Kotlin inline classes not correctly serialised in HashMap

[geertberkers]

I am using the following code:

inline class Identifier(val value: String)

data class FormInstance(
    var formId: Identifier,
    var createdAt: Date = Date(),
    var modifiedAt: Date = Date(),
    var fieldValues: HashMap<Identifier, String> = hashMapOf()
)

And the result is serialized as:

{
    "formId": "ADDISON_DIARY",
    "modifiedAt": "2019-05-01T23:44:22.703+0200"
    "createdAt": "2019-04-30T23:44:22.701+0200",
    "fieldValues": {
      "Identifier(value\u003dfrustration)": "VERYMUCH",
      "Identifier(value\u003dstress)": "NONE",
      "Identifier(value\u003dlimitations)": "ALITTLE",
      "Identifier(value\u003dsleep)": "ALOT",
      "Identifier(value\u003dnegative_effect_partner)": "ALITTLE",
      "Identifier(value\u003dsomber_feelings)": "VERYLITTLE",
}

So the problem is that the Identifier gets it value when its parsed itself, but when parsing the hashmap, it uses the following code: (screenshot line 207)

I think the code needs to check for inline classes if this is possible, because these are experimental in kotlin 1.3, and the library is written in java.

[lyubomyr-shaydariv]

I don't believe Gson should support Kotlin or its experimental features. It's probably a design problem that the Map type adapter uses static String.valueOf instead of a key-to-string strategy (and not rejecting non-String-keyed maps), but I would consider implementing a FormInstance to FormInstanceDto (having a map with true String keys, not inline classes) converter and vice versa.

[JakeWharton]

You can write a TypeAdapter which unwraps the class yourself or just override toString() and write out the wrapped property value. Gson won't be seeing updates for Kotlin.

[Marcono1234]

Relates to #1722