Data Objects in Kotlin

Data objects are a new Kotlin language feature introduced in version 1.7.20 and are currently planned to be released in version 1.9. We’ll take a closer look at what they are and what issue they are trying to solve.

What issue are data objects solving?

Below we have a typical example of a sealed class hierarchy where we are using a sealed interface (we could also use a sealed class) to define possible states for a Profile screen. We are using a data class for the success state and an object for error and loading states since we don’t need any additional information.

What if we want to log/print the current screen state for either debugging reasons or for sending it to an analytics service? The string representation of the ProfileScreenState.Success data class contains the name of the class and all its properties, which is exactly what we want.
But if we print a plain object declaration in Kotlin, we’ll get a string containing the full package name, object name, and the address of where this object is stored in memory. And since objects are singletons in Kotlin, the address part will remain the same each time we print that object and is not relevant to us.

com.dataobjects.example.ProfileScreenState$Loading@6d03e736
Success(username=exampleUser1)
com.dataobjects.example.ProfileScreenState$Error@5fd0d5ae

One solution would be to override the toString(): String function on each object and provide our implementation, but that seems like a lot of boilerplate code for such a trivial issue.

Data objects

Kotlin is planning to solve this using data objects. A data object is identical to a regular object but provides a default implementation of the toString() function that will print its name, removing the need to override it manually, and aligning behavior with a data class definition. They are especially useful for sealed class hierarchies to match behavior with data classes.

Data objects are currently an experimental feature, so to try them out, we have to tell the compiler to use version Kotlin 1.9:

kotlinOptions.languageVersion = "1.9"

After syncing the project we receive a suggestion on our existing objects that are a part of a sealed hierarchy to convert them to a data object.

IDE suggestion to convert a sealed sub-object to a data object.

IDE suggestion to convert an object with an overridden toString function to a data object.

After applying the suggestion, our objects are converted into a data object:

And if we print them now, we can see that their string representation now looks similar to a data class and only the object name is printed.

Loading
Success(username=exampleUser1)
Error

Conclusion

Data objects are a minor new language feature that improves the string representation of an object. They are most useful when we have sealed class hierarchies with other data classes and want to print/log them for debugging or analytics purposes.

Note that data objects are an experimental new language feature in Kotlin at the time of writing this post and that their definition might change in the future.

You can read more details about data objects here:

• Kotlin KEEP proposal: https://github.com/Kotlin/KEEP/blob/data-objects/proposals/data-objects.md
• Kotlin KEEP discussion: https://github.com/Kotlin/KEEP/issues/317
• Kotlin tracker: https://youtrack.jetbrains.com/issue/KT-4107/Design-and-implement-a-solution-for-toString-equals-and-hashCode-on-objects-data-object
• Official Kotlin video: https://www.youtube.com/watch?v=ovAqcwFhEGc
• Kotlin 1.7.20 changelog: https://kotlinlang.org/docs/whatsnew1720.html#preview-of-the-operator-for-creating-open-ended-ranges