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Cover image for Code Smell 61 - Coupling to Classes
Maxi Contieri
Maxi Contieri

Posted on • Updated on • Originally published at

Code Smell 61 - Coupling to Classes

Classes are handy. We can call them and invoke them any time. Is this good?


  • Coupling

  • Extensibility

  • Hard to mock


  1. Use interfaces or traits (if available).

  2. Use Dependency Injection.

  3. Favor Loose Coupling.

Sample Code




We can use almost any linter to find references to classes. We should not abuse since many uses might be false positives.


  • Coupling


Dependencies to Interfaces make a system less coupled and thus more extensible and testable.

Interfaces change less often than concrete implementations.

Some objects implement many interfaces, declaring which part depends on which interface makes the coupling more granular and the object more cohesive.


More info




Photo by Marco Bianchetti on Unsplash

When your code depends on an interface, that dependency is usually very minor and unobtrusive. Your code doesn’t have to change unless the interface changes, and interfaces typically change far less often than the code behind them. When you have an interface, you can edit classes that implement that interface or add new classes that implement the interface, all without impacting code that uses the interface.

For this reason, it is better to depend on interfaces or abstract classes than it is to depend on concrete classes. When you depend on less volatile things, you minimize the chance that particular changes will trigger massive recompilation.

Michael Feathers

Top comments (5)

chrisza4 profile image
Chakrit Likitkhajorn

I am confused. The solution said that favor loose coupling and the conclusion favor dependency on to the interface. But in the wrong example, MyDomain class depends on interface and in the right example it depends on class.

Maybe it is swapped?

mcsee profile image
Maxi Contieri

thank you very much for pointing it out

last minute change. :)
Of course was the wrong one!

michaelcurrin profile image
Michael Currin • Edited

Hi. I don't know this is an issue with your embedding or just a limitation of the platforms, but when I look at 3 different posts of yours for the first time, then initially the same file is used. On reloading immediately then it is fixed.

Perhaps its because of how multiple files exist in one gist. But doesn't explain buggy gist behavior.


mcsee profile image
Maxi Contieri


There is a bug with app version that messes up gists.
This is not happening on the website

Can you check this out?

michaelcurrin profile image
Michael Currin

It does sound like a bug. I don't think I've seen gists much before here.

I am using mobile web.

Strangely after the first 3 post problems, on the 4th post I clicked, it worked right first time around.