Top 30 Most Common Oops Concepts Interview Questions You Should Prepare For
Written by
James Miller, Career Coach
Landing a software development job often involves demonstrating a solid understanding of core programming principles. Among the most fundamental are the concepts of Object-Oriented Programming (OOP). OOPS, or Object-Oriented Programming System, is a paradigm that structures software design around objects, rather than functions and logic. Mastering these concepts is crucial not just for writing good code, but also for navigating technical interviews. Interviewers frequently test candidates on OOPS principles to gauge their foundational knowledge, their ability to think about software architecture, and how they structure their code. Whether you're a fresh graduate or an experienced developer, revisiting these concepts is always a valuable exercise. This article dives into the top 30 most common oops concepts interview questions you are likely to encounter, providing clear, concise answers to help you prepare effectively and confidently. Understanding the 'why' behind these questions and practicing articulating your answers is key to success. By preparing for these common oops concepts interview questions, you'll build a strong base to handle more complex discussions about software design patterns, architectural choices, and problem-solving approaches in an object-oriented context. Get ready to solidify your knowledge and ace your next technical interview by mastering these essential OOPS topics.
What Are oops concepts interview questions?
oops concepts interview questions delve into the core principles of Object-Oriented Programming. These questions assess a candidate's understanding of how to design and structure software using objects, classes, and the relationships between them. They cover the fundamental pillars: Encapsulation, Inheritance, Polymorphism, and Abstraction. Beyond these four, questions often explore related topics like constructors, destructors, interfaces, abstract classes, access specifiers, and design principles like coupling and cohesion. The goal is to determine if a candidate can not only define these terms but also explain their practical application, benefits, and differences, demonstrating an ability to write modular, reusable, and maintainable code. Preparing for oops concepts interview questions means understanding both the theoretical definitions and how they translate into real-world programming scenarios.
Why Do Interviewers Ask oops concepts interview questions?
Interviewers ask oops concepts interview questions for several critical reasons. Firstly, OOPS is a foundational paradigm used in many popular programming languages (Java, C++, Python, C#), so a strong grasp indicates readiness to work with these technologies. Secondly, understanding OOPS principles reveals a candidate's ability to think about software architecture in a structured way, leading to better design decisions. Encapsulation, inheritance, polymorphism, and abstraction are tools for managing complexity, promoting code reusability, and improving maintainability – all qualities employers seek. Asking about oops concepts interview questions helps evaluate problem-solving skills and how a candidate approaches structuring data and behavior. It also provides insight into their understanding of software design patterns, which are often built upon OOPS principles. Ultimately, proficiency in OOPS concepts signals that a candidate can contribute to building robust, scalable, and well-organized software systems.
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 inheritance in OOPS?
What are the types/subcategories of inheritance?
What is polymorphism?
What is abstraction?
What is method overloading and method overriding?
What is the difference between a class and an object?
What is an abstract class?
How is an abstract class different from an interface?
What are access specifiers?
What is a constructor and destructor?
What is a virtual function?
What is a friend function?
What is coupling?
What is cohesion?
What is the difference between cohesion and coupling?
What is an interface in OOPS?
Why should you program to an interface?
What is exception and exception handling?
What is garbage collection?
What is static constructor?
What is the difference between abstraction and polymorphism?
What is the difference between a class and a structure?
What is method overriding vs overloading difference?
What is a ternary operator?
Can you give an example of polymorphism in code?
Preview List
1. What does OOPS stand for?
Why you might get asked this:
This is a basic test of foundational knowledge. It checks if you know the acronym and its full form before diving deeper into the concepts.
How to answer:
State the full form clearly and briefly mention what the paradigm is based on.
Example answer:
OOPS stands for Object-Oriented Programming System. It's a programming paradigm centered around the concept of objects, which bundle data and methods that operate on that data.
2. What are the four basic concepts of OOPS?
Why you might get asked this:
Interviewers use this to see if you know the core pillars of OOPS. It's the starting point for discussing these concepts.
How to answer:
List the four main pillars: Encapsulation, Inheritance, Polymorphism, and Abstraction. You can briefly define each.
Example answer:
The four basic concepts are Encapsulation (bundling data and methods), Inheritance (creating new classes from existing ones), Polymorphism (treating objects of different classes uniformly), and Abstraction (hiding complexity).
3. What is a class?
Why you might get asked this:
Understanding classes is fundamental, as they are the blueprints for objects. This question checks your grasp of the building blocks.
How to answer:
Define a class as a template or blueprint and mention what it contains (attributes/data and behaviors/methods).
Example answer:
A class is a blueprint or template used to create objects. It defines the properties (data members) and behaviors (methods) that objects of that class will have.
4. What is an object?
Why you might get asked this:
Objects are the instances of classes and central to OOP. This tests if you know the concrete realization of a class.
How to answer:
Define an object as an instance of a class. Mention that it holds actual data and can perform actions defined by the class's methods.
Example answer:
An object is a specific instance of a class. It represents a real-world entity or concept and holds unique data values according to the class's definition.
5. What is encapsulation?
Why you might get asked this:
Encapsulation is key to data hiding and integrity. This question assesses your understanding of how OOPS protects data.
How to answer:
Explain it as bundling data and methods together within a class. Mention the concept of hiding internal details (data hiding) and controlling access.
Example answer:
Encapsulation is the process of bundling data (attributes) and the methods (functions) that operate on that data into a single unit, typically a class. It helps in data hiding by restricting direct access to data.
6. What is inheritance in OOPS?
Why you might get asked this:
Inheritance is crucial for code reuse and establishing relationships between classes. This tests your knowledge of hierarchy and specialization.
How to answer:
Define it as a mechanism where a new class (subclass) derives properties and behaviors from an existing class (superclass). Highlight code reuse.
Example answer:
Inheritance is an OOPS mechanism allowing a new class (subclass) to inherit attributes and methods from another class (superclass). This promotes code reusability and models 'is-a' relationships.
7. What are the types/subcategories of inheritance?
Why you might get asked this:
This question checks for a deeper understanding of inheritance variations supported by different languages.
How to answer:
List common types like Single, Multiple, Multilevel, Hierarchical, and Hybrid. Briefly describe each if time permits or if asked.
Example answer:
Common types include Single (one parent), Multiple (multiple parents, language dependent), Multilevel (chain of inheritance), Hierarchical (one parent, multiple children), and Hybrid (combination).
8. What is polymorphism?
Why you might get asked this:
Polymorphism is a powerful concept allowing flexibility. This tests your understanding of treating objects differently based on their type.
How to answer:
Explain "polymorphism" means "many forms." Describe how it allows objects of different classes to be treated as objects of a common superclass, with methods behaving differently.
Example answer:
Polymorphism means "many forms." It allows objects of different classes to respond to the same method call in their own specific way, typically achieved through method overloading (compile-time) or overriding (runtime).
9. What is abstraction?
Why you might get asked this:
Abstraction is about managing complexity. This question assesses your understanding of hiding details and focusing on essentials.
How to answer:
Define abstraction as hiding complex implementation details and showing only necessary features. Use an analogy like using a TV remote.
Example answer:
Abstraction is the process of hiding the complex implementation details and showing only the essential features of an object. It focuses on 'what' an object does rather than 'how' it does it.
10. What is method overloading and method overriding?
Why you might get asked this:
These are key mechanisms for achieving polymorphism. Differentiating them correctly is a standard test of understanding.
How to answer:
Clearly define both. Overloading is within the same class, different parameters (compile-time). Overriding is in a subclass, same signature (runtime).
Example answer:
Method overloading is defining multiple methods in the same class with the same name but different parameters. Method overriding is defining a method in a subclass with the same signature as a method in its superclass.
11. What is the difference between a class and an object?
Why you might get asked this:
This is a fundamental distinction in OOPS. It checks if you grasp the relationship between the blueprint and its instances.
How to answer:
Reiterate the definitions: class is the blueprint, object is the instance. Mention that a class defines structure, an object has state (data) and behavior.
Example answer:
A class is a blueprint or template that defines the structure and behavior. An object is a concrete instance of a class; it has specific data values (state) and can perform actions (behavior) defined by the class.
12. What is an abstract class?
Why you might get asked this:
Abstract classes are used for defining common interfaces and partial implementations. This tests your knowledge of classes that cannot be instantiated directly.
How to answer:
Define it as a class that cannot be instantiated directly and is meant to be a base class. Mention that it can contain both abstract (no body) and concrete methods.
Example answer:
An abstract class is a class that cannot be instantiated on its own. It's designed to be inherited by other classes and can contain a mix of regular methods and abstract methods (methods declared but not implemented).
13. How is an abstract class different from an interface?
Why you might get asked this:
This question probes your understanding of different forms of abstraction and contract definition in OOPS languages.
How to answer:
Compare key differences: Interfaces only contain abstract methods (in many languages), abstract classes can have both. Abstract classes can have instance variables and constructors.
Example answer:
An interface defines a contract with only method signatures (usually all abstract). An abstract class can have both abstract methods and concrete methods with implementations, and can contain state (instance variables).
14. What are access specifiers?
Why you might get asked this:
Access specifiers control visibility and are key to encapsulation. This tests your understanding of managing access to class members.
How to answer:
List common specifiers (public, private, protected) and briefly explain the scope of access each provides.
Example answer:
Access specifiers (or modifiers) like public, private, and protected define the visibility and accessibility of class members (data and methods). They control where a member can be accessed from.
15. What is a constructor and destructor?
Why you might get asked this:
Constructors and destructors manage object lifecycle (creation and cleanup). This tests your knowledge of these special methods.
How to answer:
Define constructor as a method called upon object creation for initialization. Define destructor (if applicable to the language) as a method called before object destruction for cleanup.
Example answer:
A constructor is a special method called automatically when an object is created; it initializes the object's state. A destructor (used in languages like C++) is called before an object is destroyed for cleanup.
16. What is a virtual function?
Why you might get asked this:
Virtual functions are essential for achieving runtime polymorphism in languages like C++. This tests your understanding of dynamic method dispatch.
How to answer:
Define it as a member function in a base class that you expect to redefine in derived classes. Explain that it enables runtime polymorphism through a base class pointer.
Example answer:
A virtual function is a member function declared in a base class that can be redefined (overridden) in a derived class. When called via a pointer or reference to the base class, the derived class's version is executed (runtime polymorphism).
17. What is a friend function?
Why you might get asked this:
Friend functions are a specific mechanism (primarily in C++) that bypasses standard encapsulation rules. This tests knowledge of language-specific features.
How to answer:
Define it as a function that is not a member of a class but is granted special permission to access the private and protected members of that class.
Example answer:
A friend function (in C++) is a function declared outside a class but given permission to access the private and protected members of that class. It's not a member function itself.
18. What is coupling?
Why you might get asked this:
Coupling and cohesion are design principles. This tests your understanding of module dependencies and their impact on maintainability.
How to answer:
Define coupling as the degree of interdependence between software modules or classes. State that low coupling is desirable for better modularity and maintainability.
Example answer:
Coupling is a measure of how interdependent two modules or classes are. High coupling means they are tightly linked, making changes harder. Low coupling is preferred for easier maintenance and reusability.
19. What is cohesion?
Why you might get asked this:
Cohesion is another key design principle, focusing on internal module consistency. This tests your understanding of how well a module's elements belong together.
How to answer:
Define cohesion as the degree to which the elements within a module or class belong together. State that high cohesion is desirable, meaning the class has a single, well-defined responsibility.
Example answer:
Cohesion measures how related and focused the responsibilities of a single module or class are. High cohesion means the class's elements work together towards one clear purpose, which is good design.
20. What is the difference between cohesion and coupling?
Why you might get asked this:
Interviewers often ask this to see if you can distinguish between these related but distinct design principles and understand their ideal balance.
How to answer:
Summarize their definitions and contrast their focus: Cohesion is about internal consistency within a module, while coupling is about external dependencies between modules.
Example answer:
Cohesion is about how well elements within a module fit together (internal focus), aiming for high cohesion. Coupling is about the degree of interdependence between modules (external focus), aiming for low coupling.
21. What is an interface in OOPS?
Why you might get asked this:
Interfaces define contracts and are crucial for achieving abstraction and supporting multiple inheritances of type (if not implementation).
How to answer:
Define it as a contract or a collection of abstract method declarations. Emphasize that classes implementing an interface must provide implementations for all its methods.
Example answer:
An interface is a blueprint of a class that defines a contract. It contains method signatures (usually abstract) that implementing classes must provide implementations for, specifying a set of behaviors.
22. Why should you program to an interface?
Why you might get asked this:
This tests your understanding of using abstraction to write flexible and maintainable code, decoupling implementation from usage.
How to answer:
Explain that depending on an interface rather than a concrete implementation promotes flexibility, allows for easy substitution of implementations, and reduces coupling.
Example answer:
Programming to an interface means interacting with objects through their defined interface rather than their concrete class. This makes your code more flexible, allowing different implementations to be used interchangeably, promoting loose coupling.
23. What is exception and exception handling?
Why you might get asked this:
Handling errors gracefully is vital for robust applications. This tests your knowledge of dealing with runtime anomalies.
How to answer:
Define an exception as an event that disrupts the normal flow of program execution. Explain exception handling as the mechanism to deal with such events to prevent crashes.
Example answer:
An exception is an abnormal event that occurs during program execution, disrupting the normal flow. Exception handling is the mechanism used to manage these runtime errors in a controlled way, preventing program termination.
24. What is garbage collection?
Why you might get asked this:
Garbage collection is a key feature in many OOPS languages (Java, C#, Python). This tests your understanding of automatic memory management.
How to answer:
Define it as an automatic process that reclaims memory occupied by objects that are no longer reachable or in use by the program.
Example answer:
Garbage collection is an automatic memory management process. It identifies and deallocates memory that was previously allocated to objects but is no longer being used by the program, preventing memory leaks.
25. What is static constructor?
Why you might get asked this:
Static constructors are specific to initializing static members. This tests knowledge of class-level initialization.
How to answer:
Define it as a constructor used to initialize static members of a class. Mention that it is called only once, before any instance is created or any static member is referenced.
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 accessed.
26. What is the difference between abstraction and polymorphism?
Why you might get asked this:
These are two different pillars of OOPS. Distinguishing them shows a clear understanding of each concept's purpose.
How to answer:
Explain that abstraction is about hiding complexity and showing essential features, while polymorphism is about allowing objects of different types to respond to the same method call differently.
Example answer:
Abstraction focuses on hiding complex implementation details and showing only relevant features to the user. Polymorphism allows a single interface to represent multiple underlying types, enabling objects to behave differently based on their type.
27. What is the difference between a class and a structure?
Why you might get asked this:
This distinction is language-dependent (e.g., C#, C++). It checks your understanding of value vs. reference types and OOPS feature support.
How to answer:
Highlight key differences based on the target language: Classes support full OOPS (inheritance, polymorphism) and are typically reference types. Structures are often value types with limited OOPS features.
Example answer:
In languages like C#, classes are reference types supporting full OOPS features, while structs are value types and do not support inheritance, though they can implement interfaces. Differences vary by language.
28. What is method overriding vs overloading difference?
Why you might get asked this:
This is a common re-phrasing of question 10, often asked to ensure the distinction is clear even when phrased differently.
How to answer:
Reiterate the key difference: Overriding is same signature, different class (inheritance). Overloading is same name, different parameters, same class.
Example answer:
Overriding occurs when a subclass provides its own implementation for a method already defined in its superclass (same signature, different class). Overloading is defining multiple methods in the same class with the same name but different parameters.
29. What is a ternary operator?
Why you might get asked this:
While not strictly an OOPS concept, it's a common language feature related to conditional logic often used in OOPS code. It checks general syntax knowledge.
How to answer:
Define it as a shorthand for an if-else statement. Provide the basic syntax: condition ? valueiftrue : valueiffalse;.
Example answer:
The ternary operator is a conditional operator that provides a concise way to write a simple if-else statement. Its syntax is condition ? valueiftrue : valueiffalse.
30. Can you give an example of polymorphism in code?
Why you might get asked this:
Theoretical understanding is good, but interviewers want to see if you can apply concepts. A simple code example demonstrates practical knowledge.
How to answer:
Provide a simple example, typically using method overriding. Show a base class with a method and a derived class overriding it, then call the method using a base class reference pointing to a derived object.
Example answer:
class Animal { void sound() { System.out.println("Animal sound"); } } class Dog extends Animal { void sound() { System.out.println("Dog barks"); } } Animal myAnimal = new Dog(); myAnimal.sound(); // Outputs "Dog barks" - demonstrates runtime polymorphism.
Other Tips to Prepare for a oops concepts interview questions
Preparing for oops concepts interview questions involves more than just memorizing definitions. It's about truly understanding the principles and being able to discuss how they apply in real-world coding scenarios. Practice explaining each concept in your own words and think about examples from projects you've worked on. Consider how OOPS principles like encapsulation or inheritance have helped you write better code or solve specific problems. As the renowned computer scientist Alan Kay, one of the pioneers of object-oriented programming, said, "OOP to me means only messaging, local retention and protection and hiding of state-process, and extreme decentralization of control." This highlights the core ideas of secure data and modularity. Don't just prepare answers; prepare to discuss and elaborate. Consider using tools like Verve AI Interview Copilot to practice articulating your understanding under timed conditions. Verve AI Interview Copilot can simulate technical interviews, giving you feedback on your responses to common oops concepts interview questions. Review code from open-source projects or your own work and identify where OOPS principles are applied. Being able to point to concrete examples will make your answers more convincing. Another expert, Grady Booch, stated, "The object model has proved to be a very powerful and general way to deal with complexity." Use this perspective: OOPS is a tool to manage complexity. Practice explaining why it helps. Leverage Verve AI Interview Copilot's features at https://vervecopilot.com to refine your explanations and boost your confidence before the big day. Utilizing Verve AI Interview Copilot can provide targeted practice, ensuring you are articulate and precise when discussing these critical software engineering fundamentals during any technical assessment.
Frequently Asked Questions
Q1: Are OOPS concepts language-specific?
A1: While core principles are universal, their implementation details and features like multiple inheritance vary significantly between languages (Java, C++, Python).
Q2: What is the difference between Association, Aggregation, and Composition?
A2: These represent relationships between objects: Association (general link), Aggregation (has-a, but parts can exist independently), Composition (has-a, parts cannot exist without the whole).
Q3: What are design patterns related to OOPS?
A3: Many patterns like Factory, Singleton, Observer, Strategy are built upon OOPS principles to solve common design problems effectively.
Q4: Is OOPS the only programming paradigm?
A4: No, other paradigms exist like procedural, functional, and logic programming. OOPS is one common approach for structuring code.
Q5: Why is low coupling and high cohesion important?
A5: They lead to more modular, understandable, testable, and maintainable code, making it easier to manage complexity and make changes.
Q6: What is the 'this' keyword?
A6: The 'this' keyword refers to the current instance of the class within instance methods or constructors, allowing access to current object members.
