Stable Abstractions Principle (SAP)

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A component's level of abstraction should be equal to its stability.

• A principle about the relationship between abstraction and stability.
• A component with high stability should also have high abstraction, and high stability should not obstruct extension.
• A low-stability component should be concrete.
• High stability makes internal concrete code harder to modify.
• To keep a component stable and still extensible, design it with interfaces and abstract classes.
• If a component is to be stable, make it extendable by interfaces and abstract classes.
• SAP + SDP corresponds to the component-level Dependency Inversion Principle.
• Stability and abstraction are correlated.

Measuring Abstraction

A measure of a component's abstraction is A.

• Nc
  • Total number of classes in the component
• Na
  • Number of abstract classes and interfaces in the component
• A
  • Abstraction
  • A = Na ÷ Nc