99 Bottles of OOP

Alignment - Steer towards the average heading of neighbors

Separation - Don’t get too close to a neighbor

Cohesion - Steer towards the average position of the flock

, if you rearrange code without changing behavior and tests begin to fail, then the tests themselves are flawed. Tests that make assertions about how things are done, rather than what actually happens, are the prime contributors to this predicament.

The "open" principle says that you should not conflate the process of moving code around, of refactoring, with the act of adding new features. You should instead separate these two operations. When faced with a new requirement, first rearrange the existing code such that it’s open to the new feature, and once that’s complete, then add the new code.