Design Philosophy: Object-Oriented Design

OO principles as thinking tools for code organization, module boundaries, and component relationships.

Core Premise

Object-oriented design is not about classes and inheritance. It is about managing the cost of change by localizing decisions behind stable interfaces. Every OO principle exists to answer one question: when a requirement changes, how many places in the code must change with it?

Good OO design means a single requirement change touches one or two files. Bad OO design means shotgun surgery across a class hierarchy that was supposed to make things "extensible."

The principles below are thinking tools, not commandments. Each one has a failure mode that is just as common as the problem it solves. Know both.

1. SOLID Principles

SOLID is a diagnostic framework. When code is hard to change, one of these principles usually explains why. But applying them prophylactically — before you feel the pain — produces the kind of over-abstracted architecture that makes senior engineers weep.