Variables

A variable is a named place to store a value so you can use it later: a price, a name, a count. In Java every variable has a type that decides what it can hold, and the compiler checks that you use it correctly.

Declaring and assigning

void main() {
    int age = 30;
    String name = "Ana";
    double price = 19.99;
    boolean member = true;

    System.out.println(name + " is " + age);
    System.out.println("Price: " + price + ", member: " + member);
}

Ana is 30
Price: 19.99, member: true

The form is type name = value;. int, double and boolean are primitive types (next lesson); String is a class for text.

Changing a value

void main() {
    int score = 10;
    System.out.println(score);
    score = 25;
    System.out.println(score);
    score = score + 5;
    System.out.println(score);
}

10
25
30

After declaring a variable, assign new values without repeating the type. score = score + 5 reads the current value, adds 5 and stores the result.

Declare first, assign later

void main() {
    int total;
    total = 7 * 6;
    System.out.println(total);
}

42

A local variable must be given a value before it's read; the compiler checks this:

void main() {
    int total;
    System.out.println(total);
}

Main.java:3: error: variable total might not have been initialized

Types are fixed

void main() {
    int count = 5;
    count = "five";
}

Main.java:3: error: incompatible types: String cannot be converted to int

A variable's type can't change. This is what "statically typed" means, and it lets the compiler catch whole classes of bugs before the program runs.

Constants with final

void main() {
    final double TAX_RATE = 0.23;
    TAX_RATE = 0.2;
}

Main.java:3: error: cannot assign a value to final variable TAX_RATE

final makes a variable unchangeable after it's assigned. Use it for values that shouldn't change; constants are conventionally named in UPPER_SNAKE_CASE.

Type inference with var

void main() {
    var city = "Lisbon";      // String
    var year = 2026;          // int
    var ratio = 0.75;         // double

    System.out.println(city + " " + year + " " + ratio);
}

Lisbon 2026 0.75

With var (Java 10+), the compiler works out the type from the value. The variable is still statically typed — city is a String forever. Use var when the type is obvious from the right-hand side; spell it out when that helps readers.

Several variables at once

void main() {
    int x = 1, y = 2, z = 3;
    System.out.println(x + y + z);
}

6

Allowed, but one declaration per line is usually clearer.

Try it yourself

Store a product name, a unit price and a quantity, then print the total cost.

4 x Notebook = 14.0