brown bag: oo design

  • dependency inversion: depend on abstractions, don’t depend on concretions.
  • Liskov substitution: don’t violate superclass by using is_kind_of; they should return the same type


  • does it have one responsibility?
  • it is DRY?
  • does everything in it changes at the same rate?
  • is it open for extension but not for modification?
  • does it depend on things that change less often than it does?