorphism: The ability of objects of different classes to respond to the same method call in different ways. This allows for flexible and dynamic behavior in programs.

These principles work together to create well-structured, maintainable, and reusable code. Encapsulation protects data integrity, abstraction simplifies object interaction, inheritance promotes code reuse, and polymorphism enables flexible method behavior.

12. Describe the use cases for inheritance.

Ans - Inheritance is beneficial when you have a set of existing functionalities (parent class) that can be reused as a foundation for creating new classes (child classes) with specialized features. Here are some common use cases:

  • Creating Class Hierarchies: Inheritance allows you to establish relationships between classes, where child classes inherit general attributes and methods from a parent class, and then add their own specific functionalities. For example, an Animal class can be a parent class for Dog and Cat child classes, inheriting common attributes like name and methods like eat(), while also having specific methods like bark() for dogs and purr() for cats.
  • Code Reusability: By inheriting common functionalities, you avoid duplicating code and streamline development.
  • Extending Functionality: Child classes can add new methods or override inherited methods to provide specialized behavior.

There are different types of inheritance to suit various needs:

  • Single Inheritance: A child class inherits from one parent class.
  • Multilevel Inheritance: A child class inherits from a parent class, which itself inherits from another parent class, creating a chain of inheritance.
  • Hierarchical Inheritance: Multiple child classes inherit from a single parent class.

13. Differentiate between method overloading and overriding.

Ans - Method Overloading: Occurs within the same class when you define multiple methods with the same name but different numbers or types of parameters. The compiler determines which overloaded method to call based on the arguments provided at runtime.

Method Overriding: Involves redefining a method inherited from a parent class within a child class. The child class method has the same name, return type, and sometimes parameter list (though covariant parameter types are allowed) as the parent class method. Overriding allows for customizing inherited behavior in child classes.

Here's an example:

class Shape {
  public void area() {
    System.out.println("Area calculation for generic shape");
  }
}

class Square extends Shape {
  @Override // Not mandatory, but good practice to indicate overriding
  public void area(int side) { // Overloaded method with parameter
    System.out.println("Area of square: " + side * side);
  }
}

 

In this example, the Shape class has a general area() method. The Square class inherits this method but overrides it with a new area(int side) method specific to calculating the area of a square.

14. Explain the benefits of encapsulation.

Ans - Encapsulation offers several advantages:

  • Data Protection: By encapsulating data (attributes) within a class and making them private, you control access to that data. Public methods within the class act as gatekeepers, ensuring data is modified only through authorized channels.
  • Information Hiding: Encapsulation hides the internal implementation details of a class, exposing only essential functionalities through public methods. This simplifies code interaction and reduces the risk of unintended modifications.
  • Modularity: Encapsulation promotes modularity by grouping related data and methods within a single unit (class). This makes code easier to understand, maintain, and reuse.
  • Improved Maintainability: Changes made within a class are less likely to impact other parts of the codebase if data access is controlled through proper encapsulation.

15. Describe a scenario where polymorphism is useful.

Ans - Polymorphism allows objects of different classes to respond to the same method call in different ways. Suppose we have a scenario i.e., animal sounds with polymorphism. Imagine you're building a program for a zoo website that displays information and plays sounds for different animals. Here's how polymorphism can be useful:

  • Base Class Animal: Create a base class Animal with attributes like name and type (e.g., mammal, bird). It can also have a generic makeSound() method that might print a message like "Animal sound".
  • Subclasses for Specific Animals: Create subclasses like Dog, Cat, and Bird that inherit from Animal. Each subclass can override the makeSound() method to provide its specific sound (e.g., Dog.makeSound() prints "Woof!", Cat.makeSound() prints "Meow!").
  • Polymorphic Array: Create an array of type Animal and add instances of Dog, Cat, and Bird to it. This allows you to treat all objects as animals initially.
  • Looping and Polymorphic Behavior: Loop through the animal array and call the makeSound() method on each object. Here's the magic of polymorphism: even though the variable holding each object is of type Animal, the actual makeSound() method called will be the overridden version specific to the object's subclass (Dog, Cat, or Bird), resulting in the appropriate animal sound being played.
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Advanced OOPs Interview Questions

16. What is polymorphism in OOP and how is it implemented in programming languages?

Ans - Polymorphism is the ability of objects to be treated as instances of their parent class rather than their actual class. It allows one interface to be used for a general class of actions. In programming languages like Java, polymorphism is implemented through method overloading (compile-time polymorphism) and method overriding (runtime polymorphism).

17. Explain the difference between abstraction and encapsulation with examples.

Ans - Abstraction is the concept of hiding the complex implementation details and showing only the essential features of an object. Encapsulation is the mechanism of wrapping the data (variables) and code (methods) together as a single unit. For example, a car’s dashboard is an abstraction that hides the engine’s complexity, while encapsulation can be seen in how the engine components are hidden inside the engine block.

18. What is the Liskov Substitution Principle in OOP?

Ans - The Liskov Substitution Principle states that objects of a superclass should be replaceable with objects of a subclass without affecting the functionality of the program. It ensures that a subclass can stand in for its superclass and function appropriately. This principle is crucial for maintaining the integrity of the OOP hierarchy.

19. How does the Single Responsibility Principle (SRP) benefit software design?

Ans - The Single Responsibility Principle states that a class should have only one reason to change, meaning it should have only one job or responsibility. SRP benefits software design by making systems more modular, easier to maintain, and less prone to bugs. Each class has a clear and focused purpose, which simplifies debugging and future enhancements.

20. What is method overriding and when would you use it?

Ans - Method overriding occurs when a subclass provides a specific implementation of a method that is already defined in its superclass. It is used when a subclass needs to tailor a method's behavior to fit its specific requirements. For example, a Bird class might have a fly method, but a subclass Penguin would override this method to indicate that penguins cannot fly.

21. Can you explain the concept of inheritance and its types?

Ans - Inheritance is a mechanism in OOP where one class (the subclass) inherits attributes and methods from another class (the superclass). There are different types of inheritance: single inheritance (one subclass inherits from one superclass), multiple inheritance (one subclass inherits from multiple superclasses, though not supported in some languages like Java), multilevel inheritance (a subclass inherits from another subclass), hierarchical inheritance (multiple subclasses inherit from one superclass), and hybrid inheritance (a combination of two or more types of inheritance).

22. What is an abstract class and how is it different from an interface?

Ans - An abstract class is a class that cannot be instantiated and is meant to be subclassed. It can have both abstract methods (without implementation) and concrete methods (with implementation). An interface is a contract that defines a set of methods without any implementation. In Java, a class can implement multiple interfaces but can inherit from only one abstract class.

23. Explain the concept of encapsulation with a real-world example.

Ans - Encapsulation is the bundling of data and methods that operate on the data within a single unit, typically a class. A real-world example of encapsulation is a capsule in medicine, which contains the drug (data) and provides a controlled way to take the medicine (methods). Similarly, a class in programming encapsulates data and provides methods to interact with that data.

24. How do constructors work in OOP?

Ans - Constructors are special methods used to initialize objects. They have the same name as the class and do not have a return type. Constructors are called when an object is created. There are two types: default constructors (no arguments) and parameterized constructors (with arguments). They set the initial state of an object.

25. What is multiple inheritance and why is it not supported in some languages like Java?

Ans - Multiple inheritance is when a class can inherit from more than one superclass. It is not supported in languages like Java because it can lead to the "diamond problem," where a class inherits conflicting implementations from multiple superclasses. Instead, Java uses interfaces to achieve similar functionality without the complexity.

26. What is a destructor and how is it different from a constructor?

Ans - A destructor is a special method called when an object is destroyed or goes out of scope, used to free resources the object may have acquired during its lifetime. Unlike constructors, destructors are not explicitly called in most programming languages; they are invoked by the garbage collector (e.g., in C++ and Python).

27. How does method overloading differ from method overriding?

Ans - Method overloading occurs when multiple methods in the same class have the same name but different parameters (number, type, or order). It is a compile-time polymorphism. Method overriding occurs when a subclass provides a specific implementation for a method already defined in its superclass. It is a runtime polymorphism.

28. What is the Open/Closed Principle in OOP?

Ans - The Open/Closed Principle states that software entities (classes, modules, functions) should be open for extension but closed for modification. This means you can add new functionality by extending existing components without altering their existing code, promoting flexibility and preventing bugs in the existing codebase.

29. Explain the concept of dynamic dispatch.

Ans - Dynamic dispatch is a mechanism by which a call to an overridden method is resolved at runtime rather than compile-time. It is used in polymorphism to ensure that the correct method implementation is called based on the actual object type, not the reference type.

30. What are design patterns and how do they relate to OOP?

Ans - Design patterns are proven solutions to common software design problems. They provide a template for how to solve a problem in various contexts. Design patterns relate to OOP as they leverage OOP principles like encapsulation, inheritance, and polymorphism to provide flexible and reusable design solutions.

Know What is Object in OOPs here in detail.

Frequently Asked Questions

What are some of the programming paradigms?

Following are some of the programming paradigms

  1. Procedural programming
  2. Imperative programming
  3. Object-oriented programming
  4. Functional programming

What are some of the languages that follow procedural programming paradigms?

The languages which use procedural programming paradigms are FORTRAN, C, ALGOL, COBOL, PASCAL, etc.

What are the different types of variables in OOPs?

There are generally three types of variables

  1. Instance variable
  2. Static variables
  3. Local variables

What are the 4 pillars of OOP interview questions?

The 4 pillars of OOP are Encapsulation, Abstraction, Inheritance, and Polymorphism. They promote code reusability, maintainability, and flexibility.

How to explain OOPs in an interview?

Briefly explain each OOP pillar (Encapsulation, Abstraction, Inheritance, Polymorphism) and provide real-world examples or code snippets to illustrate their benefits. Highlight how they work together to create well-structured code.

What are the 7 concepts of OOPs?

Traditionally, OOP is based on 4 pillars (Encapsulation, Abstraction, Inheritance, Polymorphism). Some may include additional concepts like classes, objects, and messages, but these can be considered foundational aspects related to the core pillars.

Conclusion

OOPs have always been a favorite topic for an interviewer to ask questions, and in this article, we discussed some common questions from this topic. At first, we answered the basic meaning of OOPs and saw some of the languages which use it. Then we discussed its importance in programming and also some of its features. 

Every coin has two sides so do OOPs, so we also saw its advantages and disadvantages, followed by learning the protagonist of OOPs, i.e., Classes and objects. We also saw some differences between OOPs and structural programming. And at last, we discussed the differences between class, object, and structure.

Recommended Readings: 

 

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