Why OOPs Questions Always Come Up
If you’ve ever sat in a Java interview, you know that Object-Oriented Programming (OOPs) is almost always on the table. It doesn’t matter whether you’re applying as a fresher or an experienced developer—interviewers love asking OOPs questions because they reveal how well you understand the backbone of Java.
I remember one of my first interviews for a junior Java role. I had revised data structures all night, but the very first question was, “Can you explain the difference between abstraction and encapsulation?” That’s when it clicked for me: mastering Java OOPs interview questions and answers isn’t optional—it’s essential.
In this post, I’ll walk you through some of the most common questions, along with clear, practical answers. By the end, you’ll feel more confident tackling them in your next interview.
1. What Are the Four Main Principles of OOP in Java?
This is the classic warm-up question.
Answer: The four principles are:
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Encapsulation: Binding data and methods into a single unit (class).
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Inheritance: Reusing code by deriving new classes from existing ones.
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Polymorphism: One method behaving differently depending on the object.
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Abstraction: Hiding implementation details and exposing only functionality.
👉 Example: Think of a car. You drive it (abstraction) without worrying about how the engine works. Different cars might start differently (polymorphism), but they all inherit traits like wheels and brakes from the general “vehicle” concept.
2. What Is the Difference Between a Class and an Object?
This question tests your ability to differentiate theory from implementation.
Answer:
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A class is a blueprint—a template that defines properties and behavior.
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An object is an instance of a class created in memory.
👉 Example in Java:
class Dog {
String name;
void bark() {
System.out.println(name + " is barking");
}
}
public class Main {
public static void main(String[] args) {
Dog d = new Dog(); // object created
d.name = "Bruno";
d.bark();
}
}
Here, Dog is the class, and d is the object.
3. What Is the Difference Between Method Overloading and Method Overriding?
This is one of the most frequently asked Java OOPs interview questions and answers.
Answer:
-
Overloading: Same method name, different parameter list (compile-time polymorphism).
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Overriding: Subclass provides a specific implementation of a method already defined in the parent class (runtime polymorphism).
👉 Example:
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Overloading: Two add() methods—one for integers, one for doubles.
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Overriding: A draw() method in Shape overridden by Circle or Rectangle.
4. What Is Abstraction in Java?
Interviewers often want to see how you explain this with examples.
Answer: Abstraction is hiding implementation details and exposing only the functionality. In Java, abstraction is achieved using abstract classes and interfaces.
👉 Example: You use a List in Java without worrying about whether it’s backed by an ArrayList or a LinkedList.
5. What Is Encapsulation? How Does Java Achieve It?
Answer: Encapsulation means bundling the data (variables) and methods that operate on that data into a single unit (class). Java achieves encapsulation using:
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Private variables
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Public getter and setter methods
👉 Example: In a banking app, a customer’s balance is private, and only authorized methods can access or modify it.
6. What Is Inheritance in Java?
Answer: Inheritance allows a class (child) to acquire the properties and methods of another class (parent). It promotes code reusability.
👉 Example in Java:
class Vehicle {
void run() {
System.out.println("Vehicle is running");
}
}
class Car extends Vehicle {
void run() {
System.out.println("Car is running");
}
}
Here, Car inherits from Vehicle.
7. What Is Polymorphism in Java?
Answer: Polymorphism means “many forms.” In Java, it allows one interface to be used for different underlying forms (data types).
👉 Example:
-
Compile-time (method overloading)
-
Runtime (method overriding)
class Animal {
void sound() {
System.out.println("Animal makes a sound");
}
}
class Dog extends Animal {
void sound() {
System.out.println("Dog barks");
}
}
Here, the sound() method behaves differently depending on whether the object is Animal or Dog.
8. What Are Interfaces in Java?
Answer: An interface is a blueprint of a class. It contains abstract methods (and from Java 8, default and static methods). A class implements an interface, thereby committing to provide concrete behavior.
👉 Example:
List is an interface. ArrayList and LinkedList are implementations.
9. What Is the Difference Between Abstract Class and Interface?
This is another high-frequency question.
Answer:
-
Abstract Class: Can have both abstract and non-abstract methods, constructors, and instance variables.
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Interface: Prior to Java 8, only abstract methods. From Java 8 onward, interfaces can have default and static methods.
Use an abstract class when classes share a common base. Use an interface when you want multiple classes to follow a contract.
10. Why Are OOPs Concepts Important in Java Interviews?
Interviewers ask these questions not just to test your memory but to see:
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