Using trait method in the class constructor

Question

I have a trait and a class that extends the trait. I can use the methods from the trait as follows:

trait A {
  def a = ""
}

class B(s: String) extends A {
  def b = a 
}

However, when I use the trait's method in the constructor like this:

trait A {
  def a = ""
}

class B(s: String) extends A {
  def this() = this(a) 
}

then the following error appears:

error: not found: value a

Is there some way to define default parameters for the construction of classes in the trait?

EDIT: To clarify the purpose: There is the akka-testkit:

class TestKit(_system: ActorSystem) extends { implicit val system = _system }

And each test looks like this:

class B(_system: ActorSystem) extends TestKit(_system) with A with ... {
  def this() = this(actorSystem)
  ...
}

because I want to create common creation of the ActorSystem in A:

trait A {
  val conf = ...
  def actorSystem = ActorSystem("MySpec", conf)
  ...
}

I don't see how you can have a constructor parameter that depends on a class member. The class member could (and usually does?) depend on the constructor parameters, which would allow for circular referencing. So def a isn't yet initialized. — Michael Zajac
– Michael Zajac, Commented Mar 6, 2015 at 20:22

codejitsu · Accepted Answer · 2015-03-06 19:10:55Z

2

It's a little bit tricky because of Scala initialization order. The simplest solution I found is to define a companion object for your class B with apply as factory method:

trait A {
  def a = "aaaa"
}

class B(s: String) {
 println(s)
}

object B extends A {
  def apply() = new B(a)
  def apply(s: String) = new B(s)
}

answered Mar 6, 2015 at 19:10

codejitsu

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1 Comment

mirelon Over a year ago

Nice solution, but the problem is, that B is called like new B() (it's in a generated code so I cannot modify it)

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