Kotlin: Find Max Date list of lists - kotlin

Question

In kotlin consider:
data class classA (
field1
field2
....
classBlst:List<classB>
)
data class classB (
field3
field4
...
dateField:String
)
The problem is to loop thru List, find the maximum dateField in classBlst, and return the corresponding field3 for that item. The dateField is a string and may contain nulls and also bad dates so need to try..catch.
I can build a list with all max dates and then do a second iteration on that list to find max. I can do that using iterators but looking for any lambda functional way of doing the same. Sorry new to all this.

Answer 1

Given:
data class ClassA(val list: List<ClassB>)
data class ClassB(val field3: Int, val dateField: String? = null)
You could get the latest date using mapNotNull where you'll format your date and return null if you get an exception.
Then maxBy on the field containing the formatted date in you new object created in the previous mapNotNull
I've choosed to use a Pair since it look like you only want a filter and don't want to keep the newly created object. But if that not the case consider creating a new class containing all the information needed.
val a = ClassA(
listOf(
ClassB(0, "24"),
ClassB(1, "24/03/1987"),
ClassB(2),
ClassB(3, "24/03/2024")
)
)
val res = a.list
.mapNotNull {
val dateFormater = DateTimeFormatter.ofPattern("dd/MM/yyyy")
try {
Pair(it.field3, LocalDate.parse(it.dateField, dateFormater))
} catch (e: Exception) {
null
}
}
.maxBy { it.second }