Deep copy of list with objects in Kotlin

Question

I am new to kotlin and I am trying to make a copy of a list of objects.The problem I am having is that when I change items in the new copy, the old list gets changed as well. This is the object:

class ClassA(var title: String?, var list: ArrayList<ClassB>, var selected: Boolean)
class ClassB(val id: Int, val name: String) 

I tried doing this, but it doesn't work:

val oldList:ArrayList<ClassA>


val newList :ArrayList<ClassA> = ArrayList()
newList.addAll(oldList)

Answer 1

That's bacause you are adding all the object references to another list, hence you are not making a proper copy, you have the same elements in two list. If you want diferents list and diferent references, you must clone every object in a new list:

public data class Person(var n: String)  fun main(args: Array<String>) {     //creates two instances     var anna = Person("Anna")     var Alex =Person("Alex")      //add to list     val names = arrayOf(anna , Alex)     //generate a new real clone list     val cloneNames = names.map{it.copy()}      //modify first list     cloneNames.get(0).n = "Another Anna clone"      println(names.toList())     println(cloneNames.toList()) }  [Person(n=Anna), Person(n=Alex)] [Person(n=Another Anna clone), Person(n=Alex)] 

Answer 2

var oldList: List<ClassA>?  val newList = oldList.map { it.copy() }

Answer 3

This is not related to kotlin, when you are adding the objects from the old list to the new one, it add the reference to them (no createing a new object ), whats mean it just copying the address in the memory to the new list.

To fix this problem you should create a new instance for each object. you can create a copy constructor, for example:

constructor(otherA: ClassA) {
    this.prop1 = otherA.prop1
    this.prop2 = otherA.prop2
    ...
} 

and then add them one by one to the new list:

list1.forEach { list2.add(Class(it)) }