How to use init construct in Kotlin in elegant way - oop

Question

There are many examples how to use init during class construction, but they are mostly without using actual class properties.
What I need to do is init a class property from database on object init. So far I have this:
class MyObj constructor(val id: Long){
var data: MutableMap? = null
init {
data = db.find(id) // more like pseudocode, db fetch is done and result assigned to data property
}
}
But it seems a little overcomplicated to me. Is there any better, more elegant way to do this ?

Answer 1

You can initialize the property directly in the class body:
class MyObj(val id: Long) {
val data: MutableMap = db.find(id)
}
Then you won't need to declare it as a nullable type, and may be able to use val instead of var. (I've also removed the constructor keyword which is redundant.).

Answer 2

You can just write:
class MyObj constructor(val id: Long){
val data: Map<String, String> = mapOf()
}
It's equivalent to:
class MyObj constructor(val id: Long){
val data: Map<String, String>
init {
data = mapOf()
}
}
Init block is useful if you need introduce some logic for your object initialization, like error checking etc. Ofc you can do it with or without init block. But remember what is more readable:
class MyObj constructor(val id: Long){
val data: Map<String, String> = if (id == 0L) {
mapOf(Pair("", ""))
} else {
throw IllegalStateException()
}
} // without init
class MyObj constructor(val id: Long){
val data: Map<String, String>
init {
data = if (id == 0L) {
mapOf(Pair("", ""))
} else {
throw IllegalStateException()
}
}
} // with init
You can also init this value using lazy (what means property will be initialized as function, but value will be assigned when you first ask for it, not when object is created). I guess taking value from db can be quite long, so it may be useful:
val data: Map<String, String> by lazy { mapOf() }