☕ Java

Variables & Types

Lesson 2 of 5 ~5 min

Variables are named containers that store data values. In Java, every variable has a type that determines what kind of data it can hold. Because Java is statically typed, you must declare the type of a variable before using it.

Understanding variables and types is fundamental to writing Java programs. Every piece of data — numbers, text, true/false values — needs to be stored in a properly typed variable.

Primitive types

→ int — Whole numbers: int age = 25;
→ double — Decimal numbers: double price = 9.99;
→ boolean — True or false: boolean isActive = true;
→ char — Single characters: char grade = 'A';
→ String — Text (not a primitive, but used like one): String name = "Alice";

Declaring and initializing variables

Java requires you to declare a variable's type before assigning a value. You can declare and initialize in one step, or do them separately. Once a variable is declared, you can reassign it — but only with a value of the same type.

Editor

Java

public class Main {
    public static void main(String[] args) {
        int age = 25;
        double price = 19.99;
        boolean isStudent = true;
        char grade = 'A';
        String name = "Alice";

        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
        System.out.println("Price: $" + price);
        System.out.println("Student: " + isStudent);
        System.out.println("Grade: " + grade);
    }
}

Output

Try it

Change the variable values on the left and watch the output update. Try assigning a decimal to int and see what happens.

Type casting

When you need to convert a value from one type to another, Java supports type casting. There are two kinds: widening (automatic, safe) and narrowing (manual, may lose data).

// Widening cast (automatic) — no data lost
int myInt = 9;
double myDouble = myInt;    // int → double: 9.0

// Narrowing cast (manual) — may lose precision
double myDouble2 = 9.78;
int myInt2 = (int) myDouble2; // double → int: 9

System.out.println("Widened: " + myDouble);   // 9.0
System.out.println("Narrowed: " + myInt2);     // 9

The `final` keyword

Use the final keyword to declare a constant — a variable whose value cannot be changed after assignment. This is a best practice for values that shouldn't be modified.

final double PI = 3.14159;
final String APP_NAME = "Archon";

// This would cause a compile error:
// PI = 3.0;  // ❌ Cannot assign to final variable

System.out.println("PI = " + PI);           // 3.14159
System.out.println("App: " + APP_NAME);     // Archon

Key concepts

→ Declaration — int count; creates a variable with type and name.
→ Initialization — count = 0; assigns an initial value.
→ Combined — int count = 0; does both at once.
→ final — Marks a variable as a constant that cannot be reassigned.
→ Naming convention — Variables use camelCase, constants use UPPER_SNAKE_CASE.

Quiz

Which keyword declares a constant in Java?

Challenge

Create one variable of each primitive type (int, double, boolean, char, String) and print all of them with descriptive labels using System.out.println().

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What is Java?

OOP Basics