CODEWITHSUNDEEP
scala
helpermethod
helpermethod

Reputation: 62155

Why am I able to use the parameters passed to the class definition?

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
}

Upvotes: 2

Views: 83

Answers (2)

Tomasz Nurkiewicz
Tomasz Nurkiewicz

Reputation: 340693

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)

Upvotes: 7

4e6
4e6

Reputation: 10776

In this case bar is defined as private[this] and can be acessed within the class definition. You can check it with -Xprint:typer option:

class Foo extends java.lang.Object with ScalaObject {
  <paramaccessor> private[this] val bar: Int = _;
  def this(bar: Int): $line1.$read.$iw.$iw.Foo = {
    Foo.super.this();
    ()
  }
}

Upvotes: 2

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