Must Read 40 OOPs Interview Questions & Answers For Freshers & Experienced

Beginner OOPs Interview Questions

This section covers essential questions that gauge a candidate's understanding of basic OOP concepts for interview with confidence, including terminology and core principles. Each question targets foundational knowledge to help assess readiness for more advanced topics.

Basic OOPs Concepts Interview Questions for Freshers

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

• Include in Answer:
  • Define OOP simply
  • Mention concepts like objects and classes
  • Give a relatable example

Answer:
"Object-Oriented Programming, or OOP, is a way of organizing code using objects. An object represents real-world things, and classes act like blueprints to create these objects. For example, if we have a ‘Car’ class, it might define attributes like color and speed. Specific cars (like your own car) are objects created from that blueprint. OOP makes coding easier to manage by organizing it into parts that work together."

2. Explain the four main principles of OOP.

• Include in Answer:
  • Briefly define each principle: Encapsulation, Inheritance, Polymorphism, Abstraction
  • Describe how each principle helps

Answer:
"The four main principles of OOP are:

• Encapsulation: Bundles data and methods together to keep data secure.
• Inheritance: Allows new classes to use properties from existing ones, making code reusable.
• Polymorphism: Lets the same action work differently in various contexts, making code flexible.
• Abstraction: Simplifies complex code by hiding unnecessary details, making code easier to work with."

3. How does OOP differ from procedural programming?

• Include in Answer:
  • State the main difference in approach
  • Mention OOP focuses on objects, procedural on functions
  • Example if possible

Answer:
"OOP organizes code around objects, while procedural programming organizes code around a series of steps or functions. In OOP, each object represents a real-world concept, like a ‘Car’ or a ‘Person,’ and we interact with them. In procedural programming, we solve tasks by creating a sequence of functions. OOP works well for complex systems, while procedural is simpler and can work for smaller tasks."

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Encapsulation and Data Hiding Interview Questions for Freshers

4. Define encapsulation in OOP.

• Include in Answer:
  • Definition of encapsulation
  • Its role in data security
  • Short example

Answer:
"Encapsulation is the practice of bundling data and methods in a class to keep data secure. This means that data inside a class is hidden and only accessible through specific methods. For example, in a BankAccount class, the balance would be private, accessible only through methods like deposit or withdraw to protect the data from external changes."

5. How does encapsulation help in achieving data security?

• Include in Answer:
  • Explanation of data security role
  • Example of restricting access

Answer:
"Encapsulation keeps data safe by controlling access to it. In OOP, we often make data private and provide methods (getters and setters) to access or modify it. For instance, a Person class might have a private age attribute, but we can access it only through a get_age() method. This way, we control how data is accessed and avoid unauthorized changes."

6. Provide an example of encapsulation in a simple code snippet.

• Include in Answer:
  • Code snippet showing private data
  • Example with a getter and setter method

Answer:
“Here’s an example of encapsulation in Python:

python

Copy code

class BankAccount:
    def __init__(self, balance):
        self.__balance = balance  # private attribute

    def deposit(self, amount):
        self.__balance += amount

    def get_balance(self):
        return self.__balance

In this example, __balance is private and only accessible through methods like get_balance(), ensuring the balance can’t be directly modified from outside.”

Inheritance and Code Reusability Interview Questions for Freshers

7. What is inheritance in OOP?

• Include in Answer:
  • Definition of inheritance
  • Benefits of code reuse
  • Example with parent and child class

Answer:
"Inheritance is when a class, known as the child class, inherits properties and behaviors from another class, the parent class. This allows us to reuse code without rewriting it. For example, if we have a Vehicle class, we can create a Car class that inherits its properties, saving time and avoiding duplication."

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8. Describe the types of inheritance with examples.

• Include in Answer:
  • List types: single, multiple, multilevel, hierarchical, hybrid
  • Short example for each

Answer:
"Here are the types of inheritance in OOP:

• Single Inheritance: A class inherits from one parent, e.g., Car inherits from Vehicle.
• Multiple Inheritance: A class inherits from multiple parents, like FlyingCar inherits from both Vehicle and Aircraft.
• Multilevel Inheritance: A class inherits from a class that already inherits, like ElectricCar inherits from Car, which inherits from Vehicle.
• Hierarchical Inheritance: Multiple classes inherit from one parent, e.g., Car and Bike inherit from Vehicle.
• Hybrid Inheritance: A mix of two or more types of inheritance.”

9. What are the benefits and drawbacks of using inheritance?

• Include in Answer:
  • Key benefits: code reuse, organization
  • Drawbacks: complexity, tight coupling

Answer:
"Inheritance offers benefits like reducing code duplication and organizing code, which makes it easier to read and maintain. However, it can also increase complexity and create tight coupling between classes, meaning a change in the parent class can affect all child classes. This can make maintenance harder for large projects."

Polymorphism and Method Overloading Questions for Freshers

10. Define polymorphism in OOP.

• Include in Answer:
  • Simple definition of polymorphism
  • Mention types of polymorphism: compile-time and runtime
  • Example of how polymorphism works

Answer:
"Polymorphism in OOP means ‘many forms.’ It allows a single action to perform differently based on the object or context. There are two types: compile-time polymorphism (like method overloading) and runtime polymorphism (like method overriding). For instance, if we have a draw() method, a Circle and Rectangle class can each have their own draw() method, making the action of ‘drawing’ unique for each shape."

11. Explain method overloading with an example.

• Include in Answer:
  • Definition of method overloading
  • Mention how it allows methods with the same name but different parameters
  • Example code if possible

Answer:
"Method overloading is when a class has multiple methods with the same name but different parameters. This allows us to perform similar actions with different types or numbers of inputs. For example:

python

Copy code

class Math:
    def add(self, a, b):
        return a + b

    def add(self, a, b, c):
        return a + b + c

# Usage
math = Math()
# Depending on the number of arguments, the appropriate add() method is called.
print(math.add(5, 10))       # Output: 15
print(math.add(5, 10, 15))   # Output: 30

In this case, add() is overloaded to work with both two and three numbers."

Read: Java Interview Questions & Answers

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

• Include in Answer:
  • Define both concepts
  • Key differences between overloading and overriding
  • Brief example for clarity

Answer:
"Method overloading happens within the same class and allows methods with the same name to perform different actions based on different parameters. Method overriding occurs in inheritance, where a subclass redefines a method from its parent class to provide a different implementation.

For example, overloading would be having two calculate() methods with different parameters in the same class, while overriding would be redefining the calculate() method in a subclass to change its behavior."

Classes and Objects Interview Questions for Freshers

13. What is a class, and how is it different from an object?

• Include in Answer:
  • Define class and object
  • Explain the difference: class as blueprint, object as instance
  • Simple example, if possible

Answer:
"A class is a blueprint that defines attributes and behaviors, while an object is an instance created from that class. For example, a Car class might define attributes like color and make, while an actual car you own is an object with specific values for those attributes, like ‘red’ and ‘Toyota.’"

14. Explain the relationship between a class and an instance.

• Include in Answer:
  • Define class as a template
  • Explain how instances are created from the class
  • Relatable example

Answer:
"A class is a template or blueprint for creating objects, while an instance is a specific object created from that class. For example, the class Dog might define general attributes like breed and color, and each dog (like your pet) is an instance of that class with its own specific characteristics."

15. How do you define an object in OOP?

• Include in Answer:
  • Definition of object as an instance of a class
  • Describe how it holds attributes and methods
  • Simple example if possible

Answer:
"An object in OOP is an instance of a class that has specific attributes and methods. For example, if we have a class Person with attributes like name and age, an object created from this class represents a particular person, like john = Person('John', 25). Here, john is an object with its own name and age."

Abstraction and its Importance in OOP

16. Define abstraction and its role in OOP.

• Include in Answer:
  • Definition of abstraction
  • How it hides unnecessary details
  • Example of why it’s useful

Answer:
"Abstraction in OOP is the concept of hiding complex details and showing only the essentials to the user. For example, when you use a car, you don’t need to know the engine’s inner workings; you just need to know how to drive it. Similarly, abstraction in programming hides the complexity and provides a simple interface."

17. How does abstraction help in managing complexity?

• Include in Answer:
  • Explanation of how abstraction simplifies code
  • Example of a real-world analogy

Answer:
"Abstraction helps manage complexity by hiding details and only exposing the necessary parts of code. For example, using a sendEmail() function abstracts away the underlying details of connecting to the email server. This way, users interact with a simple function without needing to understand the process behind it."

18. Give an example of abstraction in software design.

• Include in Answer:
  • Example of abstraction in real software
  • Explain how it simplifies usage

Answer:
"A good example of abstraction is a payment processing API in an e-commerce application. The API allows developers to process payments by calling a single function, processPayment(), without needing to know the detailed steps like connecting to the bank, handling responses, or managing errors. This simplifies payment handling and makes it easier to work with."

Constructors and Destructors Interview Questions for Freshers

19. What is a constructor in OOP?

• Include in Answer:
  • Definition of a constructor
  • Explain how it initializes an object
  • Example of a constructor in a class

Answer:
"A constructor is a special method in OOP that automatically runs when an object is created. It’s used to initialize the object’s properties or set initial values. For example, in Python, we use __init__() as a constructor to set up an object when it’s created:

python

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

john = Person("John", 25)

In this example, the constructor __init__ sets name to ‘John’ and age to 25 as soon as the object john is created."

20. How do destructors work in OOP, and why are they useful?

• Include in Answer:
  • Define a destructor
  • Explain how it releases resources or cleans up after an object
  • Brief example of a destructor in a class

Answer:
"A destructor is a special method that automatically runs when an object is no longer in use or goes out of scope. It’s used to release resources, such as closing files or freeing up memory, ensuring efficient memory management. In Python, the destructor method is __del__():

python

class FileHandler:
    def __init__(self, filename):
        self.file = open(filename, 'w')

    def __del__(self):
        self.file.close()
        print("File closed.")

handler = FileHandler("example.txt")

In this example, when handler goes out of scope, __del__() is called, and the file is closed. Destructors are useful for cleaning up and preventing memory leaks."

Experienced OOPs Interview Questions

These questions assess an advanced understanding of Object-Oriented Programming (OOP), challenging candidates to demonstrate technical depth and practical applications in complex scenarios. Below, we include in-depth answers covering both technical details and real-world use cases.

Advanced OOPs Concepts and Principles Interview Questions

1. What is the difference between association, aggregation, and composition?

• Include in Answer:
  • Define each term
  • Explain the strength of the relationship
  • Give examples to clarify each relationship type

Answer:
"Association, aggregation, and composition describe relationships between classes:

• Association is a loose relationship where objects know each other but don’t depend on each other for existence. For example, a Teacher and a Student have an association because a teacher teaches students, but they don’t rely on each other for existence.
• Aggregation is a ‘has-a’ relationship where a whole-part relationship exists but parts can exist independently. For example, a Classroom has Students, but students can exist without the classroom.
• Composition is a strong ‘contains-a’ relationship where parts depend on the whole and cannot exist separately. For example, a House contains Rooms; if the house is destroyed, the rooms also cease to exist."

Check out: Full stack developer interview questions

2. How does polymorphism support code extensibility?

• Include in Answer:
  • Define polymorphism in terms of extensibility
  • Explain how it allows for flexible implementations
  • Brief example demonstrating extensibility

Answer:
"Polymorphism supports code extensibility by allowing methods to behave differently based on the object calling them. This lets us add new classes that use existing methods without changing the original code. For example, a draw() method in a Shape class can be extended to handle new shapes like Circle or Square. When we add a new shape, we simply define its draw() method without modifying other classes, keeping the code flexible and extensible."

3. What is the Liskov Substitution Principle?

• Include in Answer:
  • Define the Liskov Substitution Principle (LSP)
  • Explain why it’s important in OOP
  • Simple example showing its use

Answer:
"The Liskov Substitution Principle (LSP) states that objects of a superclass should be replaceable with objects of a subclass without affecting the program's correctness. This principle ensures that derived classes extend the functionality of their base classes without changing expected behaviors. For instance, if Bird is a class and Penguin is a subclass, any code using Bird should work with Penguin as well, even though a penguin can’t fly. The LSP encourages designing classes in a way that preserves the original behavior."

Design Patterns and Best Practices in OOP

4. Describe the Singleton design pattern and its use cases.

• Include in Answer:
  • Define the Singleton pattern
  • Mention common use cases
  • Describe its benefits and potential issues

Answer:
"The Singleton pattern ensures only one instance of a class is created and provides global access to that instance. It’s often used for logging, database connections, and configuration settings, where a single shared resource is sufficient. The Singleton pattern simplifies resource management but can cause issues in multi-threaded applications if not handled properly. Lazy initialization and double-checked locking are often used to manage Singleton instances safely."

5. How would you implement a Factory pattern in a real-world scenario?

• Include in Answer:
  • Define the Factory pattern
  • Describe its use in object creation
  • Provide a relatable real-world example

Answer:
"The Factory pattern provides a way to create objects without specifying the exact class. In a real-world scenario, a VehicleFactory class could create different vehicle types like Car, Bike, or Truck based on input. This pattern makes it easy to extend the system by adding new vehicle types without modifying existing code, which promotes scalability and flexibility."

6. Explain the Observer pattern and when to use it.

• Include in Answer:
  • Define the Observer pattern
  • Explain when it’s useful
  • Give an example scenario

Answer:
"The Observer pattern allows an object (the ‘subject’) to notify other objects (the ‘observers’) when it changes. This is helpful in event-driven systems, like a stock price tracker where updates are sent to subscribers whenever prices change. The pattern decouples the subject and observers, allowing flexibility in adding or removing observers without affecting the subject."

Exception Handling and Memory Management in OOP

7. Describe how exception handling is managed in OOP.

• Include in Answer:
  • Briefly explain exception handling
  • Mention common methods (try-catch, finally)
  • Benefits in OOP for error management

Answer:
"Exception handling in OOP manages runtime errors without crashing the program. It uses structures like try-catch blocks to catch exceptions and handle them gracefully. In Java, for example, try-catch-finally ensures resources are released even if an error occurs. Exception handling makes programs more robust by allowing developers to control how errors are handled and what actions should follow."

8. Explain the role of garbage collection in OOP languages.

• Include in Answer:
  • Define garbage collection
  • Describe how it aids memory management
  • Mention example languages (Java, Python)

Answer:
"Garbage collection is an automated process in OOP languages like Java and Python that frees up memory used by objects that are no longer needed. This helps prevent memory leaks and reduces the need for manual memory management. The garbage collector automatically deallocates memory, making it easier for developers to manage resources in large programs without worrying about memory cleanup."

9. How do memory leaks occur, and how can they be prevented?

• Include in Answer:
  • Define memory leak
  • Explain common causes
  • Discuss preventive measures

Answer:
"A memory leak occurs when an application doesn’t release memory that’s no longer needed, causing available memory to decrease over time. Common causes include holding onto references to unused objects or forgetting to close resources like file streams. To prevent memory leaks, it’s essential to nullify references to objects that are no longer needed and close any open resources promptly. Using languages with garbage collection (like Java) can help manage memory automatically, but developers should still be mindful of resource handling."

Real-World Application and Case Study-Based Questions

10. How would you apply OOP concepts to design a library management system?

• Include in Answer:
  • Mention key classes (e.g., Book, Member, Library)
  • Describe how OOP concepts like inheritance, encapsulation, and polymorphism could be applied
  • Briefly explain how objects interact in this system

Answer:
"To design a library management system, ideal way would be to use OOP concepts to create classes such as Book, Member, Library, and Transaction. Each class would have specific attributes: Book might include title and author, while Member could have name and ID. Then using inheritance, create subclasses for different member types, such as StudentMember and FacultyMember, each with unique borrowing limits. Encapsulation would keep data secure; for example, a Book class’s availability status would only be modifiable through checkout or return methods. Finally, polymorphism could allow the borrow() method to work differently depending on the type of member, making the system more flexible and maintainable."

11. Explain how you’d use OOP to create an e-commerce platform.

• Include in Answer:
  • Outline essential classes (e.g., Product, Customer, Order)
  • Explain OOP principles applied (e.g., encapsulation, inheritance, polymorphism)
  • Describe how these elements work together in the system

Answer:
"In an e-commerce platform, key classes might include Product, Customer, Order, and ShoppingCart. Each class would handle its specific responsibilities: Product stores details like price and stock, while Order manages order processing. Using inheritance, we could have specialized product types like Electronics or Clothing, each with unique attributes. Encapsulation would keep data safe, so product details can only be accessed through specific methods. Polymorphism could enable payment processing by allowing different payment methods, like credit cards or PayPal, to implement a processPayment() method differently. This OOP structure keeps the system organized, flexible, and secure."

12. Describe a project where you applied inheritance to reduce code redundancy.

• Include in Answer:
  • Brief description of the project
  • How inheritance helped reduce redundancy
  • Example of base and derived classes used

Answer:
"In a project to build a content management system, we needed classes for different user roles: Admin, Editor, and Viewer. Each role shared some permissions, so next step was to create a base User class with shared properties, like username and login methods. Then, each role inherited from User and had its own unique capabilities—Admin could manage users, while Editor could only edit content. Using inheritance, repeating code for common user attributes and methods was avoided, which made the codebase more efficient and easier to maintain."

Multi-threading and Synchronization in OOP

13. Explain thread safety in OOP and its importance.

• Include in Answer:
  • Define thread safety
  • Explain why it’s important in concurrent applications
  • Briefly mention techniques to ensure thread safety

Answer:
"Thread safety ensures that multiple threads can access shared resources without causing conflicts or unexpected behaviors. In multi-threaded applications, thread safety is crucial to prevent data corruption and race conditions. Techniques to ensure thread safety include using locks, synchronized methods, and atomic variables, which help coordinate access to shared data, making sure only one thread modifies it at a time."

14. How does synchronization work in multi-threaded OOP applications?

• Include in Answer:
  • Define synchronization and its purpose
  • Explain how it prevents data conflicts in multi-threaded programs
  • Briefly mention ways to implement synchronization (e.g., locks, synchronized methods)

Answer:
"Synchronization in OOP ensures that only one thread can access a shared resource at a time, preventing conflicts and maintaining data consistency. In languages like Java, we can use the synchronized keyword to lock critical sections or methods, allowing only one thread to execute at a time. This avoids race conditions and keeps shared resources consistent across threads, especially in applications with multiple users or processes."

15. Describe a situation where you needed to manage concurrent objects.

• Include in Answer:
  • Brief description of the concurrent scenario
  • How you handled synchronization or thread safety
  • Mention any specific tools or techniques used

Answer:
"In a real-time chat application, multiple users could send messages simultaneously, requiring thread-safe message processing. Synchronized blocks were used to manage concurrent access to shared resources like the message queue, ensuring that each message was processed in the correct order. Additionally, locks and atomic variables were implemented to prevent any data conflicts and keep the system stable during high traffic."

Abstraction Layers and Interface Design in OOP

16. What is an interface, and how is it used in OOP?

• Include in Answer:
  • Definition of an interface
  • How it promotes flexibility and modularity
  • Example of when to use an interface

Answer:
"An interface in OOP is a blueprint that defines methods without implementations. It allows different classes to have a common structure while implementing their own logic. For example, a PaymentMethod interface might require a processPayment() method, which different classes (like CreditCard and PayPal) implement in their unique ways. This approach ensures flexibility and modularity."

17. Explain the concept of abstract classes and how they differ from interfaces.

• Include in Answer:
  • Definition of abstract class and interface
  • Key differences between them
  • Brief example of when to use each

Answer:
"An abstract class is a class that cannot be instantiated directly and may include both defined methods and abstract methods (those without implementation). Unlike an interface, which only defines methods that classes must implement, an abstract class can provide partial implementation. This makes abstract classes suitable when several classes share some common functionality but need additional custom implementations. 

For example, an abstract Vehicle class might have a startEngine() method that’s defined, while subclasses like Car and Bike implement their own methods specific to each vehicle type. Use an abstract class when you need some shared functionality, and an interface when you only need to define common behaviors."

18. How does abstraction improve modularity in large-scale applications?

• Include in Answer:
  • Explain how abstraction simplifies complex code
  • Mention the role of abstraction in separating functionality
  • Example of modularity benefits in large projects

Answer:
"Abstraction improves modularity by hiding complex implementation details and exposing only necessary functions. In large applications, this approach allows developers to focus on specific modules without worrying about the entire system’s internals. For instance, a Database class in a large application might expose methods like connect() and query() without detailing how connections or queries work. This way, different teams can work on specific parts of the system, making it easier to maintain and update individual components without impacting the whole."

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Advanced Polymorphism and Dynamic Binding

19. What is dynamic binding, and how does it work in OOP?

• Include in Answer:
  • Define dynamic binding
  • Explain how it enables flexibility at runtime
  • Example showing dynamic binding in action

Answer:
"Dynamic binding, or late binding, occurs when the method to be executed is determined at runtime rather than compile-time. This enables flexibility in OOP by allowing subclasses to provide their own implementations of a method defined in a parent class. For instance, if there’s a Shape class with a draw() method, a Circle or Rectangle object’s draw() method is chosen based on the object type at runtime. Dynamic binding allows polymorphic behavior, where the correct method is called according to the object’s class, enhancing flexibility."

20. How can polymorphism be used to create flexible and reusable code?

• Include in Answer:
  • Define polymorphism in terms of flexibility
  • Describe how it supports reusable code
  • Simple example demonstrating flexible and reusable code

Answer:
"Polymorphism allows methods to operate differently depending on the object calling them, making code more flexible and reusable. By using a common method name, polymorphism lets developers extend functionality without modifying the core code. For example, a print() method in a Document class can be implemented by PDFDocument and WordDocument to print in their specific formats. When a new document type is added, it simply implements print() without altering the existing structure, allowing for easy extension and code reuse."

Popular OOP Languages and Their Key Features

Here’s a quick overview of some popular OOP languages and what makes each stand out:

1. Java

• Platform-independent; runs on any system with JVM.
• Strong multithreading support.
• Automatic memory management.

2. Python

• Simple syntax, great for beginners.
• Dynamic typing for flexibility.
• Rich libraries for data science, AI, and web development.

3. C++

• High-performance, ideal for system-level applications.
• Direct memory control.
• Hybrid language; supports both OOP and procedural programming.

4. C#

• Part of the .NET ecosystem, suited for Windows apps.
• Strong type safety and automatic memory management.
• Built-in LINQ for querying data.

5. Ruby

• Dynamic, flexible syntax, ideal for web development.
• Includes garbage collection.
• Known for the powerful Ruby on Rails framework.

6. Swift

• Designed for iOS with a clean, modern syntax.
• Memory management with Automatic Reference Counting (ARC).
• Works well with older Objective-C code.

7. Kotlin

• Fully interoperable with Java.
• Null safety to prevent errors.
• Concise syntax, popular for Android development.

Top OOP Design Patterns Every Developer Should Know

Here’s a quick guide to some key OOP design patterns. These patterns help make code easier to manage and adapt by organizing how objects are created and interact. 

upGrad’s 3-Step Journey to OOP Success with Software Development Courses 

Step 1: Build Strong Foundations
Master OOP basics—encapsulation, inheritance, polymorphism, and abstraction—with interactive lessons, mini-projects, and real-world examples.

Step 2: Create Hands-On Projects
Apply OOP concepts in real projects like e-commerce apps, building a portfolio that proves your skills to employers.

Step 3: Get Interview-Ready
Prepare with mock interviews and receive feedback from industry experts, plus career coaching for resumes and job applications.

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