Top 30 Most Common Oops Interview Questions You Should Prepare For
Written by
James Miller, Career Coach
Preparing for technical interviews can feel overwhelming, especially when faced with core programming concepts like Object-Oriented Programming System (OOPS). OOPS forms the backbone of many modern programming languages like Java, C++, C#, and Python. Interviewers frequently test your understanding of OOPS principles because they reveal how you structure code, think about data relationships, and design scalable, maintainable software. A solid grasp of encapsulation, inheritance, polymorphism, and abstraction isn't just theoretical knowledge; it's a practical skill essential for effective software development. This guide covers 30 of the most common oops interview questions you're likely to encounter, providing concise, answer-ready explanations to help you build confidence and ace your next interview. Mastering these concepts is key to demonstrating your foundational programming expertise and readiness for real-world development challenges.
What Are oops interview questions?
oops interview questions are technical questions designed to assess a candidate's understanding of the Object-Oriented Programming System (OOPS) paradigm. These questions typically cover the core principles of OOPS: encapsulation, inheritance, polymorphism, and abstraction. They delve into related concepts such as classes, objects, constructors, destructors, access specifiers, abstract classes, interfaces, method overloading and overriding, and the differences between OOP and other paradigms like procedural programming. Interviewers use oops interview questions to gauge a candidate's ability to apply these concepts to design and write clean, modular, reusable, and maintainable code. Proficiency in OOPS is often seen as a fundamental requirement for software development roles, making these questions a standard part of technical interviews across various programming languages.
Why Do Interviewers Ask oops interview questions?
Interviewers ask oops interview questions for several key reasons. Firstly, they want to verify that candidates possess a strong foundation in fundamental programming concepts. OOPS principles are crucial for understanding how modern software is built. Secondly, these questions help assess a candidate's problem-solving approach and how they structure complex problems into manageable, interacting components (objects). Understanding OOPS indicates an ability to design code that is modular, reusable, and easier to maintain and debug. Furthermore, discussing OOPS allows interviewers to evaluate a candidate's communication skills as they explain technical concepts. A solid understanding of oops interview questions shows that a candidate can not only code but also understand the underlying principles that lead to good software design practices. It's a direct test of core computer science knowledge relevant to daily development tasks.
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?
What is polymorphism?
What is abstraction?
Difference between class and object?
Difference between method overloading and overriding?
What are access specifiers?
What is an abstract class?
Difference between abstract class and interface?
What is a constructor?
What is a destructor?
Difference between procedural and object-oriented programming?
What is cohesion and coupling?
What is multiple inheritance?
Explain operator overloading.
What is a virtual function?
What is a friend function?
Difference between a class and a structure (struct)?
What are interfaces and why use them?
What is exception handling in OOP?
What is garbage collection?
What is a static constructor?
What is the ternary operator?
Difference between abstraction and polymorphism?
What is an example of a token in programming?
Can you run a Java program without using OOP concepts?
Preview List
1. What does OOPS stand for?
Why you might get asked this:
This is a basic knowledge check to ensure you know the acronym's meaning. It's often the first question to start the OOPS discussion.
How to answer:
Simply state the full form of the acronym. Keep it brief and direct.
Example answer:
OOPS stands for Object-Oriented Programming System. It's a programming paradigm centered around the concept of objects, which encapsulate data and methods.
2. What are the four basic concepts of OOPS?
Why you might get asked this:
Tests your fundamental knowledge of the core pillars of OOP. This sets the stage for deeper questions on each concept.
How to answer:
List the four main principles: Encapsulation, Inheritance, Polymorphism, and Abstraction.
Example answer:
The four basic concepts, often called the pillars of OOPS, are Encapsulation, Inheritance, Polymorphism, and Abstraction. These form the foundation of object-oriented design.
3. What is a class?
Why you might get asked this:
Evaluates your understanding of the fundamental building block of OOP. It's essential to define this clearly.
How to answer:
Describe a class as a blueprint or template for creating objects, defining attributes (data) and behaviors (methods).
Example answer:
A class is a blueprint or template used to create objects. It defines the properties (data/attributes) and methods (behaviors/functions) that objects of that class will have.
4. What is an object?
Why you might get asked this:
Checks your understanding of an instance of a class, the actual entity you work with in OOP.
How to answer:
Define an object as an instance of a class, representing a real-world entity with state (data) and behavior (methods).
Example answer:
An object is an instance of a class. It's a concrete entity created from the class blueprint, having its own state (data) and exhibiting the behaviors defined by the class's methods.
5. What is encapsulation?
Why you might get asked this:
Probes your grasp of bundling data and methods and controlling access, a key principle for data integrity and modularity.
How to answer:
Explain it as wrapping data and methods into a single unit (class) and restricting direct access to data, typically via access specifiers.
Example answer:
Encapsulation is the bundling of data (variables) and the methods that operate on that data into a single unit, like a class. It also involves hiding the internal state of an object from the outside.
6. What is inheritance?
Why you might get asked this:
Assesses your knowledge of code reusability and hierarchical relationships between classes.
How to answer:
Describe it as a mechanism where a new class (subclass) derives properties and behaviors from an existing class (superclass).
Example answer:
Inheritance is an OOPS concept where one class (subclass or derived class) acquires properties and behaviors from another class (superclass or base class). It promotes code reuse and establishes IS-A relationships.
7. What is polymorphism?
Why you might get asked this:
Tests your understanding of the "many forms" concept, crucial for flexible code design.
How to answer:
Explain it as the ability for an object or method to take on many forms, often demonstrated through method overloading (compile-time) and overriding (runtime).
Example answer:
Polymorphism means "many forms." It allows a single interface to represent different types or behaviors. In OOP, this is often seen with method overloading (same method name, different parameters) and method overriding (subclass provides specific implementation).
8. What is abstraction?
Why you might get asked this:
Checks your ability to understand hiding complexity and focusing on essential features, important for managing complexity.
How to answer:
Define it as hiding complex implementation details and exposing only necessary functionalities to the user or other objects.
Example answer:
Abstraction involves hiding the complex implementation details of an object and showing only the necessary and relevant features. It focuses on what an object does rather than how it does it.
9. Difference between class and object?
Why you might get asked this:
A fundamental distinction that confirms you understand the core concepts and their relationship.
How to answer:
State that a class is a blueprint, while an object is a concrete instance created from that blueprint.
Example answer:
A class is a logical construct, a blueprint or template that defines the structure and behavior. An object is a physical or logical instance of that class, with a specific state based on the class definition.
10. Difference between method overloading and overriding?
Why you might get asked this:
Tests your knowledge of two key forms of polymorphism and their compile-time vs. runtime nature.
How to answer:
Explain overloading as having multiple methods with the same name but different parameters in the same class. Overriding is providing a specific implementation in a subclass for a method already defined in the superclass.
Example answer:
Method overloading occurs when multiple methods in the same class have the same name but different parameter lists (compile-time polymorphism). Method overriding is when a subclass provides a specific implementation for a method already defined in its superclass (runtime polymorphism).
11. What are access specifiers?
Why you might get asked this:
Assesses your understanding of how access control enforces encapsulation and data hiding.
How to answer:
Define them as keywords (like public, private, protected) that control the visibility and accessibility of class members (data and methods).
Example answer:
Access specifiers (or modifiers) are keywords in OOP languages (like public, private, protected, default) that define the accessibility level of classes, variables, methods, and constructors, controlling information hiding.
12. What is an abstract class?
Why you might get asked this:
Tests your knowledge of classes that cannot be instantiated and serve as base classes, often containing abstract methods.
How to answer:
Explain it as a class that cannot be instantiated directly and is intended to be inherited by other classes. It can contain both concrete and abstract methods.
Example answer:
An abstract class is a class that cannot be instantiated on its own. It serves as a base class for other classes and may contain abstract methods (declared but not implemented) that must be implemented by subclasses.
13. Difference between abstract class and interface?
Why you might get asked this:
A common, slightly more advanced question differentiating two mechanisms for achieving abstraction and polymorphism.
How to answer:
Highlight key differences: Abstract classes can have method implementations, interfaces usually only declarations. A class can implement multiple interfaces but inherit from only one abstract class.
Example answer:
Abstract classes can have concrete methods and variables; interfaces typically only have method declarations (in many languages). Classes can inherit from one abstract class but implement multiple interfaces, providing multiple inheritance benefits via interfaces.
14. What is a constructor?
Why you might get asked this:
Evaluates your understanding of how objects are initialized upon creation.
How to answer:
Define it as a special method automatically called when an object is created, used for initializing the object's state.
Example answer:
A constructor is a special member method of a class that is automatically invoked when an object of that class is created. Its primary purpose is to initialize the newly created object's state.
15. What is a destructor?
Why you might get asked this:
Checks your knowledge of resource cleanup when an object is destroyed.
How to answer:
Describe it as a special method called when an object is destroyed or goes out of scope, used for releasing resources held by the object.
Example answer:
A destructor is a special member method of a class that is automatically invoked when an object is destroyed or deleted. It's primarily used to perform cleanup tasks and release resources.
16. Difference between procedural and object-oriented programming?
Why you might get asked this:
Assesses your understanding of different programming paradigms and the shift towards OOP's advantages.
How to answer:
Explain procedural focuses on functions and sequential execution on data, while OOP organizes around objects combining data and methods, emphasizing modularity and reusability.
Example answer:
Procedural programming focuses on writing procedures or functions that perform operations on data. Object-oriented programming organizes code into objects, which encapsulate data and behavior together, emphasizing modularity, inheritance, and polymorphism.
17. What is cohesion and coupling?
Why you might get asked this:
These are software design principles often discussed in the context of OOP.
How to answer:
Define cohesion as how strongly related the responsibilities within a single module/class are (high cohesion is good). Define coupling as the degree of interdependence between modules/classes (low coupling is good).
Example answer:
Cohesion measures how focused a module/class is on a single task (high cohesion is desirable). Coupling measures how dependent one module/class is on another (low coupling is desirable for maintainability).
18. What is multiple inheritance?
Why you might get asked this:
Tests your knowledge of a specific type of inheritance and its implications (like the diamond problem), often noting language support differences.
How to answer:
Explain it as a class inheriting from more than one parent class. Mention that some languages like Java don't support it directly but achieve similar results via interfaces.
Example answer:
Multiple inheritance is a feature where a class can inherit attributes and behaviors from more than one parent class. Some languages (like C++) support it directly, while others (like Java) achieve it via interfaces to avoid complexity issues like the diamond problem.
19. Explain operator overloading.
Why you might get asked this:
Evaluates your understanding of extending built-in operator behavior to user-defined types.
How to answer:
Describe it as giving operators (like +, -, ==) special meaning when applied to objects of user-defined classes.
Example answer:
Operator overloading allows programmers to redefine or give additional meaning to existing operators (like +, -, *, /) when they are used with user-defined data types (objects of a class).
20. What is a virtual function?
Why you might get asked this:
Tests your understanding of achieving runtime polymorphism in languages like C++.
How to answer:
Define it as a function in a base class declared with the 'virtual' keyword, which can be overridden in derived classes. It ensures the correct derived class method is called at runtime.
Example answer:
A virtual function is a member function declared in a base class using the 'virtual' keyword. When overridden in a derived class, it allows for runtime polymorphism, ensuring the derived class's version is called via a base class pointer or reference.
21. What is a friend function?
Why you might get asked this:
A language-specific concept (primarily C++) that tests knowledge of granting special access.
How to answer:
Explain it as a function (not a member of the class) that is granted special access to the private and protected members of a class.
Example answer:
A friend function is a function that is declared with the 'friend' keyword inside a class. It's not a member of the class but has the privilege to access the private and protected members of that class.
22. Difference between a class and a structure (struct)?
Why you might get asked this:
Tests understanding of historical distinctions and default behaviors, particularly relevant in languages like C++.
How to answer:
Mention the main difference: in C++, struct members are public by default, while class members are private by default. Classes also support more OOPS features like inheritance more commonly.
Example answer:
In languages like C++, the main difference is default member access: struct members are public by default, class members are private by default. Classes are generally used for more complex data types with methods, while structs are often for simple data grouping.
23. What are interfaces and why use them?
Why you might get asked this:
Assesses understanding of defining contracts and achieving decoupling and polymorphism.
How to answer:
Define an interface as a blueprint of a class, containing only method signatures (and possibly constants), no implementation. Explain they are used to define contracts, support multiple inheritance (in Java), and achieve abstraction and decoupling.
Example answer:
An interface defines a contract that implementing classes must follow. It contains method declarations but no implementation. Interfaces are used to achieve abstraction, support multiple inheritance (in languages like Java), and design loosely coupled systems.
24. What is exception handling in OOP?
Why you might get asked this:
Checks your knowledge of dealing with runtime errors gracefully using OOPS constructs like try-catch blocks.
How to answer:
Explain it as a mechanism to manage runtime errors (exceptions) in a structured way, using blocks like try, catch, and finally to maintain program flow and prevent crashes.
Example answer:
Exception handling in OOP is a structured way to manage runtime errors. It uses constructs like try, catch, and finally blocks to detect, catch, and handle exceptions, ensuring the program can recover or terminate gracefully.
25. What is garbage collection?
Why you might get asked this:
Relevant in languages like Java, C#, and Python, testing knowledge of automatic memory management.
How to answer:
Describe it as an automatic process that identifies and reclaims memory occupied by objects that are no longer referenced by the program, preventing memory leaks.
Example answer:
Garbage collection is an automatic memory management process that reclaims memory that is no longer in use by the program. It identifies and frees up memory allocated to objects that are no longer reachable, preventing memory leaks.
26. What is a static constructor?
Why you might get asked this:
A more specific question, relevant in languages like C#, testing knowledge of initializing static members.
How to answer:
Define it as a special constructor used to initialize static members of a class. It's called automatically 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.
27. What is the ternary operator?
Why you might get asked this:
While not strictly an OOPS concept, it's a common programming construct often asked in technical interviews.
How to answer:
Explain it as a shorthand conditional operator that evaluates a boolean expression and returns one of two values based on whether the expression is true or false.
Example answer:
The ternary operator is a shorthand for a simple if-else statement. Its syntax is condition ? valueiftrue : valueiffalse. It evaluates a condition and returns the first value if true, the second if false.
28. Difference between abstraction and polymorphism?
Why you might get asked this:
Tests your ability to distinguish between two distinct but related OOPS principles.
How to answer:
Explain abstraction as hiding complexity and showing essentials, while polymorphism is the ability for an object/method to take many forms or have multiple implementations.
Example answer:
Abstraction focuses on simplifying by hiding details and showing only essentials. Polymorphism focuses on the ability of objects or methods to take on many forms or have different implementations based on context or type.
29. What is an example of a token in programming?
Why you might get asked this:
Tests basic compiler/interpreter knowledge, often included in foundational programming Q&A.
How to answer:
Provide examples like keywords (if, else, class), identifiers (variable names, function names), operators (+, -, *), literals (10, "hello"), and separators (; , { }).
Example answer:
A token is the smallest individual unit in a program's source code. Examples include keywords (e.g., class, public), identifiers (e.g., variable names, method names), operators (e.g., +, = ), literals (e.g., 10, "text"), and separators (e.g., ;, {).
30. Can you run a Java program without using OOP concepts?
Why you might get asked this:
Tests your understanding of Java's inherently object-oriented nature.
How to answer:
State clearly that no, Java is fundamentally object-oriented and requires classes and objects as its basic structure, even for a simple "Hello, World!" program.
Example answer:
No, you cannot run a Java program without using OOPS concepts. Java is designed around classes and objects. Even a simple "Hello, World!" requires a class definition and a main method within that class.
Other Tips to Prepare for a oops interview questions
Preparing for oops interview questions involves more than just memorizing definitions. You need to understand the concepts deeply and be able to explain them clearly and provide examples. Practice writing code snippets or drawing diagrams on a whiteboard (or virtual whiteboard) to illustrate concepts like inheritance hierarchies or polymorphism. Think about how these principles apply to real-world software design problems. Consider common design patterns and how they leverage OOPS principles. As tech luminary Grady Booch said, "The object model has proven to be a most durable and expressive medium for modeling complex systems." Don't just know what polymorphism is, understand why it makes code flexible. Mock interviews can be invaluable. Tools like Verve AI Interview Copilot can help you practice explaining concepts and responding to common oops interview questions, giving you immediate feedback on your clarity and confidence. Another expert tip is to review the specific OOPS implementations in the language relevant to the job you're applying for, as details can vary (e.g., multiple inheritance in C++ vs. Java interfaces). Consistent practice, focused on understanding the "why" behind the "what," is key. Verve AI Interview Copilot (https://vervecopilot.com) offers a structured way to rehearse these explanations, enhancing your preparation for oops interview questions and beyond.
Frequently Asked Questions
Q1: What's the difference between state and behavior in an object?
A1: State is the data/attributes of an object; behavior is what the object does via its methods.
Q2: What is method signature?
A2: The method signature includes the method name and the list of its parameters (type and order).
Q3: What is dynamic binding?
A3: Determining which method implementation to call at runtime, a key aspect of polymorphism (overriding).
Q4: What is static binding?
A4: Determining which method implementation to call at compile time, typically for overloaded methods.
Q5: Why is "composition over inheritance" often recommended?
A5: Composition provides flexibility and avoids issues like tight coupling and the complexities of deep inheritance hierarchies.
