Part two of my Three Steps to Code Quality via TDD series and ties very closely into step one, limiting the visibility of your classes.
The more dependencies you use the more your tests are coupled to implementation.
Consider the constructor below.
Code like this is common and difficult to work with. Each dependency you inject requires a mock, stub or fake when writing tests. This couples the implementation to the test despite the use of interfaces or abstract base classes.
Every public dependency here increases the resistance for change. If I
was to remove the builder and replace with some equivalent code to
Bar instance, the test would fail despite being
functionally equivalent. This is wrong.
A constructor is part of the public API of an object even though this is not detailed as part of interfaces in languages such as C#/Java. Every collaborator that is provided by a constructor should have a reason for being exposed as part of the the public API.
What Are Good Dependencies?
Good dependencies are things that are out of your control or process such as:
- Databases (repositories, queries)
- Web Services
- Third Parties
- Strategies (anything that needs to change dynamically)
As part three of the series will detail - isolate your tests from these sorts of dependencies, don't isolate your code from itself.
Doesn't this mean you end up with God classes?
No. As step one detailed - small, well focused classes are a good thing. They just should remain as implementation details.