Learn Java: First Steps & Hello World

void main() {
    IO.println("Hello, World!");
}

What happens when you run it?

Java is a compiled language. First, the compiler (javac) translates your code into so-called bytecode — a kind of intermediate language that the Java Virtual Machine understands.

Then the JVM (Java Virtual Machine) starts up and runs that bytecode. That's why Java behaves the same on every operating system — "Write once, run anywhere".

IO.println() prints one line. In Java 25, IO is available automatically — no import needed.

Compact vs. classic

Right now your code looks like this:

void main() {
    IO.println("Hello, World!");
}

In older code (before Java 25) and in most company projects you'll see the classic form:

public class Main {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

Both do the same thing. The compact form has been a standard feature since Java 25 (September 2025) — perfect for getting started without having to understand classes, public, static and String[] arrays right away. You'll learn the classic form later, when you want to read existing code or work in a team.

void main() {
    IO.println("Hello, World!");
}

What's a string?

In programming, text in quotation marks is called a string (a "string of characters"). "Hello" is a string literal — a text value written directly into the code.

In Java, strings are objects of the class java.lang.String. They're immutable — once created, they can't be changed. Every "change" quietly creates a new string behind the scenes.

void main() {
    IO.println("Hello");
    // Add a second line here
}

println vs. print

There's also IO.print() — without the ln. The difference: println automatically adds a line break (\n) at the end, print doesn't.

print("A") followed by print("B") gives you AB on one line. println("A") followed by println("B") gives you two lines.

void main() {
    IO.println("Fix me!")
}

Why semicolons?

Java uses ; to separate statements from each other. Unlike Python, where line breaks are enough, Java almost completely ignores whitespace (spaces, tabs, line breaks).

In theory you could write a whole program on a single line — thanks to semicolons, Java still knows where each statement ends. In practice nobody does that, of course, because it would be unreadable.

IO.println(42);

void main() {
    IO.println(42);
}

Primitive data types

Numbers without quotation marks are literals of a primitive type in Java. 42 is automatically an int (32-bit whole number). For bigger numbers there's long (64-bit).

Java strictly distinguishes between text ("42" = String) and number (42 = int). That makes Java "type-safe" — mistakes get caught early.

IO.println(3 + 4);  // prints 7

void main() {
    IO.println(???);
}

Operators and order of evaluation

Java knows the usual arithmetic operators: +, -, *, / and % (modulo/remainder). Multiplication and division come before addition and subtraction, just like in school.

Watch out for integer division: 7 / 2 gives you 3, not 3.5! If both sides are int, the result gets cut off. For decimals you need 7.0 / 2.