1. What is a Variable

1.1 Definition

A variable is a container used to store data that can be used and modified during program execution.

1.2 Purpose of Variables

• Store values (numbers, text, etc.)
• Reuse data in programs
• Make programs dynamic and flexible

1.3 Memory Representation

When a variable is created, memory is allocated in the system to store its value.

1.4 Naming Rules

• Must start with a letter, _, or $
• Cannot start with a number
• Cannot use Java keywords
• Case-sensitive

1.5 Naming Conventions

• Use meaningful names
• Follow camelCase (e.g., studentName, totalMarks)

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2. Primitive Data Types

2.1 Overview

Primitive data types are basic data types built into Java. They store simple values directly.

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2.2 int

Used to store whole numbers.

int age = 25;

• Size: 4 bytes
• Range: -2,147,483,648 to 2,147,483,647

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2.3 float

Used to store decimal numbers (single precision).

float price = 99.99f;

• Size: 4 bytes
• Requires f suffix

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2.4 double

Used for decimal numbers with higher precision.

double pi = 3.14159;

• Size: 8 bytes
• More precise than float

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2.5 char

Used to store a single character.

char grade = 'A';

• Size: 2 bytes
• Uses Unicode

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2.6 boolean

Used to store true or false values.

boolean isActive = true;

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2.7 byte

Used to store small integer values.

byte level = 10;

• Size: 1 byte
• Range: -128 to 127

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2.8 short

Used for small integer values.

short year = 2025;

• Size: 2 bytes

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2.9 long

Used for large integer values.

long population = 9000000000L;

• Size: 8 bytes
• Requires L suffix

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3. Non-Primitive Data Types

3.1 Definition

Non-primitive data types store references to objects rather than actual values.

3.2 Differences

Primitive	Non-Primitive
Stores value	Stores reference
Fixed size	Variable size
No methods	Has methods

3.3 Common Types

• String
• Arrays
• Classes
• Objects

3.4 Memory Storage

• Primitive → Stack memory
• Non-Primitive → Heap memory

3.5 Example

String name = "Pooja";

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4. Declaring and Initializing Variables

4.1 Declaration

Declaring a variable means specifying its type and name.

int number;

4.2 Initialization

Assigning a value to a variable.

number = 10;

4.3 Declaration + Initialization

int number = 10;

4.4 Multiple Variables

int a = 1, b = 2, c = 3;

4.5 Constants

Use final keyword.

final double PI = 3.14;

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5. Type Casting

5.1 Introduction

Type casting is the process of converting one data type into another.

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5.2 Implicit Casting (Widening)

Definition

Automatic conversion from smaller type to larger type.

Example

int num = 10;
double value = num;

• No data loss
• Done automatically

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5.3 Explicit Casting (Narrowing)

Definition

Manual conversion from larger type to smaller type.

Example

double num = 10.5;
int value = (int) num;

• May cause data loss
• Requires casting operator

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Common Type Conversion Order

byte → short → int → long → float → double

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Conclusion

Variables and data types are the building blocks of Java programming. Once you understand how to store data, choose the right type, and convert between types, you can start building meaningful programs. These concepts will be used in almost every Java application you create.