BeanUtils.instantiateClass is an interesting way to create a class as it takes care of kotlin features behind the scenes for you. There is a slightly mismatch with Java though.

If a parameter is not available for a primitive type, we need to specify the default value for the primitive type (i.e. 0 for int and false for boolean). With Kotlin, you have to pass null as the parameter may have a default value that we shouldn't be overriding.

Consider the following example:

class JavaExample {

  JavaExample(int counter, boolean flag, String value) { ... }
}

class KotlinExample(val counter : Int = 0, val flag: Boolean = false, val value : String?) { ... }

Let's assume we have a value for the value parameter and no information for the counter or flag.

Java requires us to provide [0, false, "my value"]
Kotlin requires us to provide [null, ,null, "my value"]

An improvement would be to provide only the latter and let the internal implementation translates null to the proper default value if the parameter is primitive.

This will benefit spring-projects/spring-boot#8762 where we're currently computing the array differently depending on the fact the target type is kotlin or not.