Practical Object-Oriented Design in Ruby: An Agile Primer

by Sandi Metz

The reason that test-first gurus can easily start writing tests is that they have so much design experience. At this stage, they have already constructed a mental map of possibilities for objects and interactions in this application. They are not attached to any specific idea and plan to use tests to discover alternatives, but they know so much about design that they have already formed an intention about the application. It is this intention that allows them to specify the first test.

This transition from class-based design to message-based design is a turning point in your design career. The message-based perspective yields more flexible applications than does the class-based perspective. Changing the fundamental design question from “I know I need this class, what should it do?” to “I need to send this message, who should respond to it?” is the first step in that direction. You don’t send messages because you have objects, you have objects because you send messages.