1. What is "Object-Oriented Programming" (OOP)?

A programming paradigm based on the concept of "objects," which can contain data (fields/attributes) and code (methods/procedures). It focuses on objects and entities rather than logic and functions.

2. What are the four main pillars of OOP?

Encapsulation
Abstraction
Inheritance
Polymorphism

3. What is a "Class"?

A blueprint or a template used to create objects. It defines the properties (data) and behaviors (methods) that all objects created from it will have.

4. What is an "Object"?

An instance of a class. It is a self-contained entity that has state (data) and behavior (functionality).

5. What is the difference between a "Class" and an "Object"?

Class: The design or plan (logical entity).
Object: The physical implementation of the plan (physical entity).

6. What is "Encapsulation"?

The practice of wrapping data (variables) and code (methods) together into a single unit (class). It provides "data hiding" by making members private and providing public accessors (getters/setters).

7. What is "Abstraction"?

The process of hiding complex implementation details and showing only the essential features of an object to the outside world.

8. What is the difference between "Encapsulation" and "Abstraction"?

Encapsulation: Focuses on HOW data is hidden and bundled.
Abstraction: Focuses on WHAT an object does, hiding the implementation.

9. What is "Inheritance"?

A mechanism where a new class (child/subclass) derives properties and behaviors from an existing class (parent/superclass). It promotes code reusability.

10. What are the different types of Inheritance?

Single Inheritance: One child from one parent.
Multiple Inheritance: One child from multiple parents (not supported in Java/C# classes).
Multilevel Inheritance: Child inherits from a class that is also a child.
Hierarchical Inheritance: Multiple children from one parent.
Hybrid Inheritance: Combination of two or more types.

11. What is "Polymorphism"?

The ability of an entity (like a method) to take on multiple forms depending on the context.

12. What is "Compile-time Polymorphism"?

Also called "Static Binding." It happens when the compiler determines which method to call at compile time (e.g., Method Overloading).

13. What is "Runtime Polymorphism"?

Also called "Dynamic Binding." It happens when the computer determines which method to call at runtime (e.g., Method Overriding).

14. What is the difference between "Overloading" and "Overriding"?

Overloading: Same method name with different parameters in the same class.
Overriding: Same method name and parameters in a child class as in a parent class.

15. What is a "Constructor"?

A special method used to initialize objects of a class. It's automatically called when an object is created and has no return type.

16. What is the difference between a "Default Constructor" and a "Parameterized Constructor"?

Default: Takes no arguments; initializes with default values.
Parameterized: Takes arguments; initializes with user-provided values.

17. What is a "Copy Constructor"?

A constructor used to create a new object by copying the data of an existing object.

18. What is a "Destructor"?

A special method used to destroy objects when they go out of scope or are explicitly deleted. (Used in C++, not in Java/Python).

19. What is an "Access Modifier"?

Keywords used to control the visibility of class members (e.g., public, private, protected).

20. What is an "Interface"?

A blueprint that defines a set of methods that a class must implement. It specifies WHAT a class should do, but not HOW.

21. What is an "Abstract Class"?

A class that cannot be instantiated and may contain abstract methods (methods without a body). It's intended to be a base class.

22. What is the difference between an "Interface" and an "Abstract Class"?

Interface: Cannot have method bodies; multiple inheritance support.
Abstract Class: Can have method bodies; single inheritance support.

23. What is "Composition"?

A relationship where one class contains an instance of another class (e.g., "Car has an Engine"). It's a "HAS-A" relationship.

24. What is "Aggregation"?

A weaker form of composition where the contained object can exist independently of the container (e.g., "Department has Students").

25. What is the difference between "Composition" and "Aggregation"?

Composition: Destruction of container kills the part (strong relationship).
Aggregation: Part survives destruction of container (weak relationship).

26. What is a "Static Method" and "Static Variable"?

Members that belong to the class rather than an individual object. They are shared across all instances.

27. What is "Method Hiding"?

Occurs when a child class defines a static method with the same signature as a static method in the parent class.

28. What is a "Virtual Function" (in C++)?

A function in a base class that can be overridden in a derived class, allowing for dynamic dispatch (polymorphism).

29. What is a "Pure Virtual Function"?

A virtual function with no implementation (e.g., = 0 in C++). It makes the class abstract.

30. What is a "Final Class" and "Final Method"?

Final Class: Cannot be inherited.
Final Method: Cannot be overridden.

31. What is "Association"?

A general relationship between two classes that defines how they interact (Composition and Aggregation are types of association).

32. What is "Coupling"?

The degree of interdependence between different classes. High-quality OOP aims for Low Coupling.

33. What is "Cohesion"?

The degree to which the elements of a class belong together. High-quality OOP aims for High Cohesion.

34. What is the "this" keyword?

A reference variable that refers to the current object instance within a class method.

35. What is the "super" keyword?

A reference variable used to refer to immediate parent class members (methods, fields, or constructors).

36. What is "Early Binding" vs "Late Binding"?

Early: Resolved at compile-time (overloading).
Late: Resolved at runtime (overriding).

37. What is "Data Hiding"?

The process of making internal data members inaccessible to the outside world using private modifiers.

38. What is "Delegation"?

A pattern where an object handles a request by handing it off to a second helper object.

39. What are "SOLID" Principles?

A set of five design principles that make software designs more understandable, flexible, and maintainable.

Single Responsibility
Open/Closed
Liskov Substitution
Interface Segregation
Dependency Inversion

40. What is "Garbage Collection"?

The process by which an OOP runtime automatically frees memory occupied by objects that are no longer referenced.

41. What is "Method Signature"?

The combination of a method's name and its parameter list (number, type, and order).

42. What is "Deep Copy" vs "Shallow Copy"?

Shallow: Copies member values; nested objects are shared.
Deep: Recursively copies members and nested objects; completely independent.

43. What is "Function Overloading" vs "Operator Overloading"?

Function: Multiple functions with the same name.
Operator: Redefining the behavior of operators (+, -, *) for objects.

44. What is "Message Passing"?

The process by which an object requests another object to perform a method.

45. What is "Friend Class" in C++?

A class that is granted access to the private and protected members of another class.

46. What is "Dynamic Dispatch"?

The process of selecting which implementation of a polymorphic operation (method) to call at runtime.

47. What is "Inline Function"?

A function whose code is expanded by the compiler at each call site, avoiding function call overhead.

48. What is "Virtual Table" (V-Table)?

An internal mechanism used by some OOP languages (like C++) to support dynamic dispatch.

49. What is "Duck Typing"?

A concept where the suitability of an object for a task is determined by the presence of certain methods and properties, rather than its class.

50. Why should we use Object-Oriented Programming?

It provides better modularity, code reusability, maintenance, and a clearer mapping between real-world entities and software code.

OOP Interview Questions

Table of Contents

What is "Object-Oriented Programming" (OOP)?

What are the four main pillars of OOP?

What is a "Class"?

What is an "Object"?

What is the difference between a "Class" and an "Object"?

What is "Encapsulation"?

What is "Abstraction"?

What is the difference between "Encapsulation" and "Abstraction"?

What is "Inheritance"?

What are the different types of Inheritance?

What is "Polymorphism"?

What is "Compile-time Polymorphism"?

What is "Runtime Polymorphism"?

What is the difference between "Overloading" and "Overriding"?

What is a "Constructor"?

What is the difference between a "Default Constructor" and a "Parameterized Constructor"?

What is a "Copy Constructor"?

What is a "Destructor"?

What is an "Access Modifier"?

What is an "Interface"?

What is an "Abstract Class"?

What is the difference between an "Interface" and an "Abstract Class"?

What is "Composition"?

What is "Aggregation"?

What is the difference between "Composition" and "Aggregation"?

What is a "Static Method" and "Static Variable"?

What is "Method Hiding"?

What is a "Virtual Function" (in C++)?

What is a "Pure Virtual Function"?

What is a "Final Class" and "Final Method"?

What is "Association"?

What is "Coupling"?

What is "Cohesion"?

What is the "this" keyword?

What is the "super" keyword?

What is "Early Binding" vs "Late Binding"?

What is "Data Hiding"?

What is "Delegation"?

What are "SOLID" Principles?

What is "Garbage Collection"?

What is "Method Signature"?

What is "Deep Copy" vs "Shallow Copy"?

What is "Function Overloading" vs "Operator Overloading"?

What is "Message Passing"?

What is "Friend Class" in C++?

What is "Dynamic Dispatch"?

What is "Inline Function"?

What is "Virtual Table" (V-Table)?

What is "Duck Typing"?

Why should we use Object-Oriented Programming?