Parallel Inheritance Hierarchies

Change Preventers Responsibility Code Smell Between Class

Add a BasicUser, and you need a BasicFunctions. Add a PremiumUser, and here comes PremiumFunctions. Every subclass in one hierarchy demands a mirror in the other, and the cost of every new feature...

2 min read April 19, 2022 1 source

Overview

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.

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.

Example

When solving, one must be cautious to not violate the Single Responsibility Principle.

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

PYTHON

Refactoring

Move Method
Move Field
Create Partial
Fold Hierarchy into One

Sources

ORIGIN
Martin Fowler
Refactoring: Improving the Design of Existing Code
1999 · ISBN 978-0201485677

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