Should one unit-test test Builder classes? If so, how?
A well-designed application is a composed network of simple components. Many developers use one or more builder classes to construct applications.
Builders can get quite complex.
In many cases we want a single component to be shared by several client components. It's easy to get this wrong. If we do so, we may build a network on which all dependencies appear to be satisfied, but we have actually created multiple copies of components when a single instance should be shared.
Builders often use lazy initialization, in which case they have state. This introduces additional opportunities for error.
Since it's easy to get builders wrong, we need to test them. End-to-end tests will usually tell us if we've build out application without all the necessary parts, but they don't pinpoint the problem, and they may not tell us if we've mistakenly introduced multiple objects instead of sharing a single one.
This suggests that we need to write unit tests for builders, but that's often difficult. The classes that are built will hide their implementation (as they should) so unit tests cannot easily verify that the right internal components are there inside them.
I'm coming to the conclusion that builder tests need to break encapsulation. That's normally a major code smell, but the nature of builders seems to leave no alternative.