🧮 Java Variables and Data Types

Understanding variables and data types is a core foundation in Java programming. Variables store values, and data types define what kind of data those variables can hold.

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📦 What is a Variable?

A variable in Java is a container that stores data during program execution.

java
int age = 25;
String name = "PrepCampusPlus";

Think of variables as labeled boxes that hold values.

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📚 Types of Variables in Java

Type	Description	Example
🧍 Local Variable	Declared inside a method	int total = 0;
🌐 Instance Variable	Declared inside a class but outside methods	String name;
🏛️ Static Variable	Declared with static keyword, shared across objects	static int count;

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🔤 Java Data Types

Java is statically typed, so you must declare the type of each variable.

java
int count = 100;
double price = 99.99;
char grade = 'A';
boolean isPassed = true;

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🧱 Java Data Types Categories

1️⃣ Primitive Data Types (8 Types)

Data Type	Size	Description	Example
byte	1 byte	Small integer (-128 to 127)	byte b = 10;
short	2 bytes	Medium integer	short s = 1000;
int	4 bytes	Default integer type	int age = 25;
long	8 bytes	Large integers	long population = 7000000000L;
float	4 bytes	Decimal numbers (less precise)	float pi = 3.14f;
double	8 bytes	Decimal numbers (more precise)	double price = 99.99;
char	2 bytes	Single character	char grade = 'A';
boolean	1 bit	true or false	boolean isValid = true;

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2️⃣ Non-Primitive (Reference) Data Types

Type	Example
String	String name = "Java";
Arrays	int[] scores = {90, 80, 70};
Classes, Objects	Student s = new Student();

These types reference memory locations and can hold multiple values or behaviors.

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🧠 Variable Naming Rules

✅ Must start with a letter, $, or _
✅ Can contain numbers but not at the start
❌ No Java keywords (like class, int)
✅ Use meaningful names (age, not a)

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📘 Best Practices

• Use camelCase for variable names: studentName

• Use UPPER_CASE for constants: PI = 3.14159

• Always initialize your variables before using them

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🧪 Code Example

java
public class DataTypeExample {
    public static void main(String[] args) {
        int age = 22;
        float gpa = 8.5f;
        boolean passed = true;
        String name = "John";

        System.out.println(name + " is " + age + " years old.");
    }
}

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📌 Summary Table

Category	Examples
Primitive Types	int, float, char, boolean
Reference Types	String, Arrays, Objects
Variable Types	Local, Instance, Static