What is the proper way to create new instance of generic class in kotlin?

Question

I use following initialization:

val entityClass = javaClass<Class<T>>() var entity = entityClass.newInstance().newInstance() 

but it's wrong and causes IllegalAccessException on java.lang.Class.newInstance(Class.java:1208)

Answer 1

If you let IntelliJ add explicit type information, you see that entityClass is actually of type Class<Class<String>>. I'm not sure if that's what you want. In line 2 you are first creating an instance of Class<T> and then one of T but that's not possible anyway, because the generic information about T is lost at runtime. Apart from that you can't instantiate class objects directly.

Solution

One possible solution would be to add a parameter of type Class<T> to your function or class and use it to instantiate objects like this.

fun <T> foo(entityClass: Class<T>) {     var entity: T = entityClass.newInstance() }  fun test() {     foo(Object::class.java) } 

But there's actually a more elegant solution without the use of reflection. Define a parameter of method type () -> T and use constructor references. Here's my related question about constructor references and here's the code:

fun <T> foo2(factory: () -> T) {     var entity: T = factory() }  fun test() {     foo2(::Object) }