This content originally appeared on DEV Community and was authored by Maxi Contieri
Double Trouble: The Curse of Redundant Structures
TL;DR: Parallel hierarchies lead to duplication and tight coupling.
Problems
Increased complexity
DRY / Code Duplication
Maintenance Nightmare
Coupling
Ripple Effect
Potential for inconsistencies across different hierarchies
Solutions
Merge hierarchies
Use composition
Extract Common Functionality
Refactorings
Context
Parallel hierarchies occur when you must make a counterpart every time you create a domain class.
The counterpart might be persistence, UI, Controller, tests, Serialization, etc
This leads to duplicate structures and tight coupling.
Changes in the domain model require changes in the parallel classes, making the system more brittle and harder to manage.
Sample Code
Wrong
// Domain classes
abstract class Transaction {
private String id;
private double amount;
}
class BankTransaction extends Transaction {
private String bankName;
}
class CreditCardTransaction extends Transaction {
private String cardNumber;
}
// Persistence classes
abstract class TransactionDAO {
private String id;
private double amount;
}
class BankTransactionDAO extends TransactionDAO {
private String bankName;
}
class CreditCardTransactionDAO extends TransactionDAO {
private String cardNumber;
}
Right
public class TransactionService {
private EntityManager entityManager;
public TransactionService(EntityManager entityManager) {
this.entityManager = entityManager;
}
public void saveTransaction(Transaction transaction) {
entityManager.getTransaction().begin();
entityManager.persist(transaction);
entityManager.getTransaction().commit();
}
public Transaction loadTransaction(
Long id, Class<? extends Transaction> transactionClass) {
return entityManager.find(transactionClass, id);
}
}
Detection
[X] Semi-Automatic
You can detect this smell by traversing the hierarchies
Exceptions
- Some frameworks force you to extend your domain using this technique
Tags
- Hierarchies
Level
[X] Intermediate
AI Generation
AI generators often create this smell by mirroring domain models in persistence layers without understanding the implications, leading to unnecessary duplication.
AI Detection
AI Assistants can fix this smell with instructions to consolidate hierarchies and use composition, reducing duplication and improving maintainability.
ChatGPT offered a solution using 'Instanceof' which is an even worse code smell
Conclusion
Parallel hierarchies create unnecessary complexity and make the codebase harder to maintain.
They bring deep hierarchies which is a symptom of subclassification for code reuse
You can merge the hierarchies and use composition to simplify the design and improve the system's robustness.
You can use Metaprogramming to manage the persistence or the unit tests.
Metaprogramming is also a code smell when you use it for domain problems, but persistence and testing are orthogonal domains.
Relations
More Info
Disclaimer
Code Smells are my opinion.
Credits
Foto de ArtisanalPhoto en Unsplash
Inheritance is surely a good answer but who knows the questions?
Michel Gauthier
This article is part of the CodeSmell Series.
This content originally appeared on DEV Community and was authored by Maxi Contieri
Maxi Contieri | Sciencx (2024-06-21T17:02:03+00:00) Code Smell 255 – Parallel Hierarchies. Retrieved from https://www.scien.cx/2024/06/21/code-smell-255-parallel-hierarchies/
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