Your First Java Program

Java is a strongly typed, object-oriented language. Java source code is compiled into bytecode, which runs on the Java Virtual Machine (JVM). Because a JVM exists for almost every operating system, the same compiled program can run on Windows, macOS and Linux without changes.

Installing Java

Download a JDK (Java Development Kit) version 17 or newer, for example from adoptium.net. After installing, open a terminal and check:

java -version
javac -version

Hello, world

In Java, all code lives inside a class. The file name must match the public class name, so save this as Main.java:

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

Hello, world!

Compile and run it:

javac Main.java
java Main

Reading the code

• public class Main declares a class called Main.
• public static void main(String[] args) is the entry point. The JVM looks for exactly this method when it starts your program.
• System.out.println(...) prints a line of text and then moves to a new line.
• Statements end with ; and blocks are wrapped in { }.

print versus println

System.out.print prints without a newline, which lets you build a line in pieces.

public class Main {
    public static void main(String[] args) {
        System.out.print("Java ");
        System.out.print("is ");
        System.out.println("everywhere.");
        System.out.println("Sum: " + (10 + 20));
    }
}

Java is everywhere.
Sum: 30

The brackets around 10 + 20 matter. Without them Java would join "Sum: " and 10 first, and print Sum: 1020.