How can you enforce a setter method to be called on creating a object that can produce compile time warning or exception? - kotlin

Question

Suppose I've a class A which looked like this before:
class A(str:String){
// body
}
Now, I want to remove the parameter from the constructor instead I'll be using a setter for that value. Like below:
class A(){
lateinit var str:String
fun setStr(paramsString:String){
str = paramsString
}
}
As I am using a setter for assigning the value, it wont give me a compile time exception while creating the object of that class.
So, I tried something like below:
class A(){
lateinit var str:String
init{
setStr(strValue:String)
}
fun setStr(paramsString:String){
str = paramsString
}
}
This init block will be executed after creating the object of the class A where the setter method will be called.
But still what I want is to warn or throwing an exception when the object of the following class is created for calling the setter method.

Answer 1

This here
class A(public str:String){
// body
}
already has a setter, since yo can do
A("Bla"); println(a.str)
compile time errors would probably need some type checking (e.g. String? allows null, String checks for not null)
if you want to execute code on a.str="Blub" you do this:
class A(){
var str:String = ""
set(str) {
// check some stuff here, throw Exception
field = str // if all is well
}
}
you can also do similar for get() BTW.
Another method to do stuff on setting is using var a: String by Delegate.vetoable.... and other Delegate methods - giving you high control over what's happening on setting a new value.
class A {
var str: String = ""
set(str) {
// check some stuff here, throw Exception
field = str // if all is well
}
var observableDelegate: String by Delegates.observable("") { p, o, n ->
println("property: [$p], Old: [$o], New [$n]")
}
var notNullValue: String by Delegates.notNull<String>()
var vetoable: String by Delegates.vetoable("") { property, oldValue, newValue ->
println("property: $property, old: $oldValue, new: $newValue")
newValue != "bla" && oldValue < newValue
}
}
fun main() {
val a = A()
a.observableDelegate = "blub"
println("Set value ${a.observableDelegate}")
runCatching { a.notNullValue }.onFailure {
println("Like expected: value not set yet, exception: ${it.message}")
}
a.notNullValue = "fuddder"
println("Now notNullValue is set: ${a.notNullValue}")
println("Vetoable now: ${a.vetoable} - setting to \"a\"")
a.vetoable = "a"
println("Vetoable now: ${a.vetoable} - setting to \"b\"")
a.vetoable = "b"
println("Vetoable now: ${a.vetoable} - setting to \"a\" again - should not work")
a.vetoable = "a"
println("Vetoable now: ${a.vetoable}")
}
output:
property: [var com.savinggrains.mwh.web.rest.A.observableDelegate: kotlin.String], Old: [], New [blub]
Set value blub
Like expected: value not set yet, exception: Property notNullValue should be initialized before get.
Now notNullValue is set: fuddder
Vetoable now: - setting to "a"
property: var com.savinggrains.mwh.web.rest.A.vetoable: kotlin.String, old: , new: a
Vetoable now: a - setting to "b"
property: var com.savinggrains.mwh.web.rest.A.vetoable: kotlin.String, old: a, new: b
Vetoable now: b - setting to "a" again - should not work
property: var com.savinggrains.mwh.web.rest.A.vetoable: kotlin.String, old: b, new: a
Vetoable now: b