Supun Wijerathne Supun Wijerathne - 1 year ago 156
Java Question

Do enums violate open/closed principle Java?

In java, If we are using an enum and eventually we want to add/remove an attribute to/from that enum, hence its usages, we are violating open/closed principle in solid principles.

If so, what is the better usage of enums?

Answer Source

The answer is no as OCP doesn't apply (and cannot apply) to Enums. Enums should be complete (contain all possible values) and static (static = final, non-mutable). You can see them as a small, finite set of Value Objects.

If you want something that can be extended, you can always use a class or build your own "type-safe enumeration" that can be extended.

About the language. I choose to use the term apply rather than violate, as principles, as many good practices, have a context where it makes sense to apply them. A violation (for me) means that a situation calls to use a principle, but it's not used or it's not implemented properly. It doesn't apply means what it says on the tin, that the principle brings no use in the current context or that it cannot be applied, as otherwise would go against other principles which have stronger forces ... in the context :).


Another reason why I think Enums don't violate OCP (now I use the word violate :D), is because OCP doesn't mean that all of the classes and methods can be extended, but rather that a conscientious developer makes informed decisions about where to put those extension points. In the case of an Enum a developer understands that there's no need to extend the possible values and further that it would be damaging to add new values, as existing code wouldn't know what to do with the new value. So he/she decides to completely close the class and not provide extension points. The same applies to any other class that is final. A final class doesn't violate OCP, but rather (or ideally) a developer made a decision of not allowing to extend it.

This is similar to what I mentioned before, that Enums are like a small, finite set of Value Objects. VOs are immutable and closed, and as such they don't violate OCP.

And to add something more, I've seen some libraries not implementing OCP correctly (Tapestry 5) and making more than needed final, which made them a PITA to use. Or just not closing anything, which made some devs make mistakes because they didn't understand the finer details of the library, and messed up the invariants.

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