Kotlin concurrent map not storing custom object

In java it is possible to write the following:

ConcurrentMap<Int, AnswerData> map = db.hashMap("map").createOrOpen();

I tried to use following in kotlin, but the objectes stored are not serialised & stored correctly. They are not filled after reading.

val db = DBMaker.fileDB("file.db").make()
val map = db.hashMap("mymap").createOrOpen() as ConcurrentMap<Int, AnswerData>
map.put(5, mayAnswerData)
db.close()

How is it done in kotlin? Would be nice if you could make more examples which show how to use for example custom objects.

Start with the Kotlin example using DBMaker.fileDB, hashMap, createOrOpen, and ConcurrentMap, then review how MapDB handles custom-object serialization across close and reopen. Done means documenting a working Kotlin custom-object example and verifying that the stored object is populated after reading it back.