Hello, World (Version 2)

While that first “Hello, World” example works just fine, Scala provides a way to write applications more conveniently. Rather than including a main method, your object can just extend the App trait, like this:

object Hello2 extends App {
    println("Hello, world")
}

If you save that code to Hello.scala, compile it with scalac and run it with scala, you’ll see the same result as the previous lesson.

What happens here is that the App trait has its own main method, so you don’t need to write one. I’ll show later on how you can access command-line arguments with this approach, but the short story is that it’s easy: they’re made available to you in a string array named args.

I haven’t mentioned it yet, but a Scala trait is similar to an abstract class in Java. (More accurately, it’s a combination of an abstract class and an interface — more on this later!)

Extra credit

If you want to see how command-line arguments work when your object extends the App trait, save this source code in a file named HelloYou.scala:

object HelloYou extends App {
    if (args.size == 0)
        println("Hello, you")
    else
        println("Hello, " + args(0))
}

Then compile it with scalac:

scalac HelloYou.scala

Then run it with and without command-line arguments. Here’s an example:

$ scala HelloYou
Hello, you

$ scala HelloYou Al
Hello, Al

This shows:

  • Command-line arguments are automatically made available to you in a variable named args.
  • You determine the number of elements in args with args.size (or args.length, if you prefer).
  • args is an Array, and you access Array elements as args(0), args(1), etc. Because args is an object, you access the array elements with parentheses (not [] or any other special syntax).

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