Hi,
Mocking is done according to the order you create the objects. If both are from the same class, the first one created will be connected to the first mock you created. Unless you use MockAll, which applies to all instances, this is how instance mocking works.
The kind of interaction you are testing, requires some "black-box" knowledge. You need to set up the expectations in the way you know how the inner code is written. So in that sense, you are correct - since the test code relies on knowledge of the inner code, it is fragile in that sense. But this is how interaction testing works - you need to know the interactions with other objects (and know which type they are and how many there are) in order to test correctly.
What you may be able to do is state-based testing. For example, test your map size after calling Clear, and see if it equals 0. This test code relies on the external interface only, and not on the internal implementation - it's more readable and less fragile.
Would you like to post the code and test (or send it to us privately) so we can understand better? We'll be glad to comment and help if we can.
