Why if I extend the App trait in Scala, I override the main method?

Question

So I read that the App trait has the following fields:

def delayedInit(body: ⇒ Unit): Unit

val executionStart: Long

def main(args: Array[String]): Unit

I know that if a trait only has one method, by "putting code" between the curly braces in the class declaration I am overriding that. But here I have two methods. So why am I overriding automatically main and not delayedInit?

Answer 1

You are not overriding main method.

Since App extends DelayedInit compiler rewrites your code like this:

// Before:
object Test extends App {
  println("test")
}

// After:
object Test extends App {
  delayedInit{println("test")}
}

From DelayedInit documentation:

Classes and objects (but note, not traits) inheriting the DelayedInit marker trait will have their initialization code rewritten as follows: code becomes delayedInit(code).

App trait implements delayedInit like this:

override def delayedInit(body: => Unit) {
  initCode += (() => body)
}

So in Test object constructor code println("test") is stored as function (() => Unit) in initCode field.

main method of App is implemented as call for all functions stored in initCode field:

for (proc <- initCode) proc()