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)
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)]
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)) }
Use 'to' for iterate object
List -> toList()
Array -> toArray()
ArrayList -> toArray()
MutableList -> toMutableList()
Example:
val array:ArrayList<String> = ArrayList()
array.add("1")
array.add("2")
array.add("3")
array.add("4")
val arrayCopy = array.toArray() // copy array to other array
Log.i("---> array " , array?.count().toString())
Log.i("---> arrayCopy " , arrayCopy?.count().toString())
array.removeAt(0) // remove first item in array
Log.i("---> array after remove" , array?.count().toString())
Log.i("---> arrayCopy after remove" , arrayCopy?.count().toString())
print log:
array: 4
arrayCopy: 4
array after remove: 3
arrayCopy after remove: 4