Why MapDB is not working with kotlin but works with Java?

Question

I am new in Kotlin. I can't understand why Map DB is not working for me with kotlin. I tried google but it was no help.

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 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 restricts use of public open fun put(key: K?, value: V?): V? defined in org.mapdb.HTreeMap

But this works with Java.

public static void main(String[] args) {
        DB db = DBMaker.fileDB("java.db").fileMmapEnable().transactionEnable().make();
        ConcurrentMap map = db.hashMap("map").createOrOpen();
        map.put("a", "b");
        map.put("a2", "b");
        System.out.println(map);
        System.out.println(map.getClass());
        db.commit();
        db.close();

        DB db2 = DBMaker.fileDB("java.db").fileMmapEnable().transactionEnable().make();
        ConcurrentMap map2 = db2.hashMap("map").open();

        System.out.println(map2);
        map2.forEach((o, o2) -> {
            System.out.println(o+" = "+o2);
        });
    }

Answer 1

@gidds is completely right about that Kotlin doesn't allow "raw" Java types and requires type parameters. So you can just cast your map like this and it will work fine.

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

If you don't like unchecked casts as I do, you can use a bit more verbose HashMapMaker constructor like this.

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