Kotlin - how to parse a string that is either in a LocalDateTime or LocalDate format into LocalDate - kotlin

Question

I have a mapper where I am getting a string that is either in this format:
"2021-06-08" or "2021-06-08T15:00"
in my mapper I should convert this string always into a LocalDate:
ExcelColumn.CHANGED_DATE -> rowData
.nullLocalDate(column.dbColumnName)
?.format(excelV2DateFormat)
nullLocalDate looks like this:
fun Entity.nullLocalDate(key: String): LocalDate? = if (this[key] == null) null else localDate(key)
fun Entity.localDate(key: String): LocalDate = when (val v = this[key]) {
is LocalDateTime -> v.toLocalDate()
is Instant -> LocalDate.ofInstant(v, ZoneId.of("UTC"))
is String -> LocalDate.parse(v)
else -> v as LocalDate
}
I also have a function that checks nullLocalDateTime:
fun Entity.nullLocalDateTime(key: String): LocalDateTime? = if (this[key] == null) null else localDateTime(key)
fun Entity.localDateTime(key: String, zoneId: ZoneId? = null): LocalDateTime = when (val v = this[key]) {
is Instant -> LocalDateTime.ofInstant(v, zoneId ?: ZoneId.of("UTC"))
is String -> LocalDateTime.parse(v)
else -> v as LocalDateTime
}
I can use one or the other, but how can I combine this two to check if it is either in localDate or localDateTime format and convert it into localDate in both cases?
As of now it works fine if the value is in format "2021-06-08", but it fails if the string is in the format of "2021-06-08T15:00":
java.time.format.DateTimeParseException: Text '2021-06-08T15:00' could not be parsed, unparsed text found at index 10
How can I accommodate both cases in one function?

Answer 1

You can define optional parts in the pattern of a DateTimeFormatter. In your case, you'd want:
val formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd['T'HH:mm]")
Where the ['T'HH:mm] defines the optional time part. You can then use this formatter when parsing your strings into LocalDate instances. There are a few ways this can be done, but the easiest and most readable is to use the LocalDate#parse(CharSequence,DateTimeFormatter) method.
Runnable example:
import java.time.LocalDate
import java.time.format.DateTimeFormatter
fun main() {
val formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd['T'HH:mm]")
val dateOnlyString = "2021-06-08"
val dateAndTimeString = "2021-06-08T15:00"
val parsedDateOnly = LocalDate.parse(dateOnlyString, formatter)
val parsedDateAndTime = LocalDate.parse(dateAndTimeString, formatter)
println("parse(\"$dateOnlyString\") --> $parsedDateOnly")
println("parse(\"$dateAndTimeString\") --> $parsedDateAndTime")
}
Output:
parse("2021-06-08") --> 2021-06-08
parse("2021-06-08T15:00") --> 2021-06-08