Java Variables

Types of Variables

Java has three kinds of variables, each with a different scope and lifetime.

Type	Declared In	Scope	Default Value
Local	Inside a method or block	Within that method/block only	None (must be initialized)
Instance	Inside a class, outside methods	Entire object	0 / null / false
Static	Inside a class with `static` keyword	Entire class (shared by all objects)	0 / null / false

Variable Types

public class VariableTypes {

    // Instance variable — belongs to each object
    String name = "Java";

    // Static variable — shared across all instances
    static int instanceCount = 0;

    public void display() {
        // Local variable — exists only inside this method
        int localVar = 10;
        instanceCount++;
        System.out.println("Name: " + name);
        System.out.println("Local: " + localVar);
        System.out.println("Count: " + instanceCount);
    }

    public static void main(String[] args) {
        VariableTypes obj1 = new VariableTypes();
        VariableTypes obj2 = new VariableTypes();
        obj1.display();
        obj2.display();
        // instanceCount is now 2 — shared by both objects
    }
}

Naming Conventions

Java follows well-established naming conventions to keep code readable and consistent.

Variables & methods: camelCase — e.g., firstName, calculateTotal()
Classes: PascalCase — e.g., BankAccount, HelloWorld
Constants: UPPER_SNAKE_CASE — e.g., MAX_SIZE, PI
Packages: all lowercase — e.g., com.example.utils
Variable names must start with a letter, $, or _ (not a digit).
Reserved keywords (int, class, for, etc.) cannot be used as names.

Constants with `final`

Use the final keyword to declare a constant. Once assigned, its value cannot be changed.

Constants and var keyword

public class Constants {

    // Class-level constant (static final)
    static final double PI = 3.141592653589793;
    static final int MAX_USERS = 100;

    public static void main(String[] args) {
        // PI = 3.14;  // ERROR: cannot assign a value to a final variable

        // var keyword (Java 10+) — type is inferred by the compiler
        var message = "Hello";      // inferred as String
        var count   = 42;           // inferred as int
        var price   = 9.99;         // inferred as double

        System.out.println("PI = " + PI);
        System.out.println("Max users = " + MAX_USERS);
        System.out.println(message + " " + count + " " + price);

        // var only works for local variables — not for fields or parameters
    }
}

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Java Variables

Types of Variables

Naming Conventions

Constants with final

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Constants with `final`