Question

I have immutable classes Outer and Inner as follows:

class Outer(val intra: Outer#Inner) {
    class Inner(val q: Int)
}

I want to create an instance of Outer and Inner which references each other, as follows:

val outer = new Outer(inner)
val inner = new outer.Inner(5)

However, of course, this code does not compile since the value “inner” is referenced in the first line before it is defined in second line.

And if I added “lazy” keywords before “val” keywords for those two lines, it compiles but it makes a stack overflow exception while running.

I know I can solve this problem if I make the class Outer mutable like this:

class Outer(var intra: Outer#Inner) {
    class Inner(val q: Int)
}
val outer = new Outer()
val inner = new outer.Inner(5)
outer.intra = inner

But I want to keep the classes immutable. How can I solve this problem?

Answer 1

You can change your definitions like this:

class Outer(_intra: => Outer#Inner) {
  lazy val intra = _intra
  class Inner(val q: Int)
}