The first question I’d ask if I were writing unit tests for their first example is why a solar calculator containing mathematical business logic should depend o

The first question I’d ask if I were writing unit tests for their first example is why a solar calculator containing mathematical business logic should depend on a location provider rather than just accepting a location as a method parameter. Unit tests help you to design your classes based on use cases—if a simple class is hard to test you should think about whether it might have a simpler design. If you have a brute forced design and each of your classes have dozens of dependencies and your tests require pages of setup and are hard to reason about and are tightly coupled to the implementation then the tests might not be providing any benefit at all.