Map DB is not working for me with kotlin.

[karandaid]

Map DB is not working for me with kotlin. gradle

dependencies {
    compile(kotlin("stdlib-jdk8"))
    implementation(group="org.mapdb", name= "mapdb", version= "3.0.7")
    testCompile("junit", "junit", "4.12")
}

File.kt

import org.mapdb.DBMaker

fun main(array: Array<String>) {
    val db = DBMaker.memoryDB().make()
    val map = db.hashMap("map").createOrOpen()
    map.put("a", "a")
    db.close()
}

Error:(7, 13) Kotlin: Type mismatch: inferred type is String but Nothing? was expected. Projected type HTreeMap<out Any?, out Any?> restricts use of public open fun put(key: K?, value: V?): V? defined in org.mapdb.HTreeMap Error:(7, 18) Kotlin: Type mismatch: inferred type is String but Nothing? was expected. Projected type HTreeMap<out Any?, out Any?> restricts use of public open fun put(key: K?, value: V?): V? defined in org.mapdb.HTreeMap

[gabrielpaim]

Hi guys, any updates on this? The same example works on Java but doesn't on Kotlin. I would like to use a nested data class as the map's value, as a use case. Version 3.0.8. Thanks

[IRus]

Can you try again with kotlin 1.4.10?

[IRus]

Cast to actual type will fix your issue:

import org.mapdb.DBMaker
import org.mapdb.HTreeMap

fun main() {
    val db = DBMaker.memoryDB().make()
    val map = db.hashMap("map").createOrOpen() as HTreeMap<String, String>
    map.put("a", "a")
    db.close()
}

Not sure is this way people use map db, like put literally Any to it, or have some specialization for every map.

Also, if it's repeated patter in your code base, some extension can be useful:

@Suppress("UNCHECKED_CAST")
fun <K, V> HTreeMap<*, *>.useAs(): HTreeMap<K, V> = this as HTreeMap<K, V>

This extension can be used this way:

import org.mapdb.DBMaker
import org.mapdb.HTreeMap

fun main() {
    val db = DBMaker.memoryDB().make()
    val map = db.hashMap("map").createOrOpen().useAs<String, String>()
    map.put("a", "a")
    db.close()
}