Now that we are writing the code to solve problems in our domain, we can approach problems in different ways: one way is to start at the topmost level we have so far discovered and let this guide us down to our lower level objects and abstractions, or we can start with the components we identified, flush them out and build up the system. Both approaches are valid and are commonly referred to as "outside-in" or "inside-out" development. The advantage of inside-out is that we always have a running working system because we build the dependents first and build up the system. The disadvantage being that it is easier to lose sight of the bigger picture and get lost in the details.

What is common to the approaches is that they follow a style based on test-driven development. We are building the tests to let us guide the design and show us when we are done. We start using our code first to get a feel for how it would behave later, and implement what we think...