Top 30 Most Common Object Oriented Programming Interview Questions You Should Prepare For
Written by
James Miller, Career Coach
Embarking on a software development career often involves demonstrating proficiency in fundamental programming paradigms. Among these, object-oriented programming (OOP) stands out as a cornerstone concept, widely adopted across languages like Java, C++, Python, C#, and many others. Its principles—encapsulation, inheritance, polymorphism, and abstraction—provide powerful tools for building complex, maintainable, and scalable software systems. Understanding these concepts is not just about theoretical knowledge; it's about applying them effectively to solve real-world problems. During technical interviews, expect a significant portion of questions to probe your understanding of OOP principles, their practical applications, and how they differ from other paradigms. Interviewers use these questions to gauge your foundational knowledge, problem-solving approach, and ability to think in an object-oriented manner, which is essential for collaborating on modern software projects. Preparing thoroughly for these common questions will significantly boost your confidence and performance.
What Are object oriented programming interview questions?
object oriented programming interview questions are designed to assess a candidate's understanding of the core principles, concepts, and terminology associated with the OOP paradigm. These questions can range from basic definitions of terms like 'class', 'object', and 'inheritance' to more complex scenarios involving design patterns, the differences between various OOP concepts (like abstraction vs. encapsulation), and how OOP features are implemented in specific programming languages. They often explore your ability to apply OOP principles to design solutions, understand code structure, and discuss the benefits and drawbacks of using an object-oriented approach. The goal is to ensure you have a solid theoretical foundation and practical grasp of how to leverage OOP effectively in software development contexts.
Why Do Interviewers Ask object oriented programming interview questions?
Interviewers ask object oriented programming interview questions for several key reasons. Firstly, they want to confirm your foundational knowledge; OOP is a fundamental concept in many software roles. Secondly, these questions reveal your thought process. Can you explain complex ideas clearly? Can you relate theoretical concepts to practical coding scenarios? Thirdly, understanding OOP is crucial for working with existing codebases and designing new systems in an object-oriented way, promoting code reusability, maintainability, and scalability. Your answers can indicate your familiarity with common design patterns and principles, which are often built upon OOP concepts. Ultimately, a strong understanding of OOP demonstrates that you can write well-structured, organized, and efficient code, which is highly valued in the software industry.
What does OOPs stand for?
What are the four basic concepts of OOPs?
How would you describe polymorphism?
What is abstraction in programming?
What is a class?
What makes OOPs different from procedural programming?
What is inheritance?
Difference between a class and an object?
Difference between method overriding and overloading?
What are the two subcategories of inheritance?
What is encapsulation?
What is a structure?
Difference between a class and a structure?
When would you use an operator keyword?
What is cohesion?
What is coupling?
Difference between cohesion and coupling?
What is an interface?
Why program to an interface?
Example of a virtual function?
What is a friend function?
Difference between a friend function and a virtual function?
What is a destructor?
What is a ternary operator?
What is another name for the ternary operator?
What is an abstract class?
Difference between an abstract class and an interface?
What is multiple inheritance?
What is the Open-Closed Design Principle?
What is the Single Responsibility Principle?
Preview List
1. What does OOPs stand for?
Why you might get asked this:
This is a basic terminology check to ensure you know the acronym. It's often the first question to ease into more detailed topics.
How to answer:
Simply state the full form of the acronym. Keep it concise and direct.
Example answer:
OOPs stands for Object-Oriented Programming System. It's a programming paradigm centered around the concept of objects and classes.
2. What are the four basic concepts of OOPs?
Why you might get asked this:
This question tests your knowledge of the fundamental pillars of OOP. It's crucial to know these four.
How to answer:
List the four concepts: abstraction, encapsulation, inheritance, and polymorphism. Briefly define each one.
Example answer:
The four basic concepts are Abstraction (hiding details), Encapsulation (bundling data/methods), Inheritance (inheriting properties), and Polymorphism (taking multiple forms).
3. How would you describe polymorphism?
Why you might get asked this:
Polymorphism is a core principle. They want to see if you understand its meaning and how it's achieved.
How to answer:
Explain that polymorphism means "many forms." Mention method overloading and overriding as key ways it's implemented.
Example answer:
Polymorphism means the ability for objects to take on multiple forms. This is commonly seen through method overloading (same name, different parameters) and method overriding (subclass providing a specific implementation of a superclass method).
4. What is abstraction in programming?
Why you might get asked this:
Abstraction is about managing complexity. Interviewers want to know if you understand how to hide details and simplify design.
How to answer:
Define abstraction as hiding implementation details and showing only essential features. Explain its purpose in reducing complexity.
Example answer:
Abstraction is the concept of hiding the complex implementation details and exposing only the necessary functionalities to the user. It helps manage complexity and focuses on 'what' an object does rather than 'how' it does it.
5. What is a class?
Why you might get asked this:
This is a fundamental building block in OOP. Your definition shows if you grasp the core structure.
How to answer:
Describe a class as a blueprint or template for creating objects. Mention that it defines properties (data) and behaviors (methods).
Example answer:
A class is a blueprint or template for creating objects. It defines the properties (like variables) and methods (like functions) that objects of that class will have, acting as a structure for data and behavior.
6. What makes OOPs different from procedural programming?
Why you might get asked this:
Comparing paradigms shows a broader understanding of programming approaches and why OOP is often preferred for large systems.
How to answer:
Contrast the focus: OOP focuses on objects (data + behavior), while procedural focuses on functions/procedures. Mention key OOP features like encapsulation.
Example answer:
OOP focuses on organizing code around objects that combine data and methods, whereas procedural programming organizes code into functions and procedures. OOP features like encapsulation, inheritance, and polymorphism distinguish it from the procedural approach.
7. What is inheritance?
Why you might get asked this:
Inheritance is crucial for code reusability and establishing relationships between classes.
How to answer:
Define inheritance as a mechanism where one class (subclass/child) acquires properties and behaviors from another class (superclass/parent). Highlight its use for code reuse.
Example answer:
Inheritance is an OOP mechanism where a new class (subclass) derives properties and methods from an existing class (superclass). It promotes code reusability and establishes an "is-a" relationship between classes.
8. Difference between a class and an object?
Why you might get asked this:
This is a core distinction. Mixing these up indicates a fundamental misunderstanding of OOP.
How to answer:
Explain that a class is the blueprint, and an object is an instance created from that blueprint. Use an analogy if helpful.
Example answer:
A class is a blueprint or template that defines the structure and behavior. An object is a concrete instance of a class, having specific data values based on the class definition. Think of a class as the cookie cutter and an object as a cookie.
9. Difference between method overriding and overloading?
Why you might get asked this:
These are two common forms of polymorphism, often confused. Distinguishing them correctly is important.
How to answer:
Define both clearly. Overloading: same name, different parameters in the same class. Overriding: same name, same parameters in parent and child classes.
Example answer:
Method overloading involves having multiple methods with the same name in the same class but different parameters. Method overriding is when a subclass provides its own implementation for a method already defined in its superclass.
10. What are the two subcategories of inheritance?
Why you might get asked this:
Tests your knowledge of different inheritance structures, though implementation varies by language (e.g., Java doesn't support multiple class inheritance).
How to answer:
Mention single inheritance (one parent) and multiple inheritance (multiple parents). Add a note about language support if relevant.
Example answer:
Common subcategories are Single Inheritance (a class inherits from one parent class) and Multiple Inheritance (a class inherits from more than one parent class, though not directly supported by class inheritance in languages like Java or C#).
11. What is encapsulation?
Why you might get asked this:
Encapsulation is key to data protection and modularity. Understanding it shows you can design safe, maintainable classes.
How to answer:
Define encapsulation as binding data (attributes) and the methods that operate on that data into a single unit (the class). Mention data hiding as a key benefit.
Example answer:
Encapsulation is the bundling of data (attributes) and the methods that work on the data into a single unit (a class). It helps hide the internal state of an object from external access and control it through methods.
12. What is a structure?
Why you might get asked this:
In languages like C++ or C#, structures exist alongside classes but have key differences. This tests your understanding of these alternatives.
How to answer:
Define a structure as a user-defined data type that groups related data members. Contrast it with classes by mentioning it's often a value type and may lack some OOP features like inheritance.
Example answer:
A structure, often a value type depending on the language, is a user-defined data type that groups different types of data members under a single name. Unlike classes, structures typically don't support inheritance and are suited for small data aggregates.
13. Difference between a class and a structure?
Why you might get asked this:
Building on the previous question, this requires you to articulate the key distinctions, especially value vs. reference types and inheritance support.
How to answer:
Highlight the main differences: Classes are typically reference types supporting inheritance; structures are typically value types and do not support inheritance.
Example answer:
Classes are reference types and support inheritance, polymorphism, etc. Structures are typically value types (copied by value) and generally do not support inheritance. Classes are suitable for complex objects, structures for small data groups.
14. When would you use an operator keyword?
Why you might get asked this:
This is specific to languages like C++ or C#. It tests your knowledge of operator overloading, a form of polymorphism.
How to answer:
Explain that the operator keyword is used to overload operators (like +, -, ==) for user-defined types (classes or structs). It allows operators to behave meaningfully with objects.
Example answer:
The operator keyword is used in languages like C++ to define how standard operators (e.g., +, -, ==) should behave when applied to objects of a custom class or struct. This is called operator overloading.
15. What is cohesion?
Why you might get asked this:
Cohesion and coupling are principles related to good design within and between modules/classes. High cohesion is desirable.
How to answer:
Define cohesion as the degree to which elements within a module or class belong together or are functionally related. Aim for high cohesion.
Example answer:
Cohesion refers to the degree to which the elements within a single module or class are related and focused towards a single purpose. High cohesion means the class is responsible for a well-defined, specific task.
16. What is coupling?
Why you might get asked this:
Coupling measures interdependence between modules. Low coupling is desirable for maintainability.
How to answer:
Define coupling as the degree of interdependence between different modules or classes. Aim for low coupling.
Example answer:
Coupling measures the degree of interdependence between different software modules or classes. Low coupling means classes are relatively independent, making the system easier to understand, modify, and test.
17. Difference between cohesion and coupling?
Why you might get asked this:
This combines the previous two concepts, asking you to contrast them and explain the goal (high cohesion, low coupling).
How to answer:
Distinguish between internal focus (cohesion) and external focus (coupling). State that good design aims for high cohesion and low coupling.
Example answer:
Cohesion is about the internal relationships within a module (how well elements inside a class work together). Coupling is about the external relationships between modules (how dependent one class is on another). Good design maximizes cohesion and minimizes coupling.
18. What is an interface?
Why you might get asked this:
Interfaces define contracts and are crucial for abstraction, polymorphism, and decoupling in many OOP languages.
How to answer:
Define an interface as a contract or blueprint that defines a set of methods that a class must implement. Mention that interfaces typically cannot contain implementation details or state.
Example answer:
An interface is a contract that defines a set of methods (or properties) that a class must implement. It specifies 'what' a class must do but not 'how' it does it, allowing for abstraction and polymorphism.
19. Why program to an interface?
Why you might get asked this:
This principle is key to flexible, maintainable, and testable code. It shows you understand decoupling.
How to answer:
Explain that programming to an interface means depending on the interface type rather than the concrete implementation type. This reduces coupling, increases flexibility, and makes code easier to change or test.
Example answer:
Programming to an interface means interacting with objects via their interface type rather than their specific class type. This reduces dependency on concrete implementations, promoting flexibility, testability, and extensibility (like swapping different implementations easily).
20. Example of a virtual function?
Why you might get asked this:
Virtual functions are central to runtime polymorphism (method overriding). Providing an example shows practical understanding.
How to answer:
Define a virtual function as a method in a base class intended to be overridden in derived classes. Give a simple example, like a draw() method in a Shape base class.
Example answer:
A virtual function is a member function in a base class that is expected to be overridden by derived classes. For example, a base Animal class might have a virtual makeSound() method, and derived classes like Dog or Cat would provide their specific implementations.
21. What is a friend function?
Why you might get asked this:
Friend functions are a specific feature (primarily C++) that break encapsulation. Understanding them shows detailed language knowledge and when encapsulation might be deliberately bypassed.
How to answer:
Define a friend function as a non-member function that is granted special access to the private and protected members of a class. Mention its use cases like operator overloading.
Example answer:
In C++, a friend function is a function that is declared as a "friend" inside a class. This allows it to access the private and protected members of that class, even though it is not a member function itself.
22. Difference between a friend function and a virtual function?
Why you might get asked this:
These concepts are unrelated but both deal with function behavior relative to classes. Comparing them tests whether you understand their distinct roles.
How to answer:
Contrast their purposes: friend functions break encapsulation for external access, while virtual functions enable runtime polymorphism through inheritance.
Example answer:
A friend function (C++) is a non-member function accessing private/protected members, breaking encapsulation. A virtual function is a member function used in base classes for method overriding in derived classes, enabling polymorphism. They serve entirely different purposes.
23. What is a destructor?
Why you might get asked this:
Destructors are important for resource management, especially in languages like C++. Understanding them shows attention to cleanup.
How to answer:
Define a destructor as a special member function called automatically when an object goes out of scope or is explicitly deleted. Explain its role in releasing resources.
Example answer:
A destructor is a special member function of a class that is automatically called when an object of that class is destroyed or goes out of scope. Its primary purpose is to release resources (like memory, file handles) acquired by the object during its lifetime.
24. What is a ternary operator?
Why you might get asked this:
While not strictly OOP, it's a common programming construct. It tests basic syntax and conditional logic knowledge.
How to answer:
Describe it as a shorthand conditional operator that takes three operands: a condition, a result if true, and a result if false. Provide its basic syntax (condition ? valueiftrue : valueiffalse).
Example answer:
The ternary operator is a conditional operator that takes three operands: a condition followed by a question mark (?), an expression to execute if the condition is true, followed by a colon (:), and an expression to execute if false. It's a shorthand for if-else.
25. What is another name for the ternary operator?
Why you might get asked this:
A simple recall question. It confirms basic familiarity with the concept.
How to answer:
Provide the alternative name.
Example answer:
The ternary operator is also commonly known as the conditional operator.
26. What is an abstract class?
Why you might get asked this:
Abstract classes are a key concept for abstraction and providing partial implementation within an inheritance hierarchy.
How to answer:
Define an abstract class as a class that cannot be instantiated and is meant to be inherited. Mention that it can contain both abstract (no implementation) and concrete (implemented) methods.
Example answer:
An abstract class is a class that cannot be instantiated directly but is intended to be a base class for others. It can contain both abstract methods (declared but not implemented) and concrete methods (with implementations), serving as a blueprint with some shared functionality.
27. Difference between an abstract class and an interface?
Why you might get asked this:
This is a frequent question comparing two key abstraction mechanisms. Articulating the differences shows a nuanced understanding.
How to answer:
Contrast their contents and purpose: abstract classes can have state, constructors, and implemented methods, while interfaces only define contracts (abstract methods, no state). Discuss multiple inheritance support.
Example answer:
Abstract classes can have constructors, state (instance variables), and both concrete and abstract methods. Interfaces, conversely, only define a contract (abstract methods) and cannot hold state or implemented methods. A class can implement multiple interfaces but usually only inherit from one abstract class.
28. What is multiple inheritance?
Why you might get asked this:
Tests your knowledge of complex inheritance scenarios and potential issues like the Diamond Problem, even if the language doesn't support it directly for classes.
How to answer:
Define multiple inheritance as a class inheriting from more than one parent class. Mention that support varies by language and discuss potential complexities like the Diamond Problem.
Example answer:
Multiple inheritance is an OOP feature where a class can inherit attributes and methods from more than one parent class. While supported in languages like C++, it can introduce complexities such as the Diamond Problem, leading languages like Java and C# to restrict it for classes.
29. What is the Open-Closed Design Principle?
Why you might get asked this:
This is one of the SOLID principles, demonstrating your understanding of good software design beyond basic syntax.
How to answer:
State the principle: software entities (classes, modules, functions) should be open for extension, but closed for modification. Explain that you add new functionality by adding new code, not changing existing, tested code.
Example answer:
The Open-Closed Principle (OCP) states that software entities should be open for extension, but closed for modification. This means you should be able to add new features or change behavior by adding new code (extending) rather than altering existing, tested code (modifying).
30. What is the Single Responsibility Principle?
Why you might get asked this:
Another key SOLID principle. It assesses your understanding of how to design classes that are focused and easier to maintain.
How to answer:
State the principle: a class should have only one reason to change. Explain that this means a class should be responsible for only one piece of functionality or one aspect of the system.
Example answer:
The Single Responsibility Principle (SRP) states that a class should have only one reason to change. This means a class should focus on doing one thing well, having a single, well-defined responsibility, which improves maintainability and understanding.
Other Tips to Prepare for a object oriented programming interview questions
Beyond memorizing definitions, truly prepare for object oriented programming interview questions by practicing applying the concepts. Write small programs that demonstrate inheritance hierarchies, polymorphism through method overriding and overloading, and encapsulate data effectively. Consider design problems and think about how you would structure a solution using OOP principles. As famed computer scientist Edsger Dijkstra said, "The principal quality objectives of a program are ... its reliability, its modifiability, and its clarity." OOP aims to achieve these. Explain why you use a certain OOP concept in a given scenario, not just what it is. Use an AI interview preparation tool like Verve AI Interview Copilot to simulate mock interviews and get feedback on your explanations and structure. Verve AI Interview Copilot can help you refine your answers to be concise and cover the key points interviewers look for. Practice explaining complex ideas simply. Leverage resources like Verve AI Interview Copilot (available at https://vervecopilot.com) to get comfortable articulating your thought process under pressure. Remember, interviews assess not just what you know, but how well you can communicate it.
Frequently Asked Questions
Q1: Is Java purely object-oriented?
A1: No, Java is not purely OOP as it includes primitive data types (like int, boolean) which are not objects.
Q2: What is the 'this' keyword?
A2: 'this' refers to the current object instance within a class's methods.
Q3: What is a static method?
A3: A static method belongs to the class itself, not any specific object instance. It can be called using the class name.
Q4: What is constructor overloading?
A4: Having multiple constructors in a class with the same name but different parameters is constructor overloading.
Q5: What is association in OOP?
A5: Association is a relationship between two separate classes, like one class 'uses' another.
Q6: What is aggregation?
A6: Aggregation is a specific type of association representing a 'has-a' relationship where the part can exist independently of the whole.
