First Java Program

Traditionally the first program in any language prints "Hello, World!". Let's walk through it from creating the file to seeing the output.

Every Java program goes through the same four steps — from a source file to output on the screen.

mkdir hello-java && cd hello-java

# Compile
javac HelloWorld.java

# Run (without .class extension)
java HelloWorld
# Hello, World!

Click on any line to see what it means.

public class HelloWorld {

    public static void main(String[] args) {

        System.out.println("Hello, World!");

    }

}

Starting with Java 11, single-file programs can be run directly — without an explicit compilation step:

java HelloWorld.java
# Hello, World!

The String[] args parameter in main() receives arguments passed after the class name. Let's explore how args is parsed line by line.

public class Greeting {

    public static void main(String[] args) {

        if (args.length == 0) {

            System.out.println("Hello, stranger!");

        } else {

            System.out.println("Hello, " + args[0] + "!");

        }

    }

}

javac Greeting.java
java Greeting          # Hello, stranger!
java Greeting Alice    # Hello, Alice!

Java provides several ways to write to standard output. Click each card to see an example.

Java has three types of comments. Click each to see what it looks like and when to use it.