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.

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: