Calculated transient value is null after deserialisation with Gson - kotlin

Question

I run into an error which I do not understand after deserializing with gson.
My usecase is way more complex, but I created this test below which shows the behaviour I see.
The issue is the calculated transient value (allObjs) that is null after deserialisation. The unit test fails on the last assert.
Apparently the #Transient prevents correct initialisation of allObjs after deserialisation.
Is this expected behaviour? Can I do anything to make the field work in this way?
If not then I have to convert all the transient fields to functions like getThemAll(), which does work.
I hope for any insights.
Regards, Rob
class CoupleDeserializeTest {
data class Couple( val objA: String, val objB: String){
#Transient
val allObjs: List<String> = listOf(objA, objB)
fun getThemAll() = listOf(objA, objB)
}
#Test
fun testDe_SerializeCouple() {
val couple = Couple("my", "text")
// This succeeds
Assert.assertNotNull(couple.allObjs)
val gson = Gson()
val json = gson.toJson(couple)
// This succeeds
Assert.assertEquals("{\"objA\":\"my\",\"objB\":\"text\"}", json)
val coupleDeserialized = gson.fromJson<Couple>(json, Couple::class.java)
// This succeeds
Assert.assertNotNull(coupleDeserialized.getThemAll())
// This fails
Assert.assertNotNull(coupleDeserialized.allObjs)
}
}

Answer 1

If you are marking a field as Transient, it will ignore the field from serialisation or deserialisation, snippet from JvmFlagAnnotations
/**
* Marks the JVM backing field of the annotated property as `transient`, meaning that it is not
* part of the default serialized form of the object.
*/
#Target(FIELD)
#Retention(AnnotationRetention.SOURCE)
#MustBeDocumented
public actual annotation class Transient
EDIT
It will ignore it in initialisation because, the value of objA and objB will be initialised using serialisation meaning objA and objB have not assigned by the time you allObjs gets initialised, to get the values later instead of directly assigning, you can use get()
data class Couple( val objA: String, val objB: String){
#Transient
val allObjs: List<String>
get() = listOf(objA, objB)
fun getThemAll() = listOf(objA, objB)
}