CODEWITHSUNDEEP
scalainner-classes
dips
dips

Reputation: 1637

Extending Inner class

I am trying to replicate my problem on a smaller example. I am getting compilation error at the shown location in the following code snippet.

class Outer {
    class Inner
}

object OuterUtil {
  val obj = new Outer
  object xyz extends obj.Inner
  //do something with xyz
}

//-------------------
object OuterUtil2 {
    var m_obj: Outer = null

    def createOuter() = {
        m_obj = new Outer
    }

    def anotherMethod() {
        //Compilation error here: stable identifier required, 
        //but OuterUtil2.this.m_obj found.
        object xyz extends m_obj.Inner
    }
}

object Test {
    OuterUtil2.createOuter
    OuterUtil2.anotherMethod
}

OuterUtil is working fine. In OuterUtil2, I am splitting the functionality into two functions. I am storing the Outer instance m_obj as a member var. The createOuter method creates and stores the Outer instance in m_obj. In anotherMethod, I am getting compilation error. How to fix OuterUtil2?

Upvotes: 1

Views: 234

Answers (2)

Christopher Chiche
Christopher Chiche

Reputation: 15325

If you just want to solve the problem with your function, here is a solution (by fixing the var to a val in the function)

def anotherMethod = {
  val obj = m_obj
  new obj.Inner
}

Another solution would be to use some options, however to overpass the stable identifier method, you have to define a val out of m_obj value. This solution is more elegant because you don't have null pointer exceptions if m_obj is not defined. 

object OuterUtil2 {
  var m_obj: Option[Outer] = None

  def createOuter {
    m_obj = Some(new Outer)
  }

  def anotherMethod = {
    m_obj match{
      case None => None
      case Some(_) => val obj = m_obj.get; Some(new obj.Inner)
    }
  }
}

Upvotes: 1

retronym
retronym

Reputation: 55028

The prefix of a type (ie, the m_obj in m_obj.Inner) must be a stable value; a var doesn't cut it. You could make that a val and move the initialization out of createOuter.

Upvotes: 3

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