📦 Java Variables – Declare, Initialize & Use Variables in Java

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🧲 Introduction – What Are Variables in Java?

Every Java program revolves around data — and variables are how we store and manipulate that data. Think of a variable as a container with a label, used to hold different kinds of information like numbers, text, or boolean values.

Whether you’re calculating tax or storing user input, understanding Java variables is a fundamental step.

By the end of this guide, you’ll be able to:

• ✅ Declare and initialize variables
• ✅ Understand Java’s variable types and naming rules
• ✅ Use variables in expressions and output
• ✅ Follow best practices for clean, readable code

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🔑 What Is a Variable in Java?

A variable in Java is a named memory location that stores a value.

Syntax:

type variableName = value;

✅ Example:

int age = 25;

✅ Explanation:

• int: data type (integer)
• age: variable name
• 25: value assigned to the variable

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📚 Types of Variables in Java

Java supports several types of variables, depending on where and how they are declared.

Type	Scope	Defined Inside	Example
Local	Method	Method or block	int count = 10;
Instance	Object	Class (outside any method, without static)	String name;
Static	Class	Class with static keyword	static int totalUsers;

💡 Tip: Always initialize local variables before using them.

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🏷️ Declaring and Initializing Variables

🔹 Declare First, Assign Later:

int score;
score = 100;

🔹 Declare and Assign Together:

String city = "Mumbai";

🔹 Multiple Variables of Same Type:

int x = 10, y = 20, z = 30;

⚠️ Warning: Avoid using undeclared or uninitialized variables — it causes a compile-time error.

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💡 Variable Naming Rules

✅ Allowed:

• Letters (a–z, A–Z)
• Digits (0–9)
• Underscore (_) and Dollar sign ($)

🚫 Not Allowed:

• Names starting with a digit
• Reserved keywords (class, int, etc.)

✅ Examples:

Valid Names	Invalid Names
userName	1user (starts with digit)
total_amount	int (Java keyword)
MAX_SIZE	@value (invalid symbol)

💡 Best Practice: Use camelCase for variables (firstName, totalAmount).

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📐 Example: Using Variables in Java

public class UserProfile {
    public static void main(String[] args) {
        String name = "Alice";
        int age = 30;
        double height = 5.7;

        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
        System.out.println("Height: " + height + " feet");
    }
}

🖨️ Output:

Name: Alice
Age: 30
Height: 5.7 feet

✅ Explanation:

• Different data types are used (String, int, double)
• All values are printed using System.out.println()

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🧪 Java Variable Data Types (Quick Overview)

Type	Description	Example Value
int	Integer numbers	100, -45
double	Decimal numbers	3.14, -9.8
char	Single character	'A', '9'
boolean	True/false value	true, false
String	Sequence of characters	"Hello"

📘 You’ll explore these in detail in the Java Data Types section.

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🧠 Summary – Java Variables Essentials

Java variables are the foundation of storing and manipulating data. You’ve learned:

• How to declare and initialize variables
• Types of variables: local, instance, static
• Naming rules and best practices
• How to use variables in Java programs.

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❓FAQs – Java Variables

❓ What is a variable in Java?

A variable is a container that holds a value — like a label on a storage box in memory.

❓ Can a Java variable name start with a number?

No. Variable names cannot begin with digits, but digits can be used after the first character.

❓ What happens if I don’t initialize a variable?

• Local variables must be initialized before use — or you’ll get a compile error.
• Instance variables get default values (0, false, null, etc.).

❓ What’s the difference between String and char?

• char: stores a single character ('A')
• String: stores a sequence of characters ("Apple")

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