Curriculum
Classes and Objects are the foundation of Object-Oriented Programming (OOP) in Java. Every Java application, from a simple student management system to a large enterprise banking platform, is built using classes and objects. Understanding Classes and Objects is essential because they allow developers to model real-world entities and create reusable, scalable, and maintainable software.
Java is an object-oriented programming language, which means most programming tasks are performed using objects created from classes. Before learning advanced OOP concepts such as inheritance, polymorphism, abstraction, and encapsulation, it is important to understand how classes and objects work.
In this lesson, you will learn what classes and objects are, why they are important, how they are created, and how they are used in real-world Java applications.
Object-Oriented Programming (OOP) is a programming paradigm that organizes software around objects rather than functions and logic.
OOP helps developers:
Java uses OOP principles to simplify application development.
Before understanding classes and objects, consider a real-world example.
Imagine a school system.
A student has:
These characteristics can be represented in Java using a class, while each individual student becomes an object of that class.
A class is a blueprint or template used to create objects.
It defines:
A class itself does not occupy memory for actual data until objects are created.
Think of a class as a design plan for constructing a building.
The blueprint describes the structure, but the actual building exists only after construction.
class Student {
}This is the simplest class definition.
A class typically contains:
Used to store data.
Example:
class Student {
String name;
int age;
}Used to define actions.
Example:
class Student {
String name;
void study() {
System.out.println("Student is studying");
}
}Methods define what an object can do.
An object is an instance of a class.
When a class is created, no memory is allocated for its variables.
Memory is allocated only when an object is created.
Example:
Student student1 = new Student();Here:
The object represents a real entity based on the class blueprint.
Consider a Car.
Every car has:
These become class attributes.
class Car {
String brand;
String color;
}Car car1 = new Car();Now car1 becomes an object of the Car class.
After creating an object, values can be assigned.
Example:
Car car1 = new Car();
car1.brand = "Toyota";
car1.color = "White";Accessing values:
System.out.println(car1.brand);
System.out.println(car1.color);Output:
Toyota
WhiteEach object can store different values independently.
One class can create multiple objects.
Example:
Car car1 = new Car();
Car car2 = new Car();Assign values:
car1.brand = "Toyota";
car2.brand = "Honda";Output:
System.out.println(car1.brand);
System.out.println(car2.brand);Result:
Toyota
HondaEach object maintains its own data.
class Student {
String name;
int age;
}Creating objects:
Student student1 = new Student();
Student student2 = new Student();Assigning values:
student1.name = "Rahul";
student1.age = 20;
student2.name = "Priya";
student2.age = 22;Displaying values:
System.out.println(student1.name);
System.out.println(student2.name);Output:
Rahul
PriyaThis demonstrates how one class can represent multiple entities.
Classes become more useful when they contain methods.
Example:
class Student {
String name;
void display() {
System.out.println(name);
}
}Object creation:
Student student1 = new Student();
student1.name = "Rahul";
student1.display();Output:
RahulMethods allow objects to perform actions.
When a class is created:
class Student {
String name;
}No memory is allocated for name.
Memory is allocated only when:
Student student1 = new Student();is executed.
This makes Java memory management more efficient.
Class:
class Employee {
String name;
double salary;
}Object:
Employee emp1 = new Employee();Values:
emp1.name = "Amit";
emp1.salary = 50000;Display:
System.out.println(emp1.name);
System.out.println(emp1.salary);Output:
Amit
50000This concept is widely used in enterprise software.
Example:
Student student1 = new Student();student1 does not directly store the object.
Instead, it stores a reference to the object’s memory location.
This allows Java to manage memory efficiently.
Example:
class Student {
String name;
void study() {
System.out.println(name + " is studying");
}
void attendClass() {
System.out.println(name + " is attending class");
}
}Object:
Student student1 = new Student();
student1.name = "Rahul";
student1.study();
student1.attendClass();Output:
Rahul is studying
Rahul is attending classObjects can perform multiple actions.
Classes and objects provide several benefits.
One class can create many objects.
Changes can be made in one location.
Applications can grow without becoming difficult to manage.
Objects represent real-world entities naturally.
Code becomes structured and easier to understand.
Classes and objects are used in:
Classes:
Customer
Account
TransactionClasses:
Product
Customer
Order
CartClasses:
Patient
Doctor
AppointmentClasses:
Student
Course
FacultyAlmost every software application uses classes and objects.
| Class | Object |
|---|---|
| Blueprint | Instance |
| Logical Entity | Physical Entity |
| No Memory for Data | Occupies Memory |
| Defines Structure | Stores Actual Values |
| Created Once | Can Be Created Many Times |
Example:
Class → Car
Object → Toyota Car
Object → Honda Car
Object → BMW CarIncorrect:
Student.name = "Rahul";Correct:
Student student = new Student();
student.name = "Rahul";Incorrect:
Student student;
student.name = "Rahul";This causes an error because no object exists.
Design classes logically and only when necessary.
These practices improve software quality.
Backend applications heavily depend on classes and objects.
Examples include:
class User {
}class Product {
}class Order {
}class Payment {
}class Response {
}Frameworks such as Spring Boot rely extensively on object-oriented design.
Classes and Objects are the foundation of Object-Oriented Programming in Java. A class acts as a blueprint that defines properties and behaviors, while objects are instances created from that blueprint.
Understanding classes and objects helps developers model real-world entities, organize code effectively, and build scalable applications. These concepts are essential for learning advanced OOP principles such as constructors, inheritance, polymorphism, abstraction, and encapsulation.
As a Java Backend Engineer, mastering classes and objects will enable you to build professional applications and enterprise software systems efficiently.
A class is a blueprint used to create objects, while an object is an instance of a class.
They help organize code, improve reusability, and model real-world systems effectively.
Yes. A single class can create any number of objects.
An object reference stores the memory address of an object.
Yes. Modern backend frameworks such as Spring Boot heavily rely on classes and objects.
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