Separate interfaces and implementations.

• One can reuse a component's interface (design) or implementation (code) or both.
• Reusable component = multiple users = single interface with multiple implementations. Each interface can have multiple implementations.
• Enhances verifiability, locality, prevention of side-effects, future evolution, subclassibility, and eases change of components,
• Languages vary widely in support of the principle. E.g., Ada package specification, C++ abstract classes, and C header files, as do techniques for exploiting them.
• Use interface specification as a barrier to access the implementation.
• Don't let client break the component abstraction defined by the interface.
• When possible, use value arguments and results, not pointers or references that reveal helpers or other internal features.
• Use authentication measures in cases where you must export (e.g., friends in C++).