MEPX
Chapter 5 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 5 of 10

Composition over inheritance

Has-a is usually safer than is-a.

The alternative

Instead of inheriting behaviour, hold an object that provides it. A Car with an Engine can change engines; a Car extending Engine cannot.

  • Composition can be changed at runtime and combined freely.
  • Reach for inheritance only when the is-a relationship is genuinely permanent.

Dependency injection

Passing collaborators in rather than creating them inside makes an object testable, because a test can pass a simple stand-in. That is most of what the term means.