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.
printf("hello world");
mock_printf(string) {
if string != "hello world" then fail;
}