private set(value) requires initilization before init{ } - kotlin

Question

I am trying to add a private set(value) to a var, but this requires to init the field before init { } has run. Why?
fun main(args: Array<String>) {
println("Hello fellow Stackoverflowers!")
val data = Data(listOf<Point>(Point(1, 1.0, 1.0), Point(2, 2.0, 2.0), Point(3, 3.0, 3.0)))
val testInit = TestInit(data)
testInit.magicMethod()
println("Relevant Point Id: ${testInit.relevantPoint.id}")
}
class TestInit(val someData: Data) {
var relevantPoint: Point // = Point(0,0.0,0.0) // or do this, but why? It is set in init{}, lateinit also not allowed
// private set // this works
private set(value) { // Why can't I do this? -> "Property must be initialized - Error"
if (value.id < 100)
field = value
else
field = someData.points.first()
}
init {
if (someData.points.size < 3) // doing validation before setting the point
throw IllegalArgumentException("Need at least three points!")
relevantPoint = someData.points.first() // here the point gets initialized
}
fun magicMethod() {
// do other calculations
relevantPoint = someData.points[someData.points.size / 2] // just assign some point
}
}
data class Data(var points: List<Point> = mutableListOf())
data class Point(val id: Int, val x: Double, val y: Double)

Answer 1

When you call relevantPoint = someData.points.first(), the field will only get initialised if it's id is less than 100 (due to your setter logic). So there is a chance that you won't have initialised the field.
You can either use a backing field if you need to keep the initialisation in init, or simply initialise the value inline, which won't go through the setter:
var relevantPoint = someData.points.first()
private set(value) {
...
}
Edit
It's interesting that this problem occurs even after you adding an else branch to your setter and I am not sure why that is. However for a solution (if you can't initialise it as above) you can easily use a backing field:
private var _relevantPoint: Point
var relevantPoint: Point
private set(value) {
if (value.id < 100) _relevantPoint = value
}
get() = _relevantPoint
init {
_relevantPoint = someData.points.first()
}

Answer 2

In Kotlin, variables must be either initialized on declaration or (only in case of var) get marked with lateinit.
According to the Documentation:
The modifier (lateinit) can be used on var properties declared inside the body of
a class (not in the primary constructor, and only when the property
does not have a custom getter or setter) as well as for top-level
properties and local variables.
So You can't do The latter
Your best bet is what Henry suggested:
var relevantPoint = someData.points.first()
private set(value) {
if (value.id < 100)
field = value
}
The other option is that you don't use custom setter and check your criteria elsewhere.

Answer 3

I think the best option is what was presented earlier.
But I thought of an alternative, this way.
data class Data(var points: List<Point> = mutableListOf())
data class Point(var id: Int, val x: Double, val y: Double)
class RelevantPoint(_point: Point) {
var point: Point = _point
set(value) {
if (value.id < 100)
field = value
}
}
fun main(args: Array<String>) {
println("Hello fellow Stackoverflowers!")
val data = Data(listOf<Point>(Point(1, 1.0, 1.0), Point(2, 2.0, 2.0), Point(3, 3.0, 3.0)))
val testInit = TestInit(data)
testInit.magicMethod()
println("Relevant Point Id: ${testInit.relevantPoint.point.id}")
}
class TestInit(val someData: Data) {
var relevantPoint: RelevantPoint
init {
if (someData.points.size < 3) // doing validation before setting the point
throw IllegalArgumentException("Need at least three points!")
relevantPoint = RelevantPoint(someData.points.first())
}
fun magicMethod() {
// do other calculations
relevantPoint.point = someData.points[someData.points.size / 2] // just assign some point
}
}