Top 30 Most Common Oops Programming Interview Questions You Should Prepare For
Written by
James Miller, Career Coach
Introduction
Preparing for technical interviews often involves mastering core concepts, and Object-Oriented Programming (OOPs) is fundamental for many programming roles. Interviewers frequently assess candidates' understanding of OOPs principles like encapsulation, inheritance, polymorphism, and abstraction. A solid grasp of these concepts is crucial not just for answering specific OOPs programming interview questions but also for demonstrating your ability to write clean, maintainable, and scalable code. This guide covers the top 30 OOPs programming interview questions you are likely to encounter. By preparing detailed, confident answers, you'll significantly boost your chances of success in your next programming interview focusing on OOPs.
What Are OOPs
OOPs, or Object-Oriented Programming System, is a programming paradigm centered around the concept of "objects." These objects are instances of "classes," which serve as blueprints. Each object encapsulates data (attributes or properties) and the procedures (methods or behaviors) that can operate on that data. The core idea is to model real-world entities or concepts as objects, making code more organized, modular, and easier to understand and manage. Mastering OOPs principles is key to tackling common OOPs programming interview questions effectively.
Why Do Interviewers Ask OOPs Questions
Interviewers ask OOPs programming interview questions to evaluate a candidate's foundational understanding of modern software design principles. Knowledge of OOPs demonstrates the ability to structure code logically, promote reusability, manage complexity, and build flexible systems. These concepts are cornerstones in many popular languages like Java, C++, Python, and C#. Your answers reveal not just theoretical knowledge but also practical application skills essential for solving complex programming problems and collaborating on large projects using OOPs methodologies. Preparing thoroughly for OOPs programming interview questions shows your readiness for the role.
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 inheritance?
What are different types of inheritance?
What is polymorphism?
What is method overloading and overriding?
What is abstraction?
What is the difference between abstraction and encapsulation?
What is an interface?
What is the difference between an abstract class and an interface?
What are access specifiers?
What is a constructor?
What is a destructor?
What is the difference between class and structure?
What is a friend function?
What is dynamic method dispatch?
What is coupling?
What is cohesion?
What is a virtual function?
What is garbage collection in OOP?
Can you run a Java application without using OOPs concepts?
What is exception handling?
What is a static constructor?
What are delegates in C#?
What are SOLID principles?
What is the advantage of programming to an interface?
How does OOP improve code maintainability?
1. What does OOPS stand for?
Why you might get asked this:
This is a basic opener to gauge your familiarity with the acronym and initiate discussion on OOPs fundamentals in a programming context.
How to answer:
State the full form clearly and briefly mention that it's a programming paradigm based on objects.
Example answer:
OOPS stands for Object-Oriented Programming System. It's a widely used programming paradigm structured around objects rather than just functions and logic, core to many modern languages.
2. What are the four basic concepts of OOPS?
Why you might get asked this:
This question checks if you know the fundamental pillars upon which OOPs is built, essential for understanding design patterns.
How to answer:
List and briefly define the four main pillars: Encapsulation, Inheritance, Polymorphism, and Abstraction.
Example answer:
The four basic concepts are Encapsulation (bundling data/methods), Inheritance (creating new classes from existing ones), Polymorphism (objects taking many forms), and Abstraction (hiding complexity).
3. What is a class?
Why you might get asked this:
Understanding the class concept is foundational, as it's the blueprint for creating objects in OOPs.
How to answer:
Define a class as a blueprint or template that defines the properties and behaviors common to all objects created from it.
Example answer:
A class is a blueprint or template for creating objects. It defines the structure (attributes) and behavior (methods) that objects of that class will possess, like a blueprint for a house.
4. What is an object?
Why you might get asked this:
This follows the class question to see if you understand the instantiated entity based on the blueprint.
How to answer:
Define an object as an instance of a class, representing a specific entity with actual data values and callable methods.
Example answer:
An object is an instance of a class. It's a concrete entity created from the class blueprint, possessing actual values for the attributes and capable of performing the actions defined by the class's methods.
5. What is encapsulation?
Why you might get asked this:
Encapsulation is a key OOPs principle for data hiding and modularity, important for system design.
How to answer:
Explain encapsulation as bundling data (variables) and the methods (functions) that operate on the data into a single unit (the class), restricting direct access to data.
Example answer:
Encapsulation is the wrapping of data and the methods that operate on that data into a single unit, which is typically a class. It helps in data hiding and prevents direct external access to internal state.
6. What is inheritance?
Why you might get asked this:
Inheritance is crucial for code reusability and establishing relationships between classes in OOPs.
How to answer:
Describe inheritance as a mechanism where a new class (child/derived) acquires properties and behaviors from an existing class (parent/base), promoting code reuse.
Example answer:
Inheritance is an OOPs mechanism allowing a class (the child) to inherit attributes and methods from another class (the parent). This promotes code reusability and establishes 'is-a' relationships between classes.
7. What are different types of inheritance?
Why you might get asked this:
This tests your knowledge of various relationship structures possible through inheritance in OOPs languages.
How to answer:
List common types like single, multiple (mention language support), multilevel, hierarchical, and hybrid inheritance.
Example answer:
Common types include Single (one parent), Multiple (multiple parents, e.g., C++), Multilevel (chain of inheritance), Hierarchical (one parent, many children), and Hybrid (combination).
8. What is polymorphism?
Why you might get asked this:
Polymorphism is a cornerstone of flexible and extensible OOPs design, allowing code to work with objects of different classes uniformly.
How to answer:
Define polymorphism as the ability of an object or method to take on many forms, primarily through method overloading and overriding.
Example answer:
Polymorphism means "many forms." In OOPs, it allows methods or objects to perform different actions in different contexts, often seen with method overloading (compile-time) and method overriding (runtime).
9. What is method overloading and overriding?
Why you might get asked this:
These are the two main ways polymorphism is achieved, and interviewers check if you know the difference and how they are applied.
How to answer:
Explain overloading as having multiple methods with the same name but different parameters in the same class. Explain overriding as redefining a parent class method in a child class.
Example answer:
Overloading is defining multiple methods with the same name but different parameters in the same class (compile-time polymorphism). Overriding is redefining a method in a child class that exists in the parent class (runtime polymorphism).
10. What is abstraction?
Why you might get asked this:
Abstraction helps manage complexity by hiding unnecessary details, crucial for large systems.
How to answer:
Describe abstraction as the principle of hiding complex implementation details and showing only the essential features to the user.
Example answer:
Abstraction focuses on showing only essential information and hiding implementation details. It simplifies complex systems by modeling classes based on their essential properties and behaviors.
11. What is the difference between abstraction and encapsulation?
Why you might get asked this:
These concepts are often confused, so explaining the distinction shows a deep understanding of OOPs principles.
How to answer:
Explain that abstraction hides complexity (what it does), while encapsulation hides state and protects data (how it's bundled).
Example answer:
Abstraction deals with hiding complexity by simplifying the view (focus on 'what' an object does). Encapsulation bundles data and methods to protect the data and manage dependencies (focus on 'how' it's done).
12. What is an interface?
Why you might get asked this:
Interfaces define contracts and support abstraction and multiple inheritance-like behavior in languages like Java.
How to answer:
Define an interface as a contract specifying a set of methods that implementing classes must provide. Note it contains no implementation details.
Example answer:
An interface is a blueprint of a class. It's a contract that declares methods without implementing them. A class implementing an interface must provide implementations for all its declared methods.
13. What is the difference between an abstract class and an interface?
Why you might get asked this:
A classic OOPs question testing your understanding of two key abstraction mechanisms.
How to answer:
Highlight key differences: Abstract classes can have concrete and abstract methods, state; Interfaces only abstract methods (historically), no state (except constants), and a class can implement multiple interfaces but extend only one abstract class.
Example answer:
An abstract class can have method implementations and state, while an interface defines only method signatures (contract). A class can inherit from one abstract class but implement multiple interfaces.
14. What are access specifiers?
Why you might get asked this:
Access specifiers relate directly to encapsulation and controlling visibility, a practical aspect of OOPs.
How to answer:
Explain that access specifiers (like public, private, protected) control the visibility and accessibility of class members (data, methods) to enforce encapsulation.
Example answer:
Access specifiers (e.g., public, private, protected) are keywords used in OOPs to control the visibility and accessibility of class members from outside the class or inheritance hierarchy, supporting encapsulation.
15. What is a constructor?
Why you might get asked this:
Constructors are essential for object creation and initialization.
How to answer:
Define a constructor as a special method used to initialize objects, having the same name as the class and no return type.
Example answer:
A constructor is a special method in a class that is automatically called when an object of that class is created. Its primary purpose is to initialize the object's state. It has the same name as the class.
16. What is a destructor?
Why you might get asked this:
Relevant in languages with manual or semi-manual memory management (like C++) for resource cleanup.
How to answer:
Define a destructor as a method called when an object is destroyed (goes out of scope or is explicitly deleted) to perform cleanup tasks, like releasing resources. Mention its naming convention (tilde).
Example answer:
A destructor is a special method (like ~ClassName() in C++) called when an object is destroyed or goes out of scope. Its purpose is to clean up resources allocated by the object during its lifetime.
17. What is the difference between class and structure?
Why you might get asked this:
Checks understanding of fundamental data grouping types and their OOPs capabilities across languages (e.g., C++ vs. C#).
How to answer:
Explain that classes support full OOPs features (inheritance, polymorphism, etc.) with members private by default, while structs are simpler value types primarily for data grouping, often public by default.
Example answer:
In languages like C++, structs are value types with public members by default, mainly for grouping data. Classes are reference types supporting full OOPs features like inheritance and polymorphism, with private members by default.
18. What is a friend function?
Why you might get asked this:
Specific to C++, tests knowledge of how to grant external functions access to private members, sometimes used for operator overloading.
How to answer:
Describe a friend function (in C++) as a function declared outside a class but granted special permission to access the class's private and protected members.
Example answer:
In C++, a friend function is a function that is not a member of a class but is declared within the class definition as a 'friend'. It is given special access to the private and protected members of that class.
19. What is dynamic method dispatch?
Why you might get asked this:
This relates to runtime polymorphism (method overriding) and how the correct method is selected.
How to answer:
Explain it as the mechanism where the method call for an overridden method is resolved at runtime based on the actual type of the object, not the reference type.
Example answer:
Dynamic method dispatch is the process where a call to an overridden method is resolved at runtime. The Java Virtual Machine or runtime environment determines which version of the overridden method to execute based on the object's actual type.
20. What is coupling?
Why you might get asked this:
Coupling and cohesion are software design principles influenced by OOPs structure, relevant for maintainability.
How to answer:
Define coupling as the degree of interdependence between software modules or classes. State that low coupling (loose coupling) is desirable for easier maintenance and modification.
Example answer:
Coupling refers to the degree to which different classes or modules are dependent on each other. Low coupling means modules are relatively independent, which is desirable for easier maintenance and changes.
21. What is cohesion?
Why you might get asked this:
Complements the coupling question, focusing on the internal unity of a module.
How to answer:
Define cohesion as the degree to which elements within a module (class) belong together. State that high cohesion is desirable, meaning a class has a single, well-defined responsibility.
Example answer:
Cohesion refers to the degree to which the elements within a single module or class belong together and work towards a single purpose. High cohesion is good design, indicating a class is focused and has one clear responsibility.
22. What is a virtual function?
Why you might get asked this:
Specific to languages like C++ and C#, crucial for implementing runtime polymorphism.
How to answer:
Explain a virtual function as a member function declared in a base class that is expected to be overridden in derived classes. Its call is resolved at runtime using dynamic dispatch.
Example answer:
A virtual function is a function in a base class (like in C++) that is declared with the 'virtual' keyword. It allows a function call to be resolved at runtime, enabling runtime polymorphism via overriding.
23. What is garbage collection in OOP?
Why you might get asked this:
Tests knowledge of memory management in languages like Java, C#, etc., which use automatic memory handling.
How to answer:
Describe garbage collection 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 (used in languages like Java or C#). It automatically finds objects that are no longer accessible or used by the program and deallocates their memory.
24. Can you run a Java application without using OOPs concepts?
Why you might get asked this:
A slightly trick question to confirm understanding that Java is fundamentally object-oriented.
How to answer:
State clearly that no, Java is inherently object-oriented, requiring the use of classes and objects even for the simplest programs (e.g., the main method is inside a class).
Example answer:
No, Java is designed around OOPs principles. Every piece of code resides within a class, and execution starts from a static method within a class, making fundamental OOPs concepts mandatory.
25. What is exception handling?
Why you might get asked this:
Important for writing robust code, exception handling allows graceful management of runtime errors.
How to answer:
Define exception handling as a mechanism to deal with runtime errors (exceptions) in a controlled way, preventing program crashes using structures like try-catch-finally blocks.
Example answer:
Exception handling is a mechanism in OOPs languages to manage runtime errors (exceptions) gracefully. It allows the program to handle unexpected situations using blocks like try, catch, and finally, preventing abrupt termination.
26. What is a static constructor?
Why you might get asked this:
Specific to languages like C#, testing knowledge of how static members are initialized.
How to answer:
Explain a static constructor as a special constructor used to initialize static members of a class. It is 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 members of a class and is executed automatically only once when the class is first loaded into memory, before any instance is created or static member is referenced.
27. What are delegates in C#?
Why you might get asked this:
Specific to C#, delegates are powerful constructs for type-safe function pointers and event handling, showcasing deeper language knowledge.
How to answer:
Define a delegate (in C#) as a type-safe reference to a method. It acts like a function pointer and is commonly used for events and callbacks.
Example answer:
In C#, a delegate is a type that represents references to methods with a particular parameter list and return type. They are used for implementing callbacks and event handling in an object-oriented way.
28. What are SOLID principles?
Why you might get asked this:
SOLID principles are fundamental guidelines for designing maintainable and scalable OOPs software.
How to answer:
List and briefly name each of the five SOLID principles: Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion.
Example answer:
SOLID is an acronym for five design principles: Single Responsibility (one reason to change), Open/Closed (open for extension, closed for modification), Liskov Substitution (derived types must be substitutable for base types), Interface Segregation (many specific interfaces are better than one general), and Dependency Inversion (depend on abstractions, not concrete implementations).
29. What is the advantage of programming to an interface?
Why you might get asked this:
Tests understanding of how interfaces promote flexible, loosely coupled designs in OOPs.
How to answer:
Explain that programming to an interface (or base class) promotes loose coupling, allowing code to work with any implementation of the interface, increasing flexibility, testability, and maintainability.
Example answer:
Programming to an interface means depending on an abstraction rather than a concrete class. This makes your code more flexible, testable, and loosely coupled because you can swap in different implementations of the interface without changing the code that uses it.
30. How does OOP improve code maintainability?
Why you might get asked this:
This is a summary question asking you to synthesize the benefits of OOPs.
How to answer:
Explain that OOPs improves maintainability through modularity (classes encapsulate logic), reusability (inheritance reduces duplication), and clear structure (abstraction hides complexity, encapsulation protects data), making large systems easier to understand, debug, and modify.
Example answer:
OOPs enhances maintainability through modularity (classes bundle code), reusability (inheritance reduces redundancy), encapsulation (data protection simplifies debugging), and abstraction (hiding complexity). This structured approach makes code easier to understand, update, and fix.
Other Tips to Prepare for a OOPs Interview
Mastering OOPs programming interview questions requires more than just memorizing definitions. Practice implementing these concepts in code. Work through examples of inheritance hierarchies, polymorphism in action, and applying encapsulation principles. Be ready to discuss real-world scenarios where you've used these OOPs concepts. As one expert notes, "Interviewers look for candidates who can not only define OOPs terms but explain why and when they are used." Consider using tools like the Verve AI Interview Copilot (https://vervecopilot.com) to practice explaining technical concepts clearly and concisely under pressure. The Verve AI Interview Copilot can provide instant feedback on your articulation and structure when answering common OOPs programming interview questions. Leverage the Verve AI Interview Copilot to refine your examples and ensure you can confidently demonstrate your practical OOPs skills. Remember, preparation using resources like Verve AI Interview Copilot is key to acing those crucial OOPs programming interview questions.
Frequently Asked Questions
Q1: Is OOPs suitable for all projects? A1: While powerful, procedural or functional paradigms might be better for very simple scripts or specific problem types.
Q2: What's the 'is-a' vs 'has-a' relationship? A2: 'Is-a' denotes inheritance (e.g., Dog IS-A Animal). 'Has-a' denotes composition (e.g., Car HAS-A Engine).
Q3: Why prefer composition over inheritance sometimes? A3: Composition offers more flexibility and avoids issues of tight coupling and fragile base class problems associated with inheritance hierarchies.
Q4: What is a class variable vs instance variable? A4: Class variables (static) are shared by all instances. Instance variables are unique to each object instance.
Q5: What is method signature? A5: The method signature typically includes the method name and the number, type, and order of its parameters. Return type is often included depending on language context.
