Question

Suppose I have this Scala code:

object Outer {

  object Inner {
    val B = "B"
  }
  class Inner {
    def b = B
  }
}

I would expect this to compile, but B cannot be accessed from the definition of b. I need to add import Inner._ in class Innerto make it work. Why is that? Is the companion object Inner not defined correctly?

Answer 1

It’s just not supposed to work this way – using import Inner._ is a consistent behavior.

---

Generally, companion object is needed, if you want to achieve the behavior similar to static members in Java. Scala way is to move all static members away to a singleton object, with the benefit that private/protected memebers of a companion class can be accessed from it:

object Outer {

  object Inner {
    def print(inner: Inner) = inner.B // compiles!
  }    
  class Inner {
    private val B = "B"
  }    
}

You can use companion object as a factory for the class with a private constructor:

scala> object Outer {
     |
     |   object Inner {
     |     def newInstance = new Inner()
     |   }
     |   class Inner private() {
     |     private val B = "B"
     |   }
     | }
defined module Outer

scala> Outer.Inner.newInstance
res1: Outer.Inner = Outer$Inner@431693

scala> new Outer.Inner
<console>:7: error: constructor Inner cannot be accessed in object $iw
       new Outer.Inner
       ^