Kotlin var does not save generic type (deserialized to LinkedTreeMap instead)

data class BigClass(var inBig: Map<String, List<SmallClass>>? = null)
data class SmallClass(var inSmall: String? = null)

fun main(args: Array<String>) {
    val json = """
{
    "inBig": {
        "key": [
            { "inSmall": "hello" }
        ]
    }
}
"""

    val deserialized = Gson().fromJson(json, BigClass::class.java)
    println(deserialized.inBig!!["key"]!![0].inSmall) // should print "hello" but throws instead
}

Executing the code above will lead to the following Exception:

Exception in thread "main" java.lang.ClassCastException: com.google.gson.internal.LinkedTreeMap cannot be cast to SmallClass
	at MyClass.main(MyClass.kt:18)

The List<SmallClass> is actually deserialized to List<LinkedTreeMap>.

However, if I changed the definition of inBig variable to be a constant, then it deserializes just fine:

data class BigClass(val inBig: Map<String, List<SmallClass>>? = null)

I changed var to val and the code prints “hello” as expected.

Kotlin 1.1.1 and Gson 2.8.1.

[piotrek1543]

after adding in data class @JvmSuppressWildcards(suppress = true) to problematic field I still get LinkedTreeMap

@JvmSuppressWildcards(suppress = true)
    @SerializedName("patient")
    @Expose
    var patient: Any? = null

[NightlyNexus]

yep, you probably want @JvmSuppressWildcards. https://github.com/google/dagger/issues/668#issuecomment-289713497