SOLID Principles

• Single Responsibility

  • CityPair
  • CityPairConverter rather than add convert method inside CityPair

• Open for Extension, Closed for Modification

  • Policy
  • NfdPolicy extend Policy, add FareBasis field

• Liskov Substitution

  • Policy interface should not have getFareBasis method, because fdPolicy has no farebasis
  • any implementation of an interface should not break our code, like throw UnsupportedOperationException

• Interface Segregation

  • Split BearKeeper into BearCleaner, BearFeeder and BearPetter

• Dependency Inversion

  • Computer-StandardKeyBoard, bad
  • Computer-KeyBoard-StandardKeyBoard, good