The calling object doesn't know it has a mock. It just makes calls to the object normally. This causes awkwardness if the object you are mocking out has getters and setters as the mock doesn't have logic like this. You would have to set expectations on the mock that you call a sequence like:
1) get() - returns 2
3) get() - returns 3
You might consider writing your own mock that can store state and just checks if the method you want to mock out is called.