Question 1

What are the four pillars of Object-Oriented Programming?

Accepted Answer

The four pillars of OOP are Encapsulation, Abstraction, Inheritance, and Polymorphism. Together they form the foundation of object-oriented design.

Encapsulation is the bundling of data (attributes) and methods (functions) that operate on that data into a single unit (class), while restricting direct access to some of the object's components. Access modifiers (private, protected, public) control visibility. This protects internal state from unintended interference and provides a clean interface

Question 2

What is the difference between abstract class and interface?

Accepted Answer

Abstract classes and interfaces are both used to achieve abstraction, but they serve different purposes and have different capabilities.

Abstract Class: A class that cannot be instantiated and may contain both abstract methods (no implementation) and concrete methods (with implementation). Can have instance variables with any access modifier. Can have constructors. A class can extend only ONE abstract class (single inheritance). Used when classes share common behavior and state. Represents an i

Question 3

Explain SOLID principles in OOP.

Accepted Answer

SOLID is an acronym for five design principles that make software designs more understandable, flexible, and maintainable.

S - Single Responsibility Principle (SRP): A class should have only one reason to change, meaning it should have only one job or responsibility. A User class should handle user data, not email sending. Violation: A class that handles both database access AND business logic AND presentation formatting.

O - Open/Closed Principle (OCP): Software entities should be open for ex

Question 4

What is polymorphism? Explain compile-time vs runtime polymorphism.

Accepted Answer

Polymorphism (Greek: many forms) is the ability of an object to take on multiple forms. It allows a single interface to represent different underlying types, enabling code to work with objects of different classes through a common interface.

Compile-Time Polymorphism (Static Binding/Early Binding): Resolved at compile time by the compiler. Achieved through method overloading and operator overloading.

Method Overloading: Same method name with different parameter lists (different number, type, o

Question 5

What are design patterns? Explain Singleton, Factory, and Observer patterns.

Accepted Answer

Design patterns are proven solutions to commonly occurring problems in software design. They are templates that can be adapted to solve particular design problems in various contexts.

Singleton Pattern (Creational): Ensures a class has only ONE instance and provides a global point of access to it. Implementation: private constructor, private static instance, public static getInstance() method. Use cases: Database connection pool, logger, configuration manager. Thread safety: Use double-checked

Question 6

What is the difference between composition and inheritance? When to use which?

Accepted Answer

Inheritance (is-a relationship): A class derives from another class, inheriting its properties and methods. The child class IS A type of parent class. Example: Dog is-a Animal, Car is-a Vehicle.

Composition (has-a relationship): A class contains instances of other classes as its members. The containing class HAS the functionality provided by the contained objects. Example: Car has-a Engine, House has-a Room.

Problems with Inheritance:
1. Tight coupling: Changes in parent class can break all su

Question 7

What is encapsulation? How is it achieved and why is it important?

Accepted Answer

Encapsulation is the OOP principle of bundling data (attributes/fields) and the methods that operate on that data into a single unit (class), while restricting direct access to the internal state. It is the mechanism of hiding internal implementation details and exposing only a controlled public interface.

How it is achieved:
1. Access Modifiers: private (accessible only within the class), protected (accessible within class and subclasses), public (accessible everywhere), package-private/intern

Question 8

What is method overriding vs method overloading?

Accepted Answer

Method Overloading (Compile-time Polymorphism): Multiple methods in the SAME class with the SAME name but DIFFERENT parameter lists. The compiler determines which method to call based on the arguments passed at compile time.

Rules for overloading:
- Parameters must differ in number, type, or order.
- Return type alone is not sufficient to distinguish overloaded methods.
- Access modifiers can vary.
- Can throw different exceptions.
- Can be in same class or inherited class.

Example: int add(in

Question 9

What is dependency injection? Why is it important?

Accepted Answer

Dependency Injection (DI) is a design technique where an object receives its dependencies from external sources rather than creating them internally. Instead of a class instantiating its own dependencies using new keyword, they are provided (injected) from outside, typically through constructors, setters, or interfaces.

Types of Dependency Injection:
1. Constructor Injection: Dependencies passed through the constructor. Most common and preferred method. Makes dependencies explicit and ensures o

Question 10

What is the diamond problem in OOP? How is it solved?

Accepted Answer

The diamond problem occurs in languages that support multiple inheritance. When a class inherits from two classes that both inherit from a common base class, it creates a diamond-shaped inheritance hierarchy. The ambiguity arises: which version of the grandparent methods and data does the grandchild class inherit?

Example: Class A has method display(). Class B extends A and overrides display(). Class C extends A and overrides display(). Class D extends both B and C. When D calls display(), whic

Question 11

What is the difference between deep copy and shallow copy?

Accepted Answer

Shallow Copy: Creates a new object but copies only the references to nested objects, not the nested objects themselves. The new object and the original share the same nested objects. Modifying a nested object through either reference affects both copies.

Deep Copy: Creates a new object and recursively copies ALL nested objects, creating completely independent copies at every level. The new object and original share nothing; modifications to one do not affect the other.

Example in Python:
origi

Question 12

What are access modifiers in OOP? Explain their scope.

Accepted Answer

Access modifiers are keywords that define the visibility and accessibility of classes, methods, and variables. They implement encapsulation by controlling who can access what.

Public: Accessible from anywhere in the program. Any class in any package can access public members. Used for the interface that a class exposes to the outside world. Classes, methods, fields, and constructors can be public.

Private: Accessible only within the declaring class. Not visible to subclasses or any other class

Object-Oriented Programming Interview Questions

What are the four pillars of Object-Oriented Programming?

What is the difference between abstract class and interface?

Explain SOLID principles in OOP.

What is polymorphism? Explain compile-time vs runtime polymorphism.

What are design patterns? Explain Singleton, Factory, and Observer patterns.

What is the difference between composition and inheritance? When to use which?

What is encapsulation? How is it achieved and why is it important?

What is method overriding vs method overloading?

What is dependency injection? Why is it important?

What is the diamond problem in OOP? How is it solved?

What is the difference between deep copy and shallow copy?

What are access modifiers in OOP? Explain their scope.

Explore More Interview Questions

Object-Oriented Programming Interview Questions

What are the four pillars of Object-Oriented Programming?

What is the difference between abstract class and interface?

Explain SOLID principles in OOP.

What is polymorphism? Explain compile-time vs runtime polymorphism.

What are design patterns? Explain Singleton, Factory, and Observer patterns.

What is the difference between composition and inheritance? When to use which?

What is encapsulation? How is it achieved and why is it important?

What is method overriding vs method overloading?

What is dependency injection? Why is it important?

What is the diamond problem in OOP? How is it solved?

What is the difference between deep copy and shallow copy?

What are access modifiers in OOP? Explain their scope.

Explore More Interview Questions