kotlin 如何对列表进行排序<Map<String, Any> &gt;?

Question

I want sort rawData according to the item date

val rawData: List<Map<String, Any>> = parseSampleData()

val sortData = rawData.sortedBy { it["date"] }

then error in sortedBy like this

Type parameter bound for R in 

inline fun <T, R : Comparable<R>> Iterable<T>.sortedBy
(
crossinline selector: (T) → R?
)
: List<T>
is not satisfied: inferred type Any is not a subtype of Comparable<Any>

also this not work

val sortedList = rawData.sortedWith(compareBy({ it.get("date") }))

with this error

Type mismatch.
Required:
Comparable<*>?
Found:
Any?

please help

Answer 1

The issue that you are facing is that kotlin doesn't know how to sort objects of type Any? , which is what is returned by it["date"] and it.get("date") . If you know the type that is returned by it["date"] , then cast it to that type, provided it implements Comparable (I assume it is some kind of a date object which should be comparable). Otherwise you need to specity what your map contains in the value slot (again provided taht it implements Comparable ).

Answer 2

You'll have to convert the date to either LocalDate or ZonedDateTime , which can then be compared.

import java.time.LocalDate

fun main() {
    val rawData: List<Map<String, Any>> = parseSampleData()
    val sortData = rawData.sortedBy { it["date"] as LocalDate }
    println(sortData)
}

fun parseSampleData() = listOf(
    mapOf("date" to LocalDate.parse("2020-01-03")),
    mapOf("date" to LocalDate.parse("2020-01-01")),
    mapOf("date" to LocalDate.parse("2020-01-02")),
)