OOP Interview Questions and Answers (Object-Oriented Programming)

Question 1

Which OOP principle hides internal details and exposes only what's needed?

Accepted Answer

Encapsulation. Encapsulation bundles data with methods and restricts direct access.

Question 2

When a child class provides its own version of a parent's method, it is called:

Accepted Answer

Method overriding. Overriding replaces inherited behaviour; overloading is same name, different parameters.

Question 3

Polymorphism means:

Accepted Answer

The same interface behaves differently for different types. e.g. shape.area() works for Circle and Square, each with its own formula.

Question 4

An abstract class:

Accepted Answer

Cannot be instantiated directly and may have abstract methods. Subclasses must implement its abstract methods before objects can be created.

Question 5

Inheritance lets a class:

Accepted Answer

Reuse and extend the behaviour of another class. class Dog(Animal) gets Animal's attributes and methods.

Question 6

A constructor is:

Accepted Answer

A special method called when an object is created. In Python it's __init__; in Java/C++ it has the class's name.

Question 7

What is an object?

Accepted Answer

An instance of a class. The class is the blueprint; objects are built from it.

Question 8

'Composition over inheritance' suggests:

Accepted Answer

Building classes by combining objects rather than deep class hierarchies. A Car has an Engine, rather than Car being a kind of Engine.

Question 9

In Python, a name starting with a double underscore (e.g. __balance) triggers:

Accepted Answer

Name mangling, to discourage access from outside. It becomes _ClassName__balance, a convention for 'private'.

Question 10

Which is an example of method overloading (in languages like Java)?

Accepted Answer

add(int, int) and add(double, double) in the same class. Same method name, different parameter lists, in the same class.

Question 11

What does super() do in Python?

Accepted Answer

Calls a method from the parent class. Commonly super().__init__(...) inside a child's constructor.

Question 12

A class method in Python receives which first argument?

Accepted Answer

cls (the class). @classmethod methods get the class; instance methods get self; @staticmethod gets neither.

Question 13

Which SOLID principle says a class should have only one reason to change?

Accepted Answer

Single Responsibility Principle. Each class should do one job well.

Question 14

The Open/Closed Principle says software entities should be:

Accepted Answer

Open for extension, closed for modification. Add new behaviour by extending, not by editing tested code.

Question 15

What is the diamond problem?

Accepted Answer

Ambiguity when a class inherits from two classes that share a common parent. Python resolves it with the Method Resolution Order (MRO).

Question 16

Which method controls what print(obj) shows in Python?

Accepted Answer

__str__. __repr__ is the fallback and is meant for developers.

Question 17

An interface (as in Java) mainly defines:

Accepted Answer

Method signatures a class promises to implement. It's a contract without (most) implementation.

Question 18

Which design pattern ensures a class has only one instance?

Accepted Answer

Singleton. Used for things like a single configuration or logger object.

Question 19

The Observer pattern is used when:

Accepted Answer

Many objects need to be notified when one object changes. Like subscribing to notifications or event listeners.

Question 20

Abstraction in OOP means:

Accepted Answer

Showing essential features while hiding implementation details. A user calls car.start() without knowing how the engine works.

Question 21

What are the four pillars of OOP?

Accepted Answer

Encapsulation, abstraction, inheritance, polymorphism. Be ready to give an example of each.

Question 22

What is an object?

Accepted Answer

An instance of a class. The class is the blueprint; objects are made from it.

Question 23

What is a constructor?

Accepted Answer

A special method that runs when an object is created. It sets up the object's initial state.

Question 24

What is encapsulation?

Accepted Answer

Keeping data private and exposing it only through methods. e.g. a private balance with deposit() and withdraw().

Question 25

What is abstraction?

Accepted Answer

Showing only essential features and hiding implementation details. You drive a car without knowing how the engine works.

Question 26

What is inheritance?

Accepted Answer

A class acquiring the properties and methods of another class. It models an 'is-a' relationship: a Car is a Vehicle.

Question 27

What is polymorphism?

Accepted Answer

One interface, many implementations. e.g. shape.area() works for circles and squares.

Question 28

Method overriding happens when:

Accepted Answer

A subclass provides its own version of a parent method with the same signature. Overriding is runtime polymorphism.

Question 29

Method overloading happens when:

Accepted Answer

Methods in the same class share a name but have different parameters. Overloading is compile-time polymorphism.

Question 30

What is an abstract class?

Accepted Answer

A class that can't be instantiated and may have abstract methods. Subclasses fill in the abstract methods.

Question 31

What is an interface?

Accepted Answer

A contract listing methods that implementing classes must provide. A class can implement many interfaces.

Question 32

What is the difference between composition and inheritance?

Accepted Answer

Composition is 'has-a'; inheritance is 'is-a'. A Car has an Engine (composition); a Car is a Vehicle (inheritance).

Question 33

Why is composition often preferred over inheritance?

Accepted Answer

It is more flexible and avoids tight coupling to a parent class. 'Favour composition over inheritance' is a classic design principle.

Question 34

What does the 'super' (or base) keyword do?

Accepted Answer

Refers to the parent class, e.g. to call its constructor or method. super(...) must be the first line of a Java constructor.

Question 35

What is a static method?

Accepted Answer

A method that belongs to the class, not to any object. Call it as ClassName.method().

Question 36

What is a destructor?

Accepted Answer

A method that runs when an object is destroyed, to free resources. C++ has destructors; Java relies on garbage collection instead.

Question 37

What does 'is-a' relationship mean in OOP?

Accepted Answer

Inheritance between a subclass and a superclass. A Dog is an Animal.

Question 38

What is multiple inheritance?

Accepted Answer

A class inheriting from more than one parent class. C++ supports it; Java allows it only through interfaces.

Question 39

What is the diamond problem?

Accepted Answer

Ambiguity when a class inherits the same member from two parents that share a base. C++ solves it with virtual inheritance; Java avoids it by disallowing multiple class inheritance.

Question 40

What is coupling in software design?

Accepted Answer

How much one class depends on another. Aim for low coupling.

Question 41

What is cohesion in a class?

Accepted Answer

How closely its responsibilities belong together. Aim for high cohesion: one class, one job.

Question 42

What does the Single Responsibility Principle say?

Accepted Answer

A class should have only one reason to change. The S in SOLID.

Question 43

What does the Open/Closed Principle say?

Accepted Answer

Classes should be open for extension but closed for modification. Add behaviour with new classes instead of editing working code.

Question 44

What does the Liskov Substitution Principle say?

Accepted Answer

Objects of a subclass should be usable wherever the parent is expected. If a Square breaks code expecting a Rectangle, the design is wrong.

Question 45

What does the Dependency Inversion Principle say?

Accepted Answer

Depend on abstractions, not on concrete classes. e.g. depend on a PaymentGateway interface, not on RazorpayClient directly.

Question 46

What is the Singleton design pattern?

Accepted Answer

A class that allows only one instance. Often used for configuration or logging.

Question 47

What is the Factory design pattern?

Accepted Answer

A method or class that creates objects without exposing the exact class. e.g. ShapeFactory.create("circle").

Question 48

What is the Observer pattern?

Accepted Answer

Objects subscribe to be notified when another object changes. Used for event listeners and notifications.

Question 49

What is a getter and setter?

Accepted Answer

Methods to read and update a private field. They let a class validate changes to its data.

Question 50

What is an instance variable?

Accepted Answer

A variable that belongs to each object separately. Each object has its own copy; static variables are shared.

Question 51

Can a constructor be private?

Accepted Answer

Yes, e.g. to implement a singleton. Then objects are created through a static method.

Question 52

What is dynamic (late) binding?

Accepted Answer

Deciding which overridden method to call at run time. This is what makes runtime polymorphism work.

Question 53

What is an access modifier?

Accepted Answer

A keyword that controls who can use a class member. public, private, protected (and default/package in Java).

Question 54

What is the purpose of the final (Java) / sealed (C#) keyword on a class?

Accepted Answer

To prevent the class from being inherited. String in Java is final.

Question 55

What is a copy constructor?

Accepted Answer

A constructor that creates an object as a copy of another object of the same class. Common in C++: ClassName(const ClassName& other).

Question 56

What is the difference between a shallow and a deep copy?

Accepted Answer

A shallow copy shares nested objects; a deep copy duplicates them too. Changing a nested list in a shallow copy affects the original.

Question 57

What is association in OOP?

Accepted Answer

A general relationship where objects use each other. Aggregation and composition are special kinds of association.

Question 58

In aggregation, what happens to the part when the whole is destroyed?

Accepted Answer

The part can live on independently. A Department has Teachers, but teachers exist without it. In composition, parts die with the whole.

Question 59

What is a virtual function table (vtable)?

Accepted Answer

A table of function pointers used to call overridden methods at run time. Each class with virtual functions has one.

Question 60

Which OOP concept is shown by a Shape reference holding a Circle object?

Accepted Answer

Polymorphism. The reference type is general; the object is specific.

OOP Interview Questions and Answers (Object-Oriented Programming)

Q1. Which OOP principle hides internal details and exposes only what's needed?

Q2. When a child class provides its own version of a parent's method, it is called:

Q3. Polymorphism means:

Q4. An abstract class:

Q5. Inheritance lets a class:

Q6. A constructor is:

Q7. What is an object?

Q8. 'Composition over inheritance' suggests:

Q9. In Python, a name starting with a double underscore (e.g. __balance) triggers:

Q10. Which is an example of method overloading (in languages like Java)?

Q11. What does super() do in Python?

Q12. A class method in Python receives which first argument?

Q13. Which SOLID principle says a class should have only one reason to change?

Q14. The Open/Closed Principle says software entities should be:

Q15. What is the diamond problem?

Q16. Which method controls what print(obj) shows in Python?

Q17. An interface (as in Java) mainly defines:

Q18. Which design pattern ensures a class has only one instance?

Q19. The Observer pattern is used when:

Q20. Abstraction in OOP means:

Q21. What are the four pillars of OOP?

Q22. What is an object?

Q23. What is a constructor?

Q24. What is encapsulation?

Q25. What is abstraction?

Q26. What is inheritance?

Q27. What is polymorphism?

Q28. Method overriding happens when:

Q29. Method overloading happens when:

Q30. What is an abstract class?

Q31. What is an interface?

Q32. What is the difference between composition and inheritance?

Q33. Why is composition often preferred over inheritance?

Q34. What does the 'super' (or base) keyword do?

Q35. What is a static method?

Q36. What is a destructor?

Q37. What does 'is-a' relationship mean in OOP?

Q38. What is multiple inheritance?

Q39. What is the diamond problem?

Q40. What is coupling in software design?

Q41. What is cohesion in a class?

Q42. What does the Single Responsibility Principle say?

Q43. What does the Open/Closed Principle say?

Q44. What does the Liskov Substitution Principle say?

Q45. What does the Dependency Inversion Principle say?

Q46. What is the Singleton design pattern?

Q47. What is the Factory design pattern?

Q48. What is the Observer pattern?

Q49. What is a getter and setter?

Q50. What is an instance variable?

Q51. Can a constructor be private?

Q52. What is dynamic (late) binding?

Q53. What is an access modifier?

Q54. What is the purpose of the final (Java) / sealed (C#) keyword on a class?

Q55. What is a copy constructor?

Q56. What is the difference between a shallow and a deep copy?

Q57. What is association in OOP?

Q58. In aggregation, what happens to the part when the whole is destroyed?

Q59. What is a virtual function table (vtable)?

Q60. Which OOP concept is shown by a Shape reference holding a Circle object?