If you are a B.Tech student like me preparing for placements, you’ve probably noticed one thing OOPs is everywhere. Whether it’s Java interviews, semester exams, coding rounds or project discussions, Object Oriented Programming is always there.
In simple words, OOPs is a way of writing programs using objects and classes so that code becomes reusable, organized, and easy to manage.
When I started learning Java, OOPs felt confusing. Too many terms class, object, inheritance, polymorphism all sounded heavy. But once I started building small projects and practicing interview questions things became much clearer.
Most companies ask OOP questions because:
- They test your programming basics
- They check your thinking and design skills
- OOP is used in real-world software development
So in this blog, I will explain the Top 20 Java OOPs Interview Questions and Answers in a very simple way like we discuss topics before exams or viva.
Important OOP Concepts
Before jumping into questions, let’s quickly revise the core pillars of OOP.
1. Class A class is a blueprint or template to create objects.
2. Object An object is a real-world entity created using a class.
3. Encapsulation Binding data and methods together inside a class.
4. Abstraction Showing only important details and hiding implementation.
5. Inheritance One class acquiring properties of another class.
6. Polymorphism One thing behaving in multiple ways.
If you understand these properly, half of the interview is already cleared.
Top 20 OOP Interview Questions and Answers
1. What is Object-Oriented Programming (OOP)?
Definition:
Object-Oriented Programming is a programming approach where software is designed using objects and classes.
Basically, instead of writing everything in one long code, we divide it into small reusable objects.
Syntax (Java Example):
class Student {
int rollNo;
String name;
}
Example:
In college ERP systems, Student, Teacher, Course are treated as objects.
Real life Understanding:
Think of a car. It has properties (color, model) and actions (start, stop). Same way objects work in OOP.
2. What is a Class in Java?
Definition:
A class is a blueprint to create objects.
Many students get confused here. Class is not the real thing — it’s just the design.
Syntax:
class Mobile {
String brand;
int price;
}
Example:
Before building a hostel, the engineer creates a design plan. That plan is like a class.
3. What is an Object?
Definition:
An object is an instance of a class.
In simple words, object is the real thing created using a class.
Syntax:
Mobile m1 = new Mobile();
Example:
If “Mobile” is a class, then Samsung phone in your hand is an object.
4. What is Encapsulation?
Definition:
Encapsulation means wrapping data and methods together in a single unit (class).
It also means restricting direct access using private variables.
Syntax:
class Student {
private int marks;
public void setMarks(int m) {
marks = m;
}
public int getMarks() {
return marks;
}
}
Example:
ATM machine hides internal process. You just enter PIN and get money.
5. What is Abstraction?
Definition:
Abstraction means hiding implementation details and showing only functionality.
Syntax:
abstract class Shape {
abstract void draw();
}
Example:
When we use a mobile app, we don’t know backend logic. We just use features.
6. Difference Between Abstraction and Encapsulation
| Feature | Abstraction | Encapsulation |
|---|---|---|
| Focus | Hiding implementation | Hiding data |
| Achieved By | Abstract class, Interface | Private variables |
| Example | Car driving mechanism | ATM PIN security |
7. What is Inheritance?
Definition:
Inheritance allows one class to use properties of another class.
Syntax:
class Animal {
void eat() {}
}
class Dog extends Animal {
void bark() {}
}
Example:
Child inheriting features from parents.
8. Types of Inheritance in Java
- Single
- Multilevel
- Hierarchical
Java does not support multiple inheritance using classes.
9. What is Polymorphism?
Definition:
Polymorphism means one method behave different in different situations.
Types:
- Compiletime (Method Overloading)
- Runtime (Method Overriding)
10. What is Method Overloading?
Definition:
Same method name but different parameters.
Syntax:
int add(int a, int b) {}
int add(int a, int b, int c) {}
Example:
Calculator performing addition of 2 or 3 numbers.
11. What is Method Overriding?
Definition:
Subclass provides specific implementation of parent method.
Syntax:
class Animal {
void sound() {}
}
class Dog extends Animal {
void sound() {}
}
12. What is Constructor?
Definition:
Constructor initializes object values.
Syntax:
class Student {
Student() {
System.out.println("Constructor called");
}
}
13. Types of Constructors
- Default Constructor
- Parameterized Constructor
14. What is the ‘this’ Keyword?
Definition:
Refers to current class object.
Syntax:
this.name = name;
15. What is the ‘super’ Keyword?
Definition:
Refers to parent class object.
Syntax:
super.show();
16. What is an Interface?
Definition:
Interface contains abstract methods that classes must implement.
Syntax:
interface Vehicle {
void start();
}
17. Difference Between Abstract Class and Interface
| Feature | Abstract Class | Interface |
|---|---|---|
| Methods | Abstract + Concrete | Only Abstract (mostly) |
| Variables | Any type | Public static final |
| Use | Partial abstraction | Full abstraction |
18. What is Association?
Definition:
Relationship between two classes.
Example:
Teacher teaches Students.
19. What is Aggregation?
Definition:
Weak relationship where one class contains another.
Example:
Department has Professors.
20. What is Composition?
Definition:
Strong relationship. One object depends fully on another.
Example:
House has Rooms. If house destroyed, rooms also gone.
Real World Examples of OOP (Easy to Relate)
When I made my mini project in 3rd year, OOP concepts became super clear.
College Management System
- Class: Student, Teacher, Course
- Object: Individual students
- Encapsulation: Student marks hidden
- Inheritance: UGStudent extends Student
- Polymorphism: Payment method (UPI/Card)
Hostel Example
- Hostel → Class
- Rooms → Objects
- Common Rules → Encapsulation
- New Hostel Block → Inheritance
FAQs (Common Student Doubts)
Is OOP only used in Java?
No. OOP is used in C++, Python, C#, etc.
Why is OOP important for interviews?
It shows your design thinking and coding structure.
Which OOP concept is most asked?
Encapsulation, Inheritance, Polymorphism.
Can we learn OOP without projects?
You can, but projects make it much clearer.
Is OOP difficult?
At first yes. But once basics are clear, it becomes easy.
Conclusion
OOPs is not just an interview topic. It’s the base of real software development.
When I started preparing for placements, I used to mug up definitions. Didn’t help much. But when I practiced coding and built projects, OOP started making real sense.
So my honest suggestion:
- Don’t just read theory
- Practice small programs
- Relate concepts to real life
- Revise interview questions regularly
Once your OOP basics are strong, Java becomes much easier. And interviews feel less scary.
All the best for your preparation. You got this 👍
