📝 What You’ll Learn

• What variables are
• How to declare and assign values
• Variable naming rules
• Constants with final
• Intro to var (type inference)

📦 What is a Variable?

A variable is a name that refers to a value stored in memory. It's like a labeled box that holds data your program needs.

int age = 25;
String name = "Alice";

✍️ Declaring a Variable

Syntax:

type variableName = value;

Examples:

int number = 10;
String city = "Tokyo";

🔁 Updating Variables

int score = 100;
score = 150;  // updated value

You can declare first, assign later:

int level;
level = 5;

🚫 Constants with final

Use the final keyword to make a variable read-only after it’s assigned.

final double PI = 3.14159;
// PI = 3.14; ❌ Error – cannot reassign

✅ Constants are usually written in UPPERCASE with underscores: MAX_SPEED, DEFAULT_TIMEOUT.

🤖 Type Inference (Java 10+)

If you're using Java 10 or newer, you can use var to let the compiler infer the type:

var name = "Java";   // String
var count = 5;       // int

⚠️ Note: var must be initialized on declaration.

✅ Naming Rules & Best Practices

• Must start with a letter, _, or $
• Can contain letters and numbers
• Cannot use spaces or start with a digit
• Avoid Java reserved words like int, class, public, etc.

✅ Good names:

userAge, totalScore, MAX_VALUE

❌ Avoid:

int, 2cool, user age

📘 Recap

• Variables store values that your program uses and updates
• Declare them with a type, or use var in modern Java
• Use final for constants
• Follow naming rules and use descriptive names