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.

So, how would I / should I clone arbitrary Kotlin object?

Should I just use clone() as in Java?

Answer 1

For a data class, you can use the compiler-generated copy() method. Note that it will perform a shallow copy.

To create a copy of a collection, use the toList() or toSet() methods, depending on the collection type you need. These methods always create a new copy of a collection; they also perform a shallow copy.

For other classes, there is no Kotlin-specific cloning solution. You can use .clone() if it suits your requirements, or build a different solution if it doesn't.

Answer 2

You can use Gson 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. 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 projectClone = originalProject.clone()

Answer 3

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).