Variables and Objects in Java

dev.to Java2026-10-30 21:550 views

What is a Variable?

A variable is a named memory location used to store data in a program.

Syntax:

dataType variableName = value;

Example:

int age = 25; String name = "John"; double salary = 25000.50;

Rules for Creating Variables

Names can contain letters, digits, _ and $.
A variable name cannot start with a digit.
Spaces are not allowed.
Java keywords cannot be used as variable names.
Variable names are case-sensitive (age and Age are different).
Use meaningful names and follow camelCase.

Types of Variables in Java

Java has three types of variables:

Local Variables
Instance Variables (Non-static)
Static Variables

1. Local Variables

Declared inside a method, constructor, or block.

Accessible only within that method or block.
Must be initialized before use.
Do not receive default values.

void display() { int age = 25; System.out.println(age); }

Here, age is a local variable.

2. Instance Variables (Non-static)

Declared inside a class but outside methods and constructors.

Each object gets its own copy.
Can have different values for different objects.
Receive default values if not initialized.

class Student { String name; int age; }

Here, name and age are instance variables.

3. Static Variables

Declared inside a class using the static keyword.

Only one shared copy exists for the class.
All objects share the same variable.
Can be accessed using the class name.

class Student { static String college = "ABC College"; }

Here, college is a static variable shared by all Student objects.

Remember: Instance variables belong to objects, whereas static variables belong to the class.

What is an Object?

An object is an instance of a class.

A class is a blueprint used to create objects.

Every object has three characteristics:

State: Properties or data (model, price).
Behavior: Actions (drive, brake).
Identity: Distinguishes one object from another.

Creating a Class and Object

Let's understand this with a simple Car example.

Example:

public class Cars { String model; // Instance variable int price; // Instance variable void drive() { // Instance method System.out.println("Car is driving..."); } public static void main(String[] args) { Cars myCar = new Cars(); // Object creation myCar.model = "XUV"; myCar.price = 600000; myCar.drive(); // Method calling } }