list - Create generic extension to make a deep copy of a List with Data Class

Question

Hi I'm trying to create a generic function to make a deep copy of list using the copy() method that a Data class provide us.

I'm using the method that I found on other post which returns a list of objects being deep copied:

listDataClass.map{ it.copy() }

But I can't seem to know how to make it as a generic function for data class so I'm really curious to see how'd you make one like this.

Answer 1

By using kotlin reflection magic, you can do it like this:

class NotADataClassException(clazz: KClass<out Any>): Exception("$clazz is not a data class")

fun List<T>.deepCopy(): List<T> {
    return map { item ->
        if (!item::class.isData) {
            throw NotADataClassException(item::class)
        } else {
            item::class.members.first { it.name == "copy" }.callBy(emptyMap()) as T
        } 
    }
}

However, this is bad because of the following reasons:

• This doesn't constrain the elements to data classes at compile time (that is what we wanted to do with the generics!) instead, it throws an exception.

• Kotlin reflection is slow and big, and reflection is used for every item.