Java Variables

A variable in Java is a container used to store data values.
Variables allow a program to store, modify, and use information during execution.

What Is a Variable?

A variable is simply a named memory location.

Example:

int age = 25;

Here:

• int → data type
• age → variable name
• 25 → value stored in the variable

Why Variables Are Important

Variables help to:

• Store data
• Perform calculations
• Manage program logic
• Make programs dynamic

Without variables, programs cannot process information effectively.

Syntax of Variable Declaration

Basic Syntax

dataType variableName = value;

Example:

int number = 10;

Types of Variables in Java

Java mainly has 3 types of variables:

1. Local Variables
2. Instance Variables
3. Static Variables

1. Local Variables

A local variable is declared inside a method, constructor, or block.

Example

class Demo {
    void show() {
        int x = 100;
        System.out.println(x);
    }
}

Characteristics

• Created when method starts
• Destroyed when method ends
• Accessible only inside that method

2. Instance Variables

Instance variables are declared inside a class but outside methods.

Each object gets its own copy.

Example

class Student {
    String name;
    int age;
}

Characteristics

• Belong to objects
• Different objects can have different values
• Stored in heap memory

3. Static Variables

Static variables belong to the class, not objects.

Only one copy exists.

Example

class College {
    static String collegeName = "ABC College";
}

Characteristics

• Shared among all objects
• Memory efficient
• Accessed using class name

Example of All Variable Types

class Employee {

    static String company = "Tech Ltd"; // Static variable

    String name; // Instance variable

    void display() {

        int salary = 50000; // Local variable

        System.out.println(name);
        System.out.println(company);
        System.out.println(salary);
    }
}

Java Data Types Used in Variables

Variables store data according to their data types.

Primitive Data Types

Examples of Variables

Integer Variable

int marks = 95;

Decimal Variable

double price = 199.99;

Character Variable

char grade = 'A';

Boolean Variable

boolean isJavaFun = true;

Rules for Naming Variables

Allowed

✅ Letters
✅ Digits
✅ _ underscore
✅ $ dollar sign

Rules

• Must start with a letter, _, or $
• Cannot start with a number
• Cannot use Java keywords
• Variable names are case-sensitive

Valid Variable Names

int age;
double salary;
String studentName;

Invalid Variable Names

int 1age;      // Invalid
int class;     // Keyword
int student-name; // Invalid symbol

Java Naming Conventions

Java follows camelCase naming style.

Good Practice

int studentAge;
double accountBalance;

Variable Initialization

Assigning value to a variable is called initialization.

Example

int number = 20;

Declaration vs Initialization

Declaration

int age;

Initialization

age = 25;

Reassigning Variables

Variable values can change.

int score = 10;
score = 20;

Constants in Java

If a value should never change, use final.

final double PI = 3.14159;

Now PI cannot be modified.

Variable Scope

Scope means where a variable can be accessed.

Local Scope

void test() {
    int x = 10;
}

x exists only inside test().

Class Scope

class Demo {
    int y = 20;
}

y can be used by all methods in the class.

Default Values of Variables

Instance and static variables get default values automatically.

Local variables do NOT get default values.

Example Program

public class VariablesDemo {

    static String company = "OpenAI";

    String employeeName = "Rahul";

    public static void main(String[] args) {

        int age = 25;

        VariablesDemo obj = new VariablesDemo();

        System.out.println("Company: " + company);
        System.out.println("Employee: " + obj.employeeName);
        System.out.println("Age: " + age);
    }
}

Output

Company: OpenAI
Employee: Rahul
Age: 25

Common Mistakes Beginners Make

1. Using Uninitialized Local Variables

int x;
System.out.println(x); // Error

2. Confusing Static and Instance Variables

Static variables belong to class.
Instance variables belong to objects.

3. Wrong Naming

int Student Age; // Invalid