When extracting implicit components, we start by looking for two conditions: arguments that are always used together in the class, and those that have the same lifetime. Once we’ve found a coincidence, we have the harder task of finding a good name that explains the concept.

We value code that is easy to maintain over code that is easy to write.1 Implementing a feature in the most direct way can damage the maintainability of the system, for example by making the code difficult to understand or by introducing hidden dependencies between components.

One of the symptoms of an unstable development environment is that there’s no obvious first place to look when something fails.

The fewer methods there are on an interface, the more obvious is its role in the calling object.

Break up an object if it becomes too large to test easily, or if its test failures become difficult to interpret. Then unit-test the new parts separately.

Driving an interface from its client avoids leaking excess information about its implementers, which minimizes any implicit coupling between objects

We should be taught not to wait for inspiration to start a thing. Action always generates inspiration. Inspiration seldom generates action. —Frank Tibolt

Loosely speaking, we use the message-passing style we’ve just described between objects, but we tend to use a more functional style within an object, building up behavior from methods and values that have no side effects.

an interface describes whether two components will fit together, while a protocol describes whether they will work together.