Should ignore properties not declared in the primary constructor of Kotlin data classes

[jibidus]

Hello,

When running the following code:

data class MyDataClass(val value: String) {
   val isEmpty = value.isEmpty()
}

EqualsVerifier.forClass(MyDataClass::class.java).verify()

… it fails with error Significant fields: equals does not use isEmpty, or it is stateless.

Note: another exception is thrown if MyDataClass is a nested class.

Do you think it would be relevant to modify EqualsVerifier in order to respect Kotlin data class contract, by ignoring properties not declared in the primary constructor?

We can contribute on this if you want.

[jqno]

I was not aware of that contract, thanks for bringing it to my attention. Yes, I do think that would be a good improvement for EqualsVerifier, and I'd absolutely welcome your contribution. If you're interested in submitting a PR, I can give you some pointers to help you do it faster!

[jibidus]

Great! Yes, if you could give me some pointers, it would be nice.

[jqno]

Awesome!

The way EqualsVerifier approaches Kotlin, is it tries to do as much as possible from Java, so as to keep the codebase consistent. (Also, I have very little Kotlin experience myself 😅) So that might be uncomfortable for you, and I apologize for that.

You might need to do some "new" reflection in order to detect if a Kotlin class is a data class. EqualsVerifier has 2 classes that do Kotlin reflection: KotlinScreen and KotlinProbe. KotlinScreen does "simple" reflection, that can be done without the kotlin-reflect library. If you do need kotlin-reflect, then you'll have to do it in KotlinProbe. The reason for this is to keep the dependency to kotlin-reflect optional: people who don't use Kotlin won't need to download it unnecessarily, and people who do can make use of it without having to add an additional equalsverifier-kotlin project to their dependencies.

There are two Maven submodules that deal with Kotlin: equalsverifier-test-kotlin and equalsverifier-test-kotlin-noreflect. They contain only tests, but here the code actually is written in Kotlin! I expect that most of your tests will be in equalsverifier-test-kotlin. equalsverifier-test-kotlin-noreflect tests use cases where deeper Kotlin reflection is needed, but for whatever reason the kotlin-reflect library is not available. In that case, EqualsVerifier should give a nice error message instead of a NoClassDefFounError or something 😄.

The place where you'll want to hook in your logic, is in Configuration's build() method. There's a small section there for Kotlin already, where it adds some fields that the compiler generates to the list of non-null fields. This is where you can also add the fields that aren't in the data class's constructor to the list of ignored fields.

Some things to watch out for: I think it would be nice if the user can side-step your logic if they want. One way to do that would be by listing fields in EqualsVerifier's withOnlyTheseFields method. Or maybe we could do it by detecting if the data class actuall defines an equals method (you could do that using ClassProbe's declaresEquals method). I don't know what makes the most sense, you're probably in a better position to judge that than I am. Also, it would be good to add a small test in equalsverifier-test-kotlin-noreflect for you scenario, just in case it wasn't already covered.

Good luck! If you need any more info, or want me to take a look at a work in progress, please let me know!

[jqno]

Something that came up in a separate ticket but that might be relevant for you too: if you want make a full build of EqualsVerifier that you can test locally, you should run from a JDK 25, and use mvn install -Prelease.

[jibidus]

What do you think of this PR #1159?
Thanks for your help.

[jqno]

Thank you for submitting it! Let's move the discussion to the PR thread.