Question

I’m just learning about classes and objects and Scala, and yesterday I saw something like this:

class Foo(bar: Int) {
  def increaseByOne = bar + 1
}

Why am I able to use bar in method increaseByOne? I would expect the the method definition complain about not knowing bar.

I though the right way to define such a class would be

class Foo(x: Int) {
  val bar = x

  def increaseByOne = bar + 1
}

Answer 1

That’s one of the wonderful features of Scala: if you reference constructor argument from any method that is not a constructor, Scala will automatically assign that constructor variable to a field. So effectively Scala translates your first code snippet into the second one for you (with private[this] modifier).

Moreover, preceding constructor argument with val/var will create getters/setters as well:

class Foo(val bar: Int)
val foo = new Foo(42);
println(foo.bar)