Top 20 OOPs Interview Questions and Answers (2025)

Interview Question Rishabh saini October 22, 2024

Answer:
The programming paradigm known as “object-oriented programming” places more emphasis on objects than on processes. It enables programmers to organize programs using reusable objects that merge methods and data, increasing the flexibility and modularity of the code. Encapsulation, abstraction, inheritance, and polymorphism are the four pillars of object-oriented programming.

2. What are the key principles of OOP?

Answer:
The key principles of OOP include:

Encapsulation: Bundling data and methods that manipulate that data into a single unit (class).
Abstraction: keeping extraneous facts hidden and just providing the user with the information that is really necessary.
Inheritance: The capability of building a new class that takes methods and properties from an existing class.
Polymorphism: Inheritance, which allows classes to be regarded as instances of the same class.

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3. What is a Class in OOP?

Answer:
A class is an object creation blueprint or template. It specifies the actions (methods) and characteristics (properties) that the objects made from it will possess. For example, a “Car” class may have attributes like color, model, and speed, and behaviors like drive and stop.

4. What is an Object in OOP?

Answer:
An object is an instance of a class. It is a real-world entity with behavior (methods) and state (attributes). If the class is “Car,” for example, an object may be a particular vehicle, such as a red Toyota Corolla.

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5. Why is encapsulation necessary, and what does it mean?

Answer:
Encapsulation is the concept of bundling the data (variables) and methods (functions) that operate on the data into a single unit, typically a class. It restricts direct access to some of the object’s components, which is crucial for protecting the integrity of the data and maintaining security.

6. What distinguishes encapsulation from abstraction?

Answer:
While Encapsulation is the process of hiding the internal workings of an object and exposing only necessary components, Abstraction is about simplifying complex systems by focusing on essential characteristics and hiding unnecessary details. Encapsulation deals with how data is accessed, and abstraction deals with what information is provided to the user.

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7. Explain Inheritance in OOP.

Answer:
A new class (subclass) can inherit methods and attributes from an existing class (superclass) through inheritance. It promotes code reusability, as you don’t need to write the same code again and again. For example, if a “Vehicle” class has basic features like “move” and “stop,” a “Car” class can inherit those features without redefining them.

8. What is Polymorphism in OOP?

Answer:
The term polymorphism refers to “many forms.” OOP permits objects of several classes to be regarded as belonging to the same superclass. There are two types of polymorphism:

Compile-time Polymorphism (Method Overloading): Permits many methods in the same class to have the same name but distinct parameters.
Run-time Polymorphism (Method Overriding): Allows a subclass to provide a specific implementation of a method that is already defined in its superclass.

9. What distinguishes overriding from overloading?

Answer:

When many methods in the same class have the same name but distinct arguments, this is known as method overloading. It takes place in compile-time.
Method overriding allows for run-time polymorphism by having a method in a subclass that has the same name and signature as the method in its superclass.

10. What does the term “super” mean?

Answer:
In OOP, the constructors or methods of the parent class are referred to by the super keyword. Calling a function or constructor of the superclass from the subclass is a frequent inheritance technique.

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11. What is an Interface?

Answer:
A class’s blueprint that solely includes abstract methods—that is, methods without implementation—is called an interface. All of the methods specified in an interface must have implementations provided by the classes that implement it. It serves as a means of enforcing specific behaviors in a class.

12. What distinguishes an abstract class from an interface?

Answer:

Interface: There is no method implementation, and all methods are abstract. A class can implement multiple interfaces.
Abstract Class: Both concrete (with implementation) and abstract (without implementation) approaches are possible. Only one abstract class may be extended by a class.

13. What is Constructor Overloading?

Answer:
Constructor overloading occurs when a class has more than one constructor with different parameters. This enables various methods to initialize a class object based on the constructor that is used.

14. What function does `this` keyword serve in OOP?

Answer:
The current instance of the class is referenced using the this keyword. It is used to differentiate between instance variables and local variables, especially when they share the same name.

15. What is Multiple Inheritance, and is it supported in all OOP languages?

Answer:
When a class may inherit from more than one superclass, this is referred to as multiple inheritance. Some OOP languages, such as Java, do not natively enable multiple inheritance because of ambiguity problems, often known as the diamond problem. Instead, Java uses interfaces to support multiple inheritance.

16. What is a Final Class in Java?

Answer:
A final class in Java cannot be subclassed. In order to ensure that no other class may change an object’s behavior, it is usually used to build immutable objects and prohibit inheritance.

17. What is an Association in OOP?

Answer:
Association represents a relationship between two objects, where one object uses or interacts with another object. It can be categorized into aggregation and composition, based on how closely objects are related.

18. What is Aggregation in OOP?

Answer:
Aggregation is a special form of association where one object contains or owns another object. However, the contained object can exist independently of the container object. For example, a “Library” may contain “Books,” but if the library is destroyed, the books can still exist.

19. What is Composition in OOP?

Answer:
Composition is a stronger form of aggregation where the contained objects are entirely dependent on the container object. If the container is destroyed, the contained objects are destroyed as well. For instance, a “Car” is composed of parts like “Engine” and “Wheels,” and if the car is destroyed, the parts lose their meaning.

20. What is the significance of the SOLID principles in OOP?

Answer:
The SOLID principles are design guidelines that help developers create more flexible, maintainable, and scalable software systems:

S: Single Responsibility Principle (SRP)
O: Open/Closed Principle (OCP)
L: Liskov Substitution Principle (LSP)
I: Interface Segregation Principle (ISP)
D: Dependency Inversion Principle (DIP)

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Top 20 OOPs Interview Questions and Answers (2025 )
Top 20 OOPs Interview Questions and Answers (2025 )
Top 20 OOPs Interview Questions and Answers (2025 )
Top 20 OOPs Interview Questions and Answers (2025 )

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