mocking / interaction / expect breaks encapsulation to perform "testing". Thus it is often a test of the implementation's assumptions when first written, and e

mocking / interaction / expect breaks encapsulation to perform "testing". Thus it is often a test of the implementation's assumptions when first written, and even worse, when the code is maintained/edited, the test is merely changed to get it to pass, because unit tests with mocks are usually: 1) fragile to implementation 2) opaque as to intent Whereas input/output integration points are more reliable, transparent, and less fragile to implementation changes if the interface is maintained. However, if you must do mock-level interaction testing, Spock has made it almost palatable in Javaland. This is one area where functional fans get to make the imperative folks eat their lunch.