To class, or to data class, that is the question

Whenever we write a new class in Kotlin, we may feel tempted about defining as a data class. But should every class which is a data holder be a data class? I don’t think so. During the last months I have seen suggestions in PRs such as “make this class a data class?”.

Kotlin data class

Let’s see first what is Kotlin compiler generating for free when we declare a data class.

• equals() / hashCode() => nothing fancy here, just the implementation for this well known functions.
• toString() => a nice to see String representation of the class.
• componentN() functions => functions that are implicitly called when using destructuring declarations.
• copy() => allows making a copy of an object (favoring inmutability!).

Are you using any of this functions? If the answer is no then there is no need to define your class as a data class.

But what if you are just interested on one of them? There is no way to tell Kotlin to just generate equals/hashCode for example. If you are coming from Java, you will have probably heard about Lombok, which allows annotating a class to generate functions as described above (and more).