Creating a new instance of a KClass

Question

I have a Kotlin class whose primary (and only) constructor is empty.

I have a reference to this class:

val kClass: KClass<MyClass> = MyClass::class 

How do I create an instance of this class using reflection?

In Java I would do myClass.newInstance() but it seems in Kotlin I need to find the constructor first:

kClass.constructors.first().call() 

I have seen mention of primaryConstructor in some bug reports but it's not showing up in my IDE.

Answer 1

You can use the Java class to create new instance:

MyClass::class.java.newInstance() 

Answer 2

In your case, Java reflection might be enough: you can use MyClass::class.java and create a new instance in the same way as you would with Java reflection (see @IngoKegel's answer).

But in case there's more than one constructor and you really need to get the primary one (not the default no-arg one), use the primaryConstructor extension function of a KClass<T>. It is a part of Kotlin reflection, which is not shipped within kotlin-stdlib.

To use it, you have to add kotlin-reflect as a dependency, e.g. a in Gradle project:

dependencies {      compile "org.jetbrains.kotlin:kotlin-reflect:$kotlin_version"     } 

^{Assuming that there is ext.kotlin_version, otherwise replace $kotlin_version with the version you use.}

Then you will be able to use primaryConstructor, for example:

fun <T : Any> construct(kClass: KClass<T>): T? {     val ctor = kClass.primaryConstructor     return if (ctor != null && ctor.parameters.isEmpty())         ctor.call() else         null }