Can't save changing class object into ArrayList properly - kotlin

Question

I created empty ArrayList of class objects and one class object. I added this class object with some parameters into ArrayList and then changed some parameters of this object. But class object that left in ArrayList also changed. That looks like I'm passing parameters by reference. Also, if I'm trying to create new object and make it equal with "=", that object becoming connected to old one, so if I'm changing first object then second one changing as well like reference to first variable. I've ran into same problem in Java (no surprise). So, how can I pass class object by value? I want to use one class object to add new objects into ArrayList and then change it without changing class object inside ArrayList.
class TestClass(var someData : Int = 0)
fun main(){
var testArrayList = arrayListOf<TestClass>()
var classObject : TestClass = TestClass(3)
testArrayList.add(classObject)
classObject.someData = 5
testArrayList.add(classObject)
classObject.someData = 7
testArrayList.add(classObject)
print(testArrayList[0].someData)
print(testArrayList[1].someData)
print(testArrayList[2].someData) //Output = 777. Needed 357
}
Same with creating new objects
class TestClass(var someData : Int = 0)
fun main(){
var testArrayList = arrayListOf<TestClass>()
var classObject : TestClass = TestClass(3)
var secondClassObject : TestClass = classObject
testArrayList.add(secondClassObject)
classObject.someData = 5
var thirdClassObject : TestClass = classObject
testArrayList.add(thirdClassObject)
classObject.someData = 7
var fourthClassObject : TestClass = classObject
testArrayList.add(classObject)
print(testArrayList[0].someData)
print(testArrayList[1].someData)
print(testArrayList[2].someData) //Output = 777. Needed 357
}
You try to make similar code in Java because arrayList in Java works the same in this situation. So, you can send answer in Java code

Answer 1

You are basically adding the same instance of TestClass over and again with modified value, so the instances in array position 3 will also point to same instance of 1, if you really want different values, you need to either create a different instances, or use data class with copy which copies the values into different instance and you can named parameters to change values you need as below
data class TestClass(var someData : Int = 0)
fun main(){
var testArrayList = arrayListOf<TestClass>()
var classObject : TestClass = TestClass(3)
testArrayList.add(classObject)
testArrayList.add(classObject.copy(someData = 5))
testArrayList.add(classObject.copy(someData = 7))
print(testArrayList[0].someData)
print(testArrayList[1].someData)
print(testArrayList[2].someData)
}

Answer 2

copy
To copy an object for changing some of its properties, but keeping the
rest unchanged, use the copy() function.
class TestClass(var someData : Int = 0) {
fun copy(someData: Int = this.someData) = TestClass(someData)
}
fun main(){
var testArrayList = arrayListOf<TestClass>()
var classObject : TestClass = TestClass(3)
testArrayList.add(classObject)
testArrayList.add(classObject.copy(someData = 5))
testArrayList.add(classObject.copy(someData = 7))
print(testArrayList[0].someData)
print(testArrayList[1].someData)
print(testArrayList[2].someData)
}