This document discusses principles of object-oriented programming maintainability such as reuse of behavior, typing and subtyping, and avoiding repetition. It covers different OOP concepts like classes, inheritance, interfaces, mixins and modules. The document suggests using traits for composing units of behavior in a way that is composable and avoids conflicts. Traits should define smaller interfaces than classes. Specifications can also be composed using shared examples. These principles can be achieved in languages like Java 8+, Ruby and Go.