How do I create a "Class" instance in Kotlin with specific type parameters? - kotlin

Question

I have this parent class and this child class:
abstract class ParentClass<T> {
protected abstract val dataClass: Class<T>
}
class ChildClass<K, V> : ParentClass<Map<K, V>>() {
override val dataClass: Class<Map<K, V>>
get() = HashMap::class.java // compilation error, invalid return type
}
I want the child class to implement the dataClass member such that I can use it to create instances of type T later on:
T data = dataClass.newInstance()
However, the return type for my implementation of dataClass is not valid in this context, so it doesn't compile:
Class<Map<K, V>> // required
Class<HashMap<*, *>> // found
Is there any way to specify the type parameters for the Class instance, or a different solution altogether to this problem? I realize I could replace the parent member with Class<*>, but I want something that is type safe.

Answer 1

I just ended up modifying the member into a function to generate a new instance, and added a second lazy member to get the class by creating a temporary instance:
abstract class ParentClass<T : Any> {
protected abstract fun createDataInstance(): T
private val dataClass: Class<T> by lazy {
createDataInstance()::class.java as Class<T>
}
}
class ChildClass<K, V> : ParentClass<Map<K, V>>() {
override fun createDataInstance(): Map<K, V> = HashMap()
}
// snippet (Java)
ChildClass<Integer, Integer> childClass = new ChildClass<>();
Map<Integer, Integer> instance = childClass.getDataInstance();
instance.put(0, 1);
System.out.println("Child instance #1: " + instance);
System.out.println("Child instance class " + childClass.getDataClass());
System.out.println("Child instance #2: " + childClass.getDataInstance());
// output (Java)
Child instance #1: {0=1}
Child instance class class java.util.HashMap
Child instance #2: {}