MEPX
Chapter 6 of 10All chapters
  1. 1Objects and classes
  2. 2Encapsulation
  3. 3Inheritance
  4. 4Polymorphism
  5. 5Composition over inheritance
  6. 6Design principles
  7. 7Design patterns
  8. 8State and mutation
  9. 9Testing object code
  10. 10When not to use it

Chapter 6 of 10

Design principles

SOLID, briefly and honestly.

The five

One reason to change per class. Open to extension, closed to modification. Subclasses substitutable for parents. Small interfaces over large ones. Depend on abstractions, not concrete types.

  • They are heuristics for managing change, not laws.
  • Applying all five to a small program produces more indirection than value.

The one that pays

Single responsibility earns its keep everywhere. If you can name two unrelated reasons a class would change, splitting it usually pays for itself within a month.