Demeter's law (suggestion) and tell don't ask - Part 2

Part 1 suggested that Tell-Don't-Ask might be a better principle to keep in mind than the Law of Demeter. There is a common problem while trying to code tell-don't-ask. The object-to-be-told may not be available at the point of making the call. For example, when a book is returned, you may want to notify another user (if any) waiting for the title. For the purpose of this example, the domain model is: every physical book (copy) is associated with a Title. Users lookup titles and checkout books. The Title knows how many copies are in stock and how many of those are lent out. To start off, here is a fairly bad implementation:

returned(){ //a method on Book
  setStatus(Book.AVAILABLE);
  title.setNumberOfCopiesAvailable(title.getNumberOfCopiesAvailable + 1);
  title.getUserInWait().notifyAvailability(title);
}

The Book asks the Title two things and tells very little. One could imagine the following conversation:

Book: How many copies are available? And who's waiting for you?
Title: Why do you ask?
Book: I was just returned to the library. I want to make sure the number of copies available reflect this and also notify whoever is waiting.
Title: Why don't you just tell me that you are back. I'll take care of the housekeeping myself.

This is a slightly contrived example, but to illustrate the point, let's start off by removing the 'ask' from the last line. By introducing a forwarding wrapper in Title, book can just tell Title to notify any waiting user. There is good support for such delegation in IntelliJ/Eclipse.

Right-Click -> Delegate
Select target object userInWait
Select target method notifyAvailability
Done

So we now have:

returned(){
  setStatus(Book.AVAILABLE);
  title.setNumberOfCopiesAvailable(title.getNumberOfCopiesAvailable + 1);
  title.notifyAvailability();
}

Now, we can push the logic of updating the number of available copies into title.notifyAvailability and rename it to title.notifyReturn

returned(){
  setStatus(Book.AVAILABLE);
  title.notifyReturn();
}

What started off as dumb forwarding (title.notifyAvailability -> user.notifyAvailability) has thus ended up as better encapsulation (title.notifyReturn).