Parallel Inheritance Hierarchies

This occurs when an inheritance tree depends on another inheritance tree by composition, and to create a subclass for a class, one finds that he has to make a subclass for another class. Fowler specified that this is a special case of Shotgun Surgery code smell.

Causation

This smell can happen naturally when trying to model a problem in a domain. The problem arises when these hierarchies are created artificially and unnecessarily (for example, by adding a standard prefix throughout the classes).

Problems

Duplication

Requires additional work to be done, which might be redundant.

Examples

class User(ABC):
    ...
    functions: Functions

class Functions(ABC):
    ...



class BasicUser(User):
    ...

class BasicFunctions(Functions):
    ...




class PremiumUser(User):
    ...

class PremiumFunctions(Functions):
    ...

# each time a new user is added, so is a new function subclass with the same prefix

class Animal(ABC):
    ...
    food: Food

class Food(ABC):
    ...



class Elephant(Animal):
    ...
    food: Vegan

class Vegan(Animal):
    ...




class Lion(Animal):
    ...
    food: Carnivore

class Carnivore(Food):
    ...

# domain-like mapping might eventually be reused

Refactoring:

• Move Method
• Move Field
• Create Partial
• Fold Hierarchy into One

Sources

• [Origin] - Martin Fowler, "Refactoring: Improving the Design of Existing Code" (1999)