Top 20 OOPs Interview Questions and Answers

Introduction

Object-Oriented Programming (OOP) is a widely used programming paradigm that focuses on the concept of objects, which can contain data and code. OOP allows for the creation of modular, reusable, and maintainable code. If you’re preparing for an OOPs interview, it’s essential to have a good understanding of the fundamental concepts and be able to answer common interview questions. In this blog post, we’ll explore the top 20 OOPs interview questions and provide detailed answers to help you prepare.

1. What is OOP?

OOP is a programming paradigm that organizes code into objects, which are instances of classes. It emphasizes the concepts of encapsulation, inheritance, and polymorphism.

2. What is a class?

A class is a blueprint or template for creating objects. It defines the properties and behaviors that objects of that class can have.

3. What is encapsulation?

Encapsulation is the process of hiding the internal details of an object and providing access to its properties and methods through well-defined interfaces. It helps in achieving data abstraction and data hiding.

4. What is inheritance?

Inheritance is a mechanism that allows a class to inherit properties and behaviors from another class. It promotes code reuse and helps in creating a hierarchical relationship between classes.

5. What is polymorphism?

Polymorphism is the ability of an object to take on many forms. It allows objects of different classes to be treated as objects of a common superclass, enabling code to be written that can work with objects of multiple types.

6. What is method overriding?

Method overriding is a feature of OOP that allows a subclass to provide a different implementation of a method that is already defined in its superclass. It is used to achieve runtime polymorphism.

7. What is method overloading?

Method overloading is a feature of OOP that allows a class to have multiple methods with the same name but different parameters. The compiler determines which method to call based on the number and types of arguments passed.

8. What is an abstract class?

An abstract class is a class that cannot be instantiated and is meant to be subclassed. It can have abstract methods, which are declared but not implemented in the abstract class. Subclasses must provide an implementation for these abstract methods.

9. What is an interface?

An interface is a collection of abstract methods. It defines a contract that implementing classes must adhere to. It allows for multiple inheritance in Java and provides a way to achieve abstraction and loose coupling.

10. What is the difference between abstract classes and interfaces?

Abstract classes can have both abstract and non-abstract methods, while interfaces can only have abstract methods. A class can implement multiple interfaces, but it can only inherit from one abstract class.

11. What is the difference between composition and inheritance?

Composition is a design technique where a class contains an instance of another class as one of its member variables. Inheritance is a mechanism where a class inherits properties and behaviors from another class. Composition promotes code reuse through object composition, while inheritance promotes code reuse through class inheritance.

12. What is the difference between method overloading and method overriding?

Method overloading is having multiple methods with the same name but different parameters in the same class, while method overriding is providing a different implementation of a method in a subclass. Method overloading is determined at compile-time, while method overriding is determined at runtime.

13. What is a constructor?

A constructor is a special method that is called when an object of a class is created. It is used to initialize the object’s state and can have parameters.

14. What is the difference between a constructor and a method?

A constructor is called when an object is created and is used to initialize the object’s state. It does not have a return type and has the same name as the class. A method, on the other hand, is called on an object and performs some action. It has a return type and can have different names.

15. What is the difference between shallow copy and deep copy?

Shallow copy creates a new object that shares the same memory as the original object, while deep copy creates a new object with its own memory and copies the values of the original object’s fields.

16. What is the final keyword?

The final keyword can be applied to classes, methods, and variables. A final class cannot be subclassed, a final method cannot be overridden, and a final variable cannot be reassigned once it is initialized.

17. What is the static keyword?

The static keyword is used to create variables and methods that belong to the class itself, rather than to instances of the class. Static variables are shared among all instances of the class, while static methods can be called without creating an object of the class.

18. What is the difference between an instance variable and a class variable?

An instance variable is a variable that is unique to each instance of a class, while a class variable is shared among all instances of a class. Instance variables are declared inside a class but outside any method, while class variables are declared with the static keyword.

19. What is the difference between method hiding and method overriding?

Method hiding occurs when a subclass declares a static method with the same name as a static method in its superclass. The subclass’s static method does not override the superclass’s static method but merely hides it. Method overriding occurs when a subclass provides a different implementation of a method that is already defined in its superclass.

20. What is the SOLID principle in OOP?

The SOLID principle is a set of five design principles that aim to make software systems more maintainable, flexible, and scalable. The principles are Single Responsibility, Open-Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion.

Conclusion

These top 20 OOPs interview questions cover a broad range of topics and concepts related to Object-Oriented Programming. By familiarizing yourself with these questions and their answers, you’ll be well-prepared to showcase your knowledge and skills during an OOPs interview. Remember to practice explaining these concepts in your own words and provide examples to demonstrate your understanding. Good luck!