Copying elemets from another list (saved in companion object) and modfying them also reflecting modifications to the original companion object

Question

I have an Activity A which has:

class ActivityA {
   companion object {
        var list: MutableList<Person> = //objects are acquired here. 
   }
}

In ActivityB, I copy this list to a variable.

class ActivityB {

    var copyList: MutableList<Person> = ActivityA.list.toMutableList() 
}

After that I am changing some data of the copyList. For example, let's change name of any element. Let's say original list has list.get(0).name = "Bruno". Now, change to something else.

 copyList.get(0).name = "Alex"

Problem is this is also causing the element at index 0 to be also changed in list. Which means list.get(0).name and copyList.get(0).name has the same names "Alex" now.

How can I make sure that original list elements are not changed even though copyList elements are changed?

score 1 · Accepted Answer · answered Jul 17 '19 at 16:30

You'll need to define a method which makes a copy of Person (and probably of any of its fields recursively or you'll run into a similar problem). If it's a data class, there is already a copy() method, but note it won't copy the fields. And on JVM you can use Cloneable to get a default implementation for non-data classes as described in https://discuss.kotlinlang.org/t/how-to-use-cloneable/2364, but it's generally not recommended.

Anyway, after you have it,

var copyList: MutableList<Person> = ActivityA.list.mapTo(mutableListOf<Person>()) { it.copy() }

But this is part of why you should prefer immutability in the first place.

Copying elemets from another list (saved in companion object) and modfying them also reflecting modifications to the original companion object

1 Answers1