Any nice way to make two immutable objects refer to eachother?

Question

Take these two Java classes:

class User {
   final Inventory inventory;
   User (Inventory inv) {
       inventory = inv;
   }
}

class Inventory {
   final User         


        

Answer 1

This is one "solution", though the loss of one final is inconvenient.

class User {
   Inventory inventory;
   User () { }
   // make sure this setter is only callable from where it should be,
   // and is called only once at construction time
   setInventory(inv) {
       if (inventory != null) throw new IllegalStateException();
       inventory = inv;
   }
}

class Inventory {
   final User owner;
   Inventory (User own) {
       owner = own;
   }
}