Learn Java Variables – Exercises Online

int age = 25;

void main() {
    int age = 25;
    IO.println(age);
}

How variables work in memory

When you write int age = 25;, Java sets aside exactly 4 bytes (32 bits) of memory and stores the value 25 there as a binary number.

The name age is just a label for you — the compiled bytecode works with a numbered slot in the method's stack frame instead. The name only survives as optional debug info.

int can store values from about -2.1 billion to +2.1 billion. For bigger numbers: long (8 bytes).

String name = "Anna";

void main() {
    String name = "Anna";
    IO.println(name);
}

String vs. int — reference vs. value

int stores the value directly. String stores a reference (a pointer) to an object in the so-called heap memory.

That's why you compare strings with .equals() instead of ==: == only compares the references (do both point to the same object?), not the content.

double price = 4.99;

void main() {
    double price = 4.99;
    IO.println(price);
}

Floating-point numbers and their quirks

double uses 8 bytes and stores numbers in the IEEE 754 floating-point format. That means: some decimal numbers can't be represented exactly!

For example: 0.1 + 0.2 gives you 0.30000000000000004 in Java. That's why money amounts are usually handled with BigDecimal.

String name = "Java";
IO.println("Hello " + name);

void main() {
    int age = 25;
    IO.println(???);
}

String concatenation

When you write "Text" + number, Java automatically converts the number into a string (string conversion) and glues the two together.

Strings are immutable, so every concatenation produces a new string object. Inside a loop that adds up fast and gets slow. In that case pros use a StringBuilder, or for formatted output String.format("I am %d years old", age).

int x = 5;
x = 10;  // x is now 10

void main() {
    int points = 0;
    // Set points to 100
    IO.println(points);
}

Declaration vs. assignment

int x = 5; is a declaration with initialization — the type is fixed and a starting value is set. x = 10; is a plain assignment.

If you write int x = 10; again, Java thinks you want to create a new variable — and complains, because x already exists. So: state the type only once!

boolean isSunny = true;
boolean isRaining = false;

void main() {
    boolean javaIsCool = false;
    IO.println(javaIsCool);
}

Booleans and logic

In theory a boolean needs just 1 bit. The JVM doesn't actually pin down its size, though — in practice it usually takes a whole byte (in arrays) or even 4 bytes (as a local variable), because that's faster to work with.

Boolean values are the foundation of every decision in a program. You can combine them with && (AND), || (OR) and ! (NOT).

Java uses short-circuit evaluation: with a && b, b isn't even checked if a is already false.