I'm sorta new to kotlinx.serailization. Here's something that blew up at runtime that surprised me:...

# serialization

[Colton Idle]

I'm sorta new to kotlinx.serailization.

Here's something that blew up at runtime that surprised me:

@Serializable
data class MyResponseBody(
    val id: Long,
    val email: String,
    val username: String?
)

I got an error of Field 'username' is required for type with serial name

Why is that. Doesn't the ? indicate that it's not required? If not, then how can I catch this at compile time? Seems like marking something with @Serializable should enforce that any ? field should have a default, no?

[jw]

The question mark makes the type nullable, meaning it is still required but accepts null

[jw]

What you're asking for is behavior that supports absence which are not the same

[jw]

That is what a default expression provides (and is useful regardless of nullability)

[jw]

Consider {"presentNotNull":"present","presentNull":null} and the three properties that would bind to it: presentNotNull, presentNull, and absent

[jw]

presentNotNull can be of type String or String? and if it has a default expression it won't be used

[jw]

presentNull can only be String? but not String and the default expression will not be used

[jw]

absent can be of type String or String? and the default expression will be used

[sindrenm]

You should be able to set a default value, which would be used if the field was not present in the serialized data.

@Serializable
data class MyResponseBody(
  val id: Long,
  val email: String,
  val username: String? = null,
)

Alternatively, at least if you're using kotlinx.serialization.json, you can configure Json to not require explicit nulls by setting explicitNulls = false.

https://kotlinlang.org/api/kotlinx.serialization/kotlinx-serialization-json/kotlinx.serialization.json/-json-builder/explicit-nulls.html

[Colton Idle]

Makes sense. Thank you both!