How to clone object in Kotlin?

Question

The question is that simple.

Kotlin documentation describes cloning only in accessing Java and in enum class. In latter case clone is just throwing an exception.

Answer 1

I've voted for @yole for nice answer, but other ways if you don't (or can't) use data class. You can write helper method like this:

object ModelHelper {

    inline fun <reified T : Serializable> mergeFields(from: T, to: T) {
        from::class.java.declaredFields.forEach { field ->
            val isLocked = field.isAccessible
            field.isAccessible = true

            if (field.get(from) != null) {
                field.set(to, field.get(from))
            }

            field.isAccessible = isLocked
        }
    }

}

So you can "copy" instance A into B by:

val bInstance = AClassType()
ModelHelper.mergeFields(aInstance, bInstance)

Sometimes, I use this way to merge data from many instances into one object which value available (not null).

Answer 2

If the class you are trying to clone does not implement Cloneable or is not a data class and is a part of an outside library, you can create an extension method that returns a new instance. For example:

class Person {
  var id: String? = null
  var name: String? = null
} 
fun Person.clone(): Person {
  val person = Person()
  person.id = id
  person.name = name
  return person 
}

Answer 3

You can use Gson library to convert the original object to a String and then convert back that String to an actual Object type, and you'll have a clone. Although this is not the intended usage of the Gson library which is actually used to convert between JSON and other object types, but I have devised this method to solve the cloning problem in many of my Kotlin based Android applications. See my example. Put this function in the class/model of which you want to create a clone. In my example I'm cloning a Project type object so I'll put it in the Project class

class Project{
 fun clone(): Project {
                val stringProject = Gson().toJson(this, Project::class.java)
                return Gson().fromJson<Project>(stringProject, Project::class.java)
            }
}

Then use it like this:

val originalProject = Project()
val clonedProject = originalProject.clone()

Answer 4

It requires to implement Cloneable for your class then override clone() as a public like:

public override fun clone(): Any {<your_clone_code>}

https://discuss.kotlinlang.org/t/how-to-use-cloneable/2364/3

Answer 5

fun <T : Any> clone (obj: T): T {
  if (!obj::class.isData) {
    println(obj)
    throw Error("clone is only supported for data classes")
  }

  val copy = obj::class.memberFunctions.first { it.name == "copy" }
  val instanceParam = copy.instanceParameter!!
  return copy.callBy(mapOf(
    instanceParam to obj
  )) as T
}

Answer 6

It's also possible to clone an object using kotlinx.serialization

import kotlinx.serialization.Serializable
import kotlinx.serialization.json.Json
import kotlinx.serialization.json.JsonConfiguration

@Serializable
class A
{
    val name: String = "Cloneable class A"

    fun clone(): A {
        val json = Json(JsonConfiguration.Stable)
        val jsonStr = json.stringify(serializer(), this)
        return json.parse(serializer(), jsonStr)
    }
}