Java OOPs Interview Questions and Answers in 2025 (With Real Examples & My Experience)
If you’re preparing for Java interviews in 2025, you must master Object-Oriented Programming (OOPs). After spending 15+ years in Java development, interviewing hundreds of candidates, and working across real enterprise systems, I can confidently say:
👉 OOPs is still the #1 filtering round in Java interviews.
👉 90% of developers fail because they memorize, not understand.
This guide fixes that.
You’ll get clear explanations, real-life examples, diagrams, use cases, and my personal tips that interviewers actually expect.
Blog is divided into 2 different sections, they are :
1. What is OOPs in Java?
OOPs (Object-Oriented Programming System) is a programming paradigm based on the concept of objects, which contain data (fields) and behavior (methods).
Four pillars of OOPs:
-
Encapsulation
-
Inheritance
-
Polymorphism
-
Abstraction
2. Java OOPs Interview Questions & Answers (2025)
Q1. What is a Class and an Object?
Answer:
A class is a blueprint.
An object is a real-time instance of that blueprint.
Example:
class Car {
String color;
void drive() { }
}
Car myCar = new Car(); // object
Diagram
Class (Car)
├── color
└── drive()
Object (myCar)
├── color = "Red"
└── drive()
Real Use Case
👉 In microservices, each service instance is like an object created from a service template/class.
Q2. What is Encapsulation? Why is it important?
Answer:
Encapsulation is binding data with methods while keeping details private using private, public, or protected.
Example:
class BankAccount {
private double balance;
public void deposit(double amt) {
balance += amt;
}
}
[BankAccount]
+ deposit()
- balance (hidden)
Personal Insight:
While building fintech apps, encapsulation protected sensitive fields such as
accountNumber, avoiding accidental modifications.
Q3. What is Inheritance?
Inheritance allows one class to acquire properties of another.
Example:
class Vehicle { }
class Bike extends Vehicle { }
Vehicle
↑
Bike
Use Case: Every Spring Boot REST controller you write indirectly inherits from DispatcherServlet mappings. Q4. What is Polymorphism? (Runtime & Compile-time)
Answer:
Polymorphism = one name, many forms
Two Types
Compile-time → Method Overloading
Run-time → Method Overriding
Example (Overriding):
class Animal { void sound(){ } }
class Dog extends Animal { void sound(){ System.out.println("Bark"); } }
Real Use Case:
Polymorphism makes frameworks like Spring powerful:
PaymentService service;Spring injects different implementations using polymorphism.
Q5. What is Abstraction?
Hiding complex logic and showing only essential details.
Example with Interface
interface Payment {
void pay();
}
Use Case:UPI payment apps hide:
- Encryption
- Routing
- Authentication
You only see “Pay”.
Q6. What is the difference between an Interface and an Abstract Class?
| Feature | Interface | Abstract Class |
|---|---|---|
| Default methods | Yes | Yes |
| Constructors | No | Yes |
| Multiple inheritance | Yes | No |
| Fields | static + final | non-final, inherited |
Personal Tip:
If you want to define capabilities, use interface.
If you want partial implementation, use abstract class.
Q7. What is Association, Aggregation, Composition?
Association: Loose relationship
Aggregation: “Has-a”, but independent
Composition: “Part-of”, strongly dependent
Diagram
If you want to define capabilities, use interface.
If you want partial implementation, use abstract class.
Composition: Human ──► Heart (strong)
Aggregation: Library ──▷ Books (weak)
Q8. What is Constructor Overloading?
Defining multiple constructors with different parameters.
class Student {
Student(){}
Student(String name){}
}
Q9. What is ‘super’ keyword used for?
-
Call parent constructor
-
Access parent methods/variables
Q10. What is Method Overriding Rules (2025 version)?
-
Same name
-
Same parameters
-
Cannot reduce visibility
-
Can throw unchecked exceptions
-
Should use
@Override
Q11. What is the Diamond Problem? How does Java solve it?
Java doesn’t allow multiple inheritance with classes → avoids diamond problem.
Interfaces default methods + conflict resolution rules fix this.
Q12. What is Object Cloning?
Creating a copy using:
Object clone() throws CloneNotSupportedException;
But in real projects → avoid clones. Use copy constructors.
Q13. What is the difference between Object Equality vs Reference Equality?
== → reference check
.equals() → value check
Q14. Can Java achieve 100% abstraction?
Yes → with Interfaces only.
Q15. Why is OOP still relevant in 2025?
Because Java frameworks (Spring, Hibernate, Quarkus) still rely on OOP fundamentals such as:
-
Dependency Injection
-
Polymorphism
-
Encapsulation
-
Abstraction
Real-World OOP Use Cases (From My Experience)
1. Banking System
-
Encapsulation protects balance
-
Inheritance for different account types
-
Polymorphism for different interest calculations
2. Spring Boot Microservices
-
Interfaces for service contracts
-
Polymorphism for injecting implementations
-
Abstraction in Feign clients
3. E-commerce Platforms
-
Product → inheritance
-
Payment → abstraction
-
Cart → composition
ASCII OOP Diagram: Complete Picture
┌──────────────────┐
│ Object │
└──────────────────┘
▲
│
┌───────────────┼───────────────┐
│ │ │
Encapsulation Inheritance Abstraction
│ │ │
└───────────────┼───────────────┘
▼
Polymorphism
Example Code (Interview Friendly)
Inheritance + Polymorphism
abstract class Notification {
abstract void send();
}
class Email extends Notification {
void send() { System.out.println("Sending Email"); }
}
class SMS extends Notification {
void send() { System.out.println("Sending SMS"); }
}
FAQs
1. Is OOP enough for Java interviews?
Yes, but combine with Collections, Multithreading, Streams, and Spring.
2. Why do interviewers ask so many OOP questions?
It reveals your architecture and problem-solving skills.
3. Should I give textbook definitions?
No — interviewers prefer examples + real-life logic.
4. What is the most misunderstood concept?
Polymorphism — especially runtime vs compile-time.
5. How to master OOP quickly?
Build 2–3 micro projects:
Payment System
Library Management
E-commerce Cart
Final Tips to Crack Java OOPs Interviews
✔ Don’t memorize definitions — explain with examples
✔ Draw diagrams if asked (big + immediate impression)
✔ Showcase your architecture skills
✔ Demonstrate why OOP matters in real-world systemsWant to prepare for more Java interview concepts?
👉 Check out my guides on Java Lambda Expressions, Generics, and Pattern Matching.
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