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Hello World

Let's write and run your first Onion program!

The Classic Hello World

Create a file named Hello.on:

println("Hello, World!")

That's it! This single line is a complete Onion program.

Running Your Program

There are two ways to run Onion programs:

Method 1: Direct Execution with onion

The onion command compiles and runs your program in memory:

onion Hello.on

Output: 

Hello, World!

Method 2: Compilation with onionc

Compile to .class files and run with Java:

# Compile
onionc Hello.on

# Run with Java
java Hello

Understanding the Code

println("Hello, World!")
  • IO - The built-in I/O module
  • :: - Static method access operator (similar to Java's .)
  • println() - Print with newline function
  • "Hello, World!" - String literal

More Examples

Using Variables

val message: String = "Hello, Onion!"
println(message)

Getting User Input

val name: String = IO::readln("What's your name? ")
println("Hello, " + name + "!")

Run it: 

onion greeting.on

Output: 

What's your name? Alice
Hello, Alice!

With a Main Class

You can also write programs with explicit class definitions:

class HelloWorld {
  public:
    static def main(args: String[]): void {
      println("Hello, World!")
    }
}

This follows the traditional Java main method pattern.

Command-Line Arguments

Access command-line arguments through the main method:

class ArgsDemo {
  public:
    static def main(args: String[]): void {
      println("Number of arguments: " + args.length)
      for var i: Int = 0; i < args.length; i = i + 1 {
        println("Arg " + i + ": " + args[i])
      }
    }
}

Run with arguments: 

onion ArgsDemo.on hello world 123

Output: 

Number of arguments: 3
Arg 0: hello
Arg 1: world
Arg 2: 123

Next Steps

Now that you've written your first Onion program, continue with: