Object Thinking and Confusion over Collaboration

David West, the author of "Object Thinking", has a different definition of collaboration than I had ever seen.

The main difference (found on page 130) is noted in a scenario where object A is collaborating with object B. He states that the object being collaborated with (B) is "not an object occupying one of object A's instance variables, a temporary variable declared in the method that object A is executing in order to satisfy the original request, or an object supplied to object A as an argument".

Then on page 254, the author states, "Collaborating objects are very tightly coupled. For this reason, collaborations should occur inside the encapsulation barrier, with objects occupying the instance variables, objects being received along with messages, and objects occupying temporary variables".

At this point, I was confused, thinking I had found a contradiction. Let's see if other references to collaboration clear things up...

On page 226, the author talks about the airplane collaborating with an instrumentCluster which on page 227 is shown as being held in an instance variable (Figure 8-2). Okay, maybe that first definition just had a typo...

On the previous page, page 225, the author restates the definition of collaboration as found on page 130, the definition where collaborations don't take place between arguments, instance and temporary variables.

If you're keeping score, that's two references to collaborations taking place between arguments, temporary variables, and instance variables and two references for collaborations not being between arguments, and the rest.

My thinking on the matter is that any time one object uses the services of another object, there is collaboration. If collaboration is not occurring between instance variables, temporary variables, and arguments, then what is it called when you use the services of these other objects?

After some more thought, I think we have a catch-22 situation here. When you are filling out a CRC card, and you are about to write down a collaborator, you must first think whether or not that collaborator will be stored in an instance variable, temporary variable, or argument. If the collaborator will be referenced via an instance variable, then it's not a collaborator at all and you should not write it down. That sounds like jumping to implementation details during the "Object Discovery" process.

If collaboration is something that is to be discovered during the "Object Discovery" process, then you cannot make restrictions on what is and what isn't a collaboration based on implementation details.