Somebody explained it to me once that, In spring framework, we program to the interface rather than programming to concerete class, because it leads to tight coupling, and that's problematic in unit testing while doing the mocking.
I understand the tight coupling part based on the example here, but I don't understand how tight coupling is a problem with unit testing and mocking.
Just like the way we mock an interface, we can mock a concerte class. So, how tight coupleing hurts the unit testing?