Your first Java program

A program is a list of instructions that a computer carries out one after another. You write the instructions as text, called source code. Java can't run that text directly: first a tool called the compiler (named javac) checks it and translates it into a compact form, and then the Java virtual machine runs the result. When you press Check on this site, both happen right in your browser, with the real Java 21 compiler.

The shape of a Java program

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

Hello, Java!

Almost every line here is a frame that you'll write the same way every time:

• public class Main { ... } creates a class called Main. All Java code lives inside a class. The class name has to match the name of the file, and on this site the file is always Main.java, so the class is always Main.
• public static void main(String[] args) { ... } is the main method: the place where the program starts running. The words around main all have a meaning that later modules explain. For now, copy the line exactly as it is.
• System.out.println("Hello, Java!"); is the one line that does something: it prints the text between the double quotes and then ends the line. The quotes themselves are not printed.
• Every { has a matching }. The class's braces wrap the main method, and the main method's braces wrap the instructions.

In other Java courses and tools the class often has a different name, such as Sandbox or Program. That's fine: the rule is only that the class name and the file name match.

Capital letters matter

Java reads System and system as two different words. System.out.println has a capital S and nothing else in capitals. If one letter is off, the compiler stops and says what it couldn't understand. Step 5 of this module shows how to read those messages.