Key Points

• Class interfaces should provide a consistent abstraction. Many problems arise from violating this single principle.

• A class interface should hide something a system interface, a design decision, or an implementation detail.

• Containment is usually preferable to inheritance unless you're modeling an "is a" relationship.

• Inheritance is a useful tool, but it adds complexity, which is counter to Software's Primary Technical Imperative of managing complexity.

• Classes are your primary tool for managing complexity. Give their design as much attention as needed to accomplish that objective.