Example.scala

import scala.collection.mutable

object Example {
    case class Parent(private val childLocator: ChildLocator) {
        lazy val child = childLocator.locate(from = this)
    }

    case class Child(parent: Parent)

    class ChildLocator { // mutable
        private val mapping = mutable.HashMap[Parent, Child]()

        def locate(from: Parent) = mapping(from)

        def insert(from: Parent, to: Child): Unit = {
            mapping(from) = to
        }
    }

    def main(args: Array[String]): Unit = {
        val childLocator = new ChildLocator()

        val parent = Parent(childLocator)
        val child = Child(parent)

        childLocator.insert(parent, child)

        println(parent.child)
        println(child.parent)
    }
}