Top 30 Most Common Oops Basic Interview Questions You Should Prepare For
Written by
James Miller, Career Coach
Preparing for technical interviews can feel daunting, especially when revisiting foundational concepts like Object-Oriented Programming (OOP). OOP is a cornerstone of modern software development and a frequent topic in technical interviews across various programming languages like Java, C++, Python, and C#. Mastering the basic oops interview questions demonstrates your understanding of core principles that govern how software is structured, maintained, and scaled. This knowledge is crucial not just for coding but for designing robust and flexible systems. Interviewers often probe these areas to gauge your fundamental grasp of software design paradigms. This article will guide you through the top 30 most common oops basic interview questions, providing concise yet comprehensive answers and tips on how to approach them. By thoroughly understanding these concepts, you can build confidence and articulate your knowledge effectively during your next interview, setting a strong foundation for discussing more complex topics and demonstrating your readiness for the role.
What Are oops basic interview questions?
oops basic interview questions revolve around the fundamental principles and concepts of Object-Oriented Programming. This includes topics like classes, objects, encapsulation, abstraction, inheritance, and polymorphism. Interviewers ask these questions to assess a candidate's foundational knowledge of OOP, which is essential for working with many modern programming languages and frameworks. These questions often cover definitions, differences between concepts (like overloading vs. overriding, abstract classes vs. interfaces), and practical application scenarios. They test your ability to explain core ideas clearly and understand why OOP principles are important in software design. A solid understanding of these basics is often seen as a prerequisite for tackling more advanced programming challenges and designing scalable, maintainable codebases. Preparing for oops basic interview questions ensures you can articulate these core ideas confidently.
Why Do Interviewers Ask oops basic interview questions?
Interviewers ask oops basic interview questions for several key reasons. Firstly, they want to confirm that candidates possess a solid theoretical understanding of fundamental programming paradigms. OOP principles like encapsulation and polymorphism are not just academic concepts; they directly influence code structure, readability, and maintainability. Secondly, these questions help gauge a candidate's problem-solving approach. Understanding inheritance or polymorphism shows an ability to think about relationships between code entities and design flexible systems. Thirdly, knowledge of OOP basics is often necessary for proficiency in object-oriented languages commonly used in the industry. By asking these questions, interviewers can assess if a candidate has the foundational knowledge required to write idiomatic and effective code in languages like Java, C#, or Python. Demonstrating a strong grasp of oops basic interview questions indicates you can contribute to well-structured and collaborative projects.
Preview List
What does OOPs stand for?
What are the four basic concepts of OOPs?
What is a class?
What is an object?
What is encapsulation?
What is abstraction?
What is inheritance?
What is polymorphism?
What is the difference between a class and an object?
What is a constructor?
What is the difference between method overloading and method overriding?
What is type casting?
What is an interface?
Why should you program to an interface?
What is an abstract class?
What is the difference between an abstract class and an interface?
What is multiple inheritance?
What are the subcategories of inheritance?
Explain inheritance with an example.
What is a static constructor?
What is cohesion?
What is coupling?
Differentiate between cohesion and coupling.
What is a destructor?
What is a friend function?
What is a virtual function?
What is the difference between abstraction and polymorphism?
What is inheritance versus composition?
What are access specifiers?
What are access modifiers, and what is their significance?
1. What does OOPs stand for?
Why you might get asked this:
This is a fundamental check to ensure you know the acronym for Object-Oriented Programming, the core topic of discussion.
How to answer:
State the full form clearly and concisely. Mention it's a programming paradigm.
Example answer:
OOPs stands for Object-Oriented Programming System (or just Object-Oriented Programming). It's a paradigm based on the concept of objects which can contain data and methods.
2. What are the four basic concepts of OOPs?
Why you might get asked this:
Tests your knowledge of the foundational pillars that define the OOP paradigm.
How to answer:
List the four main pillars: Encapsulation, Abstraction, Inheritance, and Polymorphism.
Example answer:
The four basic concepts of OOPs are Encapsulation, Abstraction, Inheritance, and Polymorphism. These principles help in designing maintainable and scalable software.
3. What is a class?
Why you might get asked this:
Checks understanding of the primary building block in OOP - the blueprint for objects.
How to answer:
Define a class as a template or blueprint, describing properties (attributes/data) and behaviors (methods).
Example answer:
A class is a blueprint or a template for creating objects. It defines the structure and behavior that objects of that class will possess, including attributes (data) and methods (functions).
4. What is an object?
Why you might get asked this:
Tests your understanding of the concrete realization of a class.
How to answer:
Define an object as an instance of a class, representing a real-world entity with state and behavior defined by the class.
Example answer:
An object is an instance of a class. It's a concrete entity based on the class blueprint, having its own specific state (values of attributes) and behavior (actions it can perform).
5. What is encapsulation?
Why you might get asked this:
Probes your understanding of bundling data and methods and controlling access.
How to answer:
Explain it as wrapping data and methods within a single unit (class) and restricting direct access to internal details.
Example answer:
Encapsulation is the binding of data (attributes) and the methods that operate on that data into a single unit (a class). It also involves controlling access to the data, typically making data private and providing public methods to interact with it.
6. What is abstraction?
Why you might get asked this:
Checks your grasp of hiding complexity and showing only relevant details.
How to answer:
Describe it as the process of simplifying complex reality by modeling classes based on essential properties and behaviors, hiding implementation details.
Example answer:
Abstraction is about showing only the essential features of an object while hiding the unnecessary background details. It focuses on what an object does rather than how it does it, simplifying the user's interaction.
7. What is inheritance?
Why you might get asked this:
Assesses your knowledge of creating new classes based on existing ones, promoting code reuse.
How to answer:
Explain it as a mechanism where a new class (subclass/derived class) acquires properties and behaviors from an existing class (superclass/base class).
Example answer:
Inheritance is an OOP mechanism where a new class inherits attributes and methods from an existing class. The new class is called the subclass or derived class, and the existing class is the superclass or base class.
8. What is polymorphism?
Why you might get asked this:
Tests your understanding of an object taking on multiple forms or having methods behave differently based on context.
How to answer:
Define polymorphism as the ability of an object to take on many forms, often related to method calls resolving differently based on the object's type at runtime.
Example answer:
Polymorphism means "many forms." In OOP, it allows objects of different classes to respond to the same method call in their own specific ways. It often involves method overriding or overloading.
9. What is the difference between a class and an object?
Why you might get asked this:
A fundamental distinction testing your grasp of the template vs. instance relationship.
How to answer:
Clearly state that a class is the blueprint (type), while an object is a concrete instance created from that blueprint.
Example answer:
A class is a blueprint or template that defines the structure and behavior. An object is a real, tangible instance of a class, created from that blueprint, occupying memory and having a specific state.
10. What is a constructor?
Why you might get asked this:
Tests knowledge of object initialization.
How to answer:
Explain it as a special method called automatically when an object is created, used to initialize its state.
Example answer:
A constructor is a special member function of a class that is automatically invoked when an object of the class is created. Its main purpose is to initialize the object's attributes.
11. What is the difference between method overloading and method overriding?
Why you might get asked this:
A classic oops basic interview questions, testing compile-time vs. runtime polymorphism concepts.
How to answer:
Differentiate based on signature (overloading has different parameters, same name within same class) vs. implementation (overriding has same signature, different implementation in subclass).
Example answer:
Method overloading occurs within the same class, where multiple methods have the same name but different parameter lists. Method overriding occurs in inheritance, where a subclass provides its own implementation of a method defined in its superclass, with the same signature.
12. What is type casting?
Why you might get asked this:
Checks understanding of converting data types, which is relevant in polymorphism and inheritance contexts.
How to answer:
Define it as converting a value from one data type to another, mentioning upcasting (child to parent) and downcasting (parent to child).
Example answer:
Type casting is the process of converting a variable of one data type into another data type. In OOP, this often applies to converting between base and derived class types (upcasting/downcasting).
13. What is an interface?
Why you might get asked this:
Assesses knowledge of defining contracts for behavior, a key abstraction mechanism.
How to answer:
Describe it as a collection of abstract methods and constant values that defines a contract that implementing classes must fulfill.
Example answer:
An interface is a contract that specifies a set of methods that implementing classes must provide. It declares 'what' a class must do, without specifying 'how'. It contains only method signatures and constants.
14. Why should you program to an interface?
Why you might get asked this:
Tests your understanding of applying abstraction for flexibility and loose coupling.
How to answer:
Explain that it promotes flexibility, allows swapping implementations easily, and reduces coupling between components.
Example answer:
Programming to an interface means interacting with objects through their interface type rather than their concrete class type. This makes code more flexible, decoupled, and easier to maintain or extend, as implementations can be changed without affecting the code that uses the interface.
15. What is an abstract class?
Why you might get asked this:
Checks understanding of partial abstraction, combining abstract and concrete members.
How to answer:
Define it as a class that cannot be instantiated directly and may contain both abstract (no implementation) and concrete (with implementation) methods.
Example answer:
An abstract class is a class that cannot be instantiated on its own. It can contain abstract methods (methods without a body that must be implemented by subclasses) and concrete methods (methods with a body). It serves as a base class for other classes.
16. What is the difference between an abstract class and an interface?
Why you might get asked this:
A common oops basic interview questions comparing two key abstraction tools.
How to answer:
Highlight key differences: abstract classes can have state and concrete methods, interfaces only abstract methods (pre-Java 8/C# 8), a class can extend one abstract class but implement multiple interfaces.
Example answer:
An abstract class can have both abstract and concrete methods, and can contain fields. An interface traditionally only defined abstract methods (Java 8+ allows defaults), and cannot hold state. A class can inherit from one abstract class but implement multiple interfaces.
17. What is multiple inheritance?
Why you might get asked this:
Checks understanding of inheriting from more than one class and awareness of the diamond problem.
How to answer:
Define it as a class inheriting from more than one superclass. Mention that some languages (like C++) support it directly, while others (like Java) achieve it via interfaces.
Example answer:
Multiple inheritance is a feature where a class can inherit properties and behaviors from more than one parent class. It can lead to issues like the diamond problem, which is why languages like Java don't support it directly but offer interfaces instead.
18. What are the subcategories of inheritance?
Why you might get asked this:
Tests your knowledge of different ways classes can inherit.
How to answer:
List common types: Single, Multilevel, Hierarchical, and (less common or via interfaces) Multiple inheritance.
Example answer:
The main types of inheritance are Single inheritance (one base class), Multilevel inheritance (A -> B -> C), Hierarchical inheritance (one base class, multiple derived classes), and Multiple inheritance (inheriting from multiple base classes).
19. Explain inheritance with an example.
Why you might get asked this:
Assesses your ability to apply the concept to a real-world or code scenario.
How to answer:
Provide a simple class hierarchy example, explaining how the subclass inherits from the superclass.
Example answer:
Consider a Vehicle base class with properties like speed and a method move(). A Car class can inherit from Vehicle, gaining these properties and methods. The Car class might add its own specific properties like numDoors or override the move() method.
20. What is a static constructor?
Why you might get asked this:
Checks knowledge of how static members are initialized.
How to answer:
Describe it as a constructor that initializes static members of a class and is called automatically only once, before the first instance is created or any static members are accessed.
Example answer:
A static constructor is used to initialize any static data, or to perform a particular action that needs to be performed once only. It is called automatically before the first instance is created or any static members are referenced.
21. What is cohesion?
Why you might get asked this:
Tests understanding of good design principles regarding the internal focus of a module.
How to answer:
Define cohesion as the degree to which elements within a module (like a class) belong together or are functionally related. High cohesion is desirable.
Example answer:
Cohesion is a qualitative measure indicating the degree to which a class or module's elements belong together. High cohesion means the elements within a module are tightly related and focused on a single task or purpose.
22. What is coupling?
Why you might get asked this:
Tests understanding of good design principles regarding dependencies between modules.
How to answer:
Define coupling as the degree of interdependence between software modules or classes. Low coupling is desirable.
Example answer:
Coupling is a measure of how dependent modules or classes are on each other. Low coupling indicates that modules are relatively independent, making the system easier to understand, modify, and test.
23. Differentiate between cohesion and coupling.
Why you might get asked this:
A common question comparing two critical design metrics.
How to answer:
Contrast them: Cohesion is about elements within a module working together (internal strength); Coupling is about dependencies between modules (external reliance). Aim for high cohesion, low coupling.
Example answer:
Cohesion measures the relatedness of elements within a single module, aiming for high internal focus. Coupling measures the interdependence between different modules, aiming for low external reliance to minimize impact of changes.
24. What is a destructor?
Why you might get asked this:
Checks knowledge of object cleanup, especially relevant in languages with manual memory management or specific resource handling needs.
How to answer:
Explain it as a special method called automatically when an object is destroyed (or goes out of scope), typically used for cleanup or releasing resources.
Example answer:
A destructor is a special member function that is automatically called when an object is no longer needed or goes out of scope. It's primarily used for releasing resources like memory or file handles held by the object.
25. What is a friend function?
Why you might get asked this:
Specific to C++, tests understanding of controlled access outside the class interface.
How to answer:
Define it as a non-member function granted special access to the private and protected members of a class.
Example answer:
A friend function is a function that, although not a member of a class, is granted access to the private and protected members of that class. It's declared using the friend keyword inside the class definition.
26. What is a virtual function?
Why you might get asked this:
Tests understanding of runtime polymorphism in languages like C++ or C#.
How to answer:
Define it as a member function in a base class that is declared using the virtual keyword, allowing it to be overridden in derived classes and enabling runtime polymorphism (dynamic dispatch).
Example answer:
A virtual function is a base class member function that is expected to be redefined in derived classes. Using the virtual keyword ensures that the correct derived class function is called when referenced via a base class pointer or reference (runtime polymorphism).
27. What is the difference between abstraction and polymorphism?
Why you might get asked this:
Compares two different OOP principles often confused, testing your ability to distinguish their roles.
How to answer:
Explain that abstraction is about hiding complexity and focusing on essential characteristics, while polymorphism is about the ability of objects to take multiple forms or for operations to exhibit different behaviors depending on context.
Example answer:
Abstraction is the concept of hiding implementation details and showing only necessary features. Polymorphism is the concept of using a single interface to represent different underlying types, allowing objects to respond differently to the same message.
28. What is inheritance versus composition?
Why you might get asked this:
A critical design question assessing your understanding of "is-a" relationships (inheritance) vs. "has-a" relationships (composition).
How to answer:
Contrast them: Inheritance is an "is-a" relationship (e.g., Car IS-A Vehicle), while Composition is a "has-a" relationship (e.g., Car HAS-A Engine). Explain that composition often provides more flexibility.
Example answer:
Inheritance represents an "is-a" relationship (e.g., a Dog is a Mammal). Composition represents a "has-a" relationship (e.g., a Car has an Engine). Composition is often preferred for flexibility and avoiding the complexities of inheritance hierarchies.
29. What are access specifiers?
Why you might get asked this:
Tests understanding of how encapsulation is enforced.
How to answer:
List and briefly describe common access specifiers (public, private, protected) that control the visibility and accessibility of class members.
Example answer:
Access specifiers (like public, private, protected) control the accessibility of members of a class. They determine which parts of the program can access a member, supporting encapsulation by hiding implementation details.
30. What are access modifiers, and what is their significance?
Why you might get asked this:
Similar to access specifiers, reinforcing understanding of visibility control.
How to answer:
Explain they are keywords used to specify the declared accessibility of a member or a type. Their significance lies in enforcing encapsulation and controlling the interface exposed by a class.
Example answer:
Access modifiers are keywords used to define the scope and visibility of class members. They are significant because they enforce encapsulation, preventing unauthorized access to data and methods and allowing developers to control the public interface of their classes.
Other Tips to Prepare for a oops basic interview questions
Beyond memorizing definitions, practicing application is key for oops basic interview questions. Think about how these principles are used in real-world code examples. Code small programs demonstrating inheritance or polymorphism. Review design patterns that heavily rely on OOP concepts. "The object-oriented approach is really about how you think about problems," says software architect and author, Grady Booch. Understanding the why behind each principle is as important as the what. Consider using tools like Verve AI Interview Copilot (https://vervecopilot.com) to practice articulating your answers clearly and concisely. Verve AI Interview Copilot provides simulated interview environments where you can test your knowledge of oops basic interview questions and receive feedback on your delivery. Rehearsing with Verve AI Interview Copilot can significantly boost your confidence and fluency. Additionally, review the core libraries or frameworks in your target programming language to see how OOP principles are applied in practice. As Edsger Dijkstra famously said, "Simplicity is a prerequisite for reliability." OOP aims for simpler, more reliable systems, and your ability to discuss these benefits will impress interviewers. Prepare thoroughly, practice your explanations, and leverage resources like Verve AI Interview Copilot to hone your communication skills around oops basic interview questions.
Frequently Asked Questions
Q1: What is the main benefit of using OOP?
A1: Increased code reusability, maintainability, and flexibility through principles like inheritance and polymorphism.
Q2: Is OOP the only programming paradigm?
A2: No, other paradigms exist like procedural, functional, and logic programming. OOP is just one approach.
Q3: Can a class have multiple constructors?
A3: Yes, a class can have multiple constructors with different parameter lists (constructor overloading).
Q4: What is an instance variable?
A4: A variable defined within a class but outside any method, belonging to each object instance.
Q5: What is a class variable (static variable)?
A5: A variable defined within a class with the static keyword, belonging to the class itself, shared by all instances.
