Do you need help preparing for your Java OOPS interview? Object-oriented programming (OOP) is a core aspect of Java, and a strong understanding of its principles is essential. This guide covers the top 50 Java OOPS interview questions and answers, addressing both fundamental and advanced topics.
With clear explanations and practical examples, these questions will help you approach technical rounds with confidence.
Fun Fact: According to the TIOBE Index (2025), Java remains a top 5 programming language, with over 9 million developers using it. A major reason for its popularity is its strong OOPS foundation.
Basic Java OOPS Interview Questions
Here is a list of basic Java Object Oriented Programming questions and answers for interviews:
- What are the four main principles of Object-Oriented Programming?
The four main principles of OOPS are:
- Encapsulation – Wrapping data and methods into a single unit (class) to restrict direct access to data.
- Abstraction – Hiding implementation details and exposing only necessary functionalities using abstract classes or interfaces.
- Inheritance – Allowing one class (child) to inherit properties and methods from another (parent) to promote reusability.
- Polymorphism – Allowing a single method or operator to have multiple implementations (method overloading and method overriding).
- How is abstraction different from encapsulation in Java?
Abstraction hides unnecessary details and exposes only the essential parts. It is implemented using abstract classes and interfaces. Encapsulation, on the other hand, restricts direct access to an object’s data by using access modifiers like private, protected, and public. While abstraction is about hiding implementation, encapsulation is about data security and integrity.
- What is method overloading and method overriding? Provide examples.
- Method Overloading: When multiple methods in the same class have the same name but different parameter lists.
Example:
class MathOperations {
int add(int a, int b) { return a + b; }
double add(double a, double b) { return a + b; }
}
- Method Overriding: When a subclass provides a specific implementation of a method already defined in its superclass.
Example:
class Parent {
void display() { System.out.println(“Parent class method”); }
}
class Child extends Parent {
@Override
void display() { System.out.println(“Child class method”); }
}
- Why is multiple inheritance not supported in Java?
Multiple inheritance is not supported in Java to avoid ambiguity issues caused by the diamond problem. If two parent classes have the same method, the compiler cannot determine which one to inherit. Instead, Java provides interfaces, allowing a class to implement multiple interfaces without ambiguity.
Java OOPS Interview Questions for Freshers
Here are some common Java Object Oriented Programming interview questions for freshers:
- What is the difference between a class and an object?
A class is a blueprint for creating objects. It defines attributes (variables) and behaviors (methods). An object is an instance of a class with specific values assigned to its attributes.
Example:
class Car {
String brand;
void drive() { System.out.println(“Car is driving”); }
}
Car myCar = new Car(); // Object creation
- What is the significance of the ‘this’ keyword in Java?
The this keyword refers to the current instance of a class. It is used to:
- Differentiate instance variables from local variables when they have the same name.
- Call another constructor in the same class.
- Pass the current instance as a parameter.
Example:
class Employee {
String name;
Employee(String name) { this.name = name; }
}
- How does Java achieve runtime polymorphism?
Java achieves runtime polymorphism through method overriding. The overridden method in a subclass is called at runtime based on the object type, even when referenced by a parent class.
Example:
class Animal {
void sound() { System.out.println(“Animal makes a sound”); }
}
class Dog extends Animal {
void sound() { System.out.println(“Dog barks”); }
}
Animal obj = new Dog();
obj.sound(); // Outputs: Dog barks
- What is the purpose of the ‘super’ keyword in Java?
The super keyword is used to refer to the immediate parent class. It can be used to:
- Call the parent class constructor.
- Access parent class methods.
- Access parent class variables.
Example:
class Parent {
void display() { System.out.println(“Parent method”); }
}
class Child extends Parent {
void show() { super.display(); }
}
Java OOPS Interview Questions for Experienced
Let’s go through important Java OOPS programming interview questions and answers for experienced candidates:
- How does Java manage memory with respect to objects?
Java uses automatic memory management with the help of the Garbage Collector (GC). When an object is no longer referenced, the GC removes it to free up memory. Java memory consists of:
- Heap (stores objects).
- Stack (stores method calls and local variables).
- Method area (stores class structures).
- What are the different types of constructors in Java?
- Default Constructor – No parameters, initializes objects with default values.
- Parameterized Constructor – Takes arguments to initialize instance variables.
- Copy Constructor – Copies values from one object to another.
Example:
class Student {
String name;
Student(String name) { this.name = name; }
}
- How is object cloning implemented in Java?
Java supports shallow cloning using the clone() method from the Cloneable interface.
Example:
class Employee implements Cloneable {
String name;
Employee(String name) { this.name = name; }
protected Object clone() throws CloneNotSupportedException {
return super.clone();
}
}
Deep cloning requires manual copying of referenced objects.
- What is a shallow copy and deep copy in Java?
- Shallow Copy: Copies field values but does not create new referenced objects. Changes in the original object affect the copied object.
- Deep Copy: Creates a new copy of referenced objects, making them independent.
Example of deep copy:
class Address {
String city;
Address(String city) { this.city = city; }
}
class Person {
String name;
Address address;
Person(String name, Address address) {
this.name = name;
this.address = new Address(address.city); // Deep Copy
}
}
Java OOPS Interview Questions for 2 Years Experienced
If you have 2 years of experience, you might come across such Java and OOPS interview questions:
- Why did you choose Java for your career?
- Describe a situation where you had to debug a complex object-oriented issue. How did you solve it?
- If you had to redesign an existing system to improve reusability, which OOPS principles would you focus on and why?
Java OOPS Interview Questions for 3 Years Experienced
These interview questions for OOPS in Java are for candidates with three years of experience:
- What is the most challenging Java project you have worked on?
- How do you handle a situation where your team disagrees on the best OOPS approach for a project?
- If you need to implement a flexible payment system, which OOPS concepts would you apply and how?
Java OOPS Interview Questions for 5 Years Experienced
These Java Object Oriented interview questions are for candidates with 5 years of experience:
- What design patterns have you used in your Java projects, and why?
- How do you mentor junior developers on OOPS principles?
- You need to refactor a monolithic Java application into a microservices-based architecture. How would you approach the OOPS design?
Java OOPS Interview Questions for 10 Years Experienced
If you are at a senior level and have around 10 years of experience, you might come across such Java OOPS interview questions:
- How has your understanding of OOPS evolved over the years?
- Have you ever had to optimize an object-oriented Java system for performance? How did you do it?
- Given an existing Java application with tight coupling, how would you refactor it to follow SOLID principles?
OOPS Concepts Interview Questions in Java
You might also come across OOPS concepts in Java interview questions like these:
- What is the difference between an interface and an abstract class?
An interface defines a contract that classes must follow. It contains only abstract methods (before Java 8) and allows default and static methods (from Java 8). Interfaces support multiple inheritance since a class can implement multiple interfaces.
An abstract class can have both abstract and concrete methods. It can include constructors and instance variables. Unlike interfaces, abstract classes can have method implementations but do not support multiple inheritance.
- How does Java implement multiple inheritance?
Java does not support multiple inheritance through classes to avoid ambiguity (diamond problem). Instead, it uses interfaces. A class can implement multiple interfaces, allowing it to inherit behaviors from different sources without conflicts.
- What is dynamic method dispatch in Java?
Dynamic method dispatch, also called runtime polymorphism, is the process where method calls are resolved at runtime based on the object’s actual type, not the reference type.
Core Java OOPS Interview Questions
These are some core Java OOP questions and answers for interviews:
- What is an association, aggregation, and composition in Java?
- Association: A relationship between two classes where both objects exist independently (e.g., Student and Teacher).
- Aggregation: A weaker relationship where the child can exist independently, but the parent owns it (e.g., Department and Employee).
- Composition: A strong relationship where the child object’s existence depends on the parent (e.g., Car and Engine).
- What is the role of access modifiers in OOPS?
Access modifiers control visibility of class members:
- private – Accessible only within the class.
- default – Accessible within the same package.
- protected – Accessible in the same package and subclasses.
- public – Accessible from anywhere.
Advanced Java and OOPs Interview Questions
Let’s go through some advanced Java Object Oriented interview questions and answers:
- What are the different types of design patterns in Java?
- Creational Patterns – Singleton, Factory, Builder.
- Structural Patterns – Adapter, Composite, Proxy.
- Behavioral Patterns – Strategy, Observer, Command.
- How do lambda expressions fit into Java’s object-oriented model?
Lambda expressions provide a concise way to implement functional interfaces (interfaces with a single abstract method). They allow inline implementations without creating a separate class.
Example:
interface Calculator {
int operate(int a, int b);
}
Calculator add = (a, b) -> a + b;
System.out.println(add.operate(5, 3)); // Outputs: 8
- What is the function of reflection in Java OOPS?
Reflection allows introspection and manipulation of classes, methods, and fields at runtime. It is useful in frameworks, serialization, and dependency injection.
Example:
Class<?> obj = Class.forName(“java.util.ArrayList”);
System.out.println(obj.getMethods());
Also Read - Top 25+ Python OOPs Interview Question (2025)
JavaScript Object Oriented Interview Questions
You should also take a look at these OOPS in JavaScript interview questions:
- How is object-oriented programming implemented in JavaScript?
JavaScript is prototype-based, meaning objects inherit from other objects instead of classes. Objects can be created using constructors, prototypes, or ES6 classes.
- What is prototypal inheritance, and how does it differ from classical inheritance?
Prototypal inheritance allows objects to inherit properties directly from another object using the prototype chain. Unlike classical inheritance, which relies on class hierarchies, JavaScript objects inherit dynamically.
Example:
let parent = { greet: function() { console.log(“Hello”); } };
let child = Object.create(parent);
child.greet(); // Outputs: Hello
Also Read - Top 20 PHP OOPs Interview Questions and Answers
OOPs Coding Questions in Java
Here are some coding Java object oriented interview questions:
- Write a Java program to demonstrate method overriding.
class Parent {
void show() { System.out.println(“Parent class method”); }
}
class Child extends Parent {
@Override
void show() { System.out.println(“Child class method”); }
}
public class Test {
public static void main(String[] args) {
Parent obj = new Child();
obj.show(); // Outputs: Child class method
}
}
- Implement a singleton class in Java.
class Singleton {
private static Singleton instance;
private Singleton() {} // Private constructor
public static Singleton getInstance() {
if (instance == null) {
instance = new Singleton();
}
return instance;
}
}
- Create an interface and implement it in multiple classes with different behaviors.
interface Animal {
void sound();
}
class Dog implements Animal {
public void sound() { System.out.println(“Dog barks”); }
}
class Cat implements Animal {
public void sound() { System.out.println(“Cat meows”); }
}
public class Test {
public static void main(String[] args) {
Animal a1 = new Dog();
Animal a2 = new Cat();
a1.sound();
a2.sound();
}
}
Also Read - Top 20 C++ OOPs Interview Questions and Answers
OOPS Practice Questions in Java
- Implement a real-world example of polymorphism in Java.
- Create a Java class that follows the principle of encapsulation.
- Write a Java program to demonstrate the Factory Design Pattern.
- Implement an abstract class with a concrete method and abstract methods.
Also Read - Top 30+ C# OOPs Interview Questions and Answers
Java OOPS Viva Questions
- What is the difference between instance and static methods?
- How does Java handle object destruction?
- Can a constructor be private? If yes, when would you use it?
- What is the difference between early binding and late binding in Java?
Also Read - Top 20 OOPs ABAP Interview Questions and Answers
OOPS MCQ Questions in Java
Here are some common interview questions for OOPS in Java in MCQ form:
- Which of the following is not an OOPS principle?
a) Encapsulation
b) Inheritance
c) Compilation
d) Polymorphism
Answer: c) Compilation
- What is the default access modifier for a class in Java if none is specified?
a) Private
b) Public
c) Protected
d) Package-private
Answer: d) Package-private
- Which keyword is used to prevent a class from being inherited?
a) Final
b) Static
c) Abstract
d) Private
Answer: a) Final
- What will happen if a subclass does not override an abstract method of its superclass?
a) It will compile successfully
b) It will throw a runtime error
c) It will result in a compilation error
d) It will execute with a warning
Answer: c) It will result in a compilation error
- Which of the following is true about Java constructors?
a) A class can have only one constructor
b) Constructors cannot be overloaded
c) A constructor does not have a return type
d) Constructors must be explicitly defined in a class
Answer: c) A constructor does not have a return type
Also Read - Top 50+ OOPs Interview Questions and Answers for 2025
Wrapping Up
So, these are the top Java OOPS interview questions you should prepare for. Understanding these concepts will help you confidently tackle technical interviews and show your expertise in object-oriented programming.Looking for Java OOPS job opportunities? Check out Hirist—India’s top job portal for tech professionals, where you can find roles that match your skills.
