Classes and Objects Interview Questions & Answers

Introduction

Everything in Object-Oriented Programming revolves around Classes and Objects.

Every Java application—from a simple console application to a large Spring Boot microservice—is built using classes and objects.

Examples include:

• Controller classes
• Service classes
• Repository classes
• Entity classes
• DTO classes
• Configuration classes

Understanding how classes and objects work internally is essential for writing maintainable Java applications and succeeding in technical interviews.

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1. What is a Class?

Answer

A Class is a blueprint or template used to create objects.

It defines:

• Fields (State)
• Methods (Behavior)
• Constructors
• Nested Types

Example

public class Employee {

    private String name;

    public void work() {

        System.out.println("Working");

    }

}

A class itself does not occupy Heap memory for object data until objects are created.

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2. What is an Object?

Answer

An Object is a runtime instance of a class.

Example

Employee employee =

new Employee();

Illustration

Employee Class

        │

        ▼

new Employee()

        │

        ▼

Employee Object

Objects contain actual data and execute methods defined in the class.

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3. What is the difference between a Class and an Object?

Answer

Class	Object
Blueprint	Runtime instance
Logical definition	Physical entity in memory
Defines structure	Holds actual data
Created once	Multiple objects can be created
No object data until instantiated	Occupies Heap memory

A single class can create thousands of objects.

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4. How is an object created in Java?

Answer

Objects are typically created using the new keyword.

Example

Employee employee =

new Employee();

Execution

Class Loaded

↓

Heap Memory Allocated

↓

Constructor Invoked

↓

Reference Returned

↓

Object Ready

The object resides in Heap memory, while the local reference is stored on the thread's Stack.

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5. What happens internally when an object is created?

Answer

When the JVM executes:

Employee employee =

new Employee();

it performs the following steps:

1. Verifies the class is loaded.
2. Allocates Heap memory.
3. Initializes fields with default values.
4. Executes constructors.
5. Returns the object reference.

Memory

Stack

employee

     │

     ▼

Heap

Employee Object

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6. What are instance variables?

Answer

Instance variables belong to individual objects.

Example

class Employee {

    String name;

}

Each object maintains its own copy.

Employee e1 =

new Employee();

Employee e2 =

new Employee();

Memory

Heap

Employee1

name = "John"

-----------------

Employee2

name = "David"

Changing one object does not affect another.

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7. What are static variables?

Answer

Static variables belong to the class rather than individual objects.

Example

class Employee {

    static String company = "ABC";

}

Memory

Employee Class

↓

company = "ABC"

↓

Shared By All Objects

There is only one copy regardless of the number of objects created.

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8. What is a Constructor?

Answer

A constructor initializes an object when it is created.

Example

class Employee {

    Employee() {

        System.out.println("Created");

    }

}

Execution

new Employee()

↓

Constructor

↓

Object Initialized

Constructors have the same name as the class and do not declare a return type.

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9. What is the difference between Constructors and Methods?

Answer

Constructor	Method
Initializes objects	Performs operations
Same name as class	Any valid name
No return type	Has a return type or void
Invoked during object creation	Invoked explicitly

Constructors prepare objects for use, while methods implement business logic.

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10. Where are objects stored in memory?

Answer

Objects are stored in Heap Memory.

Example

Employee employee =

new Employee();

Memory

Stack

employee

      │

      ▼

Heap

Employee Object

Local references reside in Stack memory.

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11. Explain a production use case.

Answer

Scenario

A Spring Boot application receives a customer registration request.

HTTP Request

↓

CustomerDTO

↓

Customer Entity

↓

CustomerService

↓

CustomerRepository

↓

Database

Each request creates multiple objects:

• DTO
• Entity
• Response
• Validation objects

After request completion, temporary objects become eligible for Garbage Collection.

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12. What are common mistakes related to Classes and Objects?

Answer

Common mistakes include:

Creating unnecessary objects.

Using static variables for instance-specific data.

Exposing internal fields directly.

Creating God classes with too many responsibilities.

Ignoring constructors for proper initialization.

Good object-oriented design keeps classes small and focused.

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13. What are the best practices?

Answer

Recommended practices

• Design classes with a single responsibility.
• Keep fields private.
• Use constructors for mandatory initialization.
• Minimize mutable state.
• Prefer immutable objects where appropriate.
• Use static members only for shared data.
• Avoid creating unnecessary objects.
• Follow naming conventions consistently.

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14. How are Classes and Objects used in Spring Boot?

Answer

Spring Boot applications consist of many specialized classes.

Controller

↓

Service

↓

Repository

↓

Entity

↓

DTO

The Spring IoC container manages many of these objects (beans), creating and injecting them where required.

Developers focus on object behavior while Spring manages object lifecycle.

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15. What interview tips should you remember?

Answer

Interviewers commonly ask:

• Class
• Object
• Class vs Object
• Object creation
• Constructors
• Instance variables
• Static variables
• Heap vs Stack
• Spring Beans
• Production scenarios

Remember

• A class is a blueprint.
• An object is a runtime instance.
• Objects are created using new.
• Constructors initialize objects.
• Objects live in Heap memory.
• Local references live in Stack memory.
• Static variables are shared.
• Use real-world examples to explain classes and objects.

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Summary

Classes define the structure and behavior of objects, while objects represent real runtime entities in memory. Understanding object creation, constructors, instance variables, static members, and JVM memory allocation is fundamental for building enterprise Java applications and answering OOP interview questions confidently.

Key Takeaways

• Understand what Classes and Objects are.
• Learn the difference between Classes and Objects.
• Know how objects are created.
• Understand constructors and initialization.
• Learn instance vs static variables.
• Understand Heap and Stack memory usage.
• Apply object-oriented design principles.
• Understand how Spring Boot manages objects.
• Follow enterprise Java best practices.
• Support interview answers with production examples.

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