thenBy

inline fun <T> Comparator<T>.thenBy(
    crossinline selector: (T) -> Comparable<*>?
): Comparator<T>
(source)

Creates a comparator comparing values after the primary comparator defined them equal. It uses the function to transform value to a Comparable instance for comparison.

import kotlin.test.*

fun main(args: Array<String>) {
//sampleStart
val list = listOf("aa", "b", "bb", "a")

val lengthComparator = compareBy<String> { it.length }
println(list.sortedWith(lengthComparator)) // [b, a, aa, bb]

val lengthThenString = lengthComparator.thenBy { it }
println(list.sortedWith(lengthThenString)) // [a, b, aa, bb]
//sampleEnd
}
inline fun <T, K> Comparator<T>.thenBy(
    comparator: Comparator<in K>,
    crossinline selector: (T) -> K
): Comparator<T>
(source)

Creates a comparator comparing values after the primary comparator defined them equal. It uses the selector function to transform values and then compares them with the given comparator.

import kotlin.test.*

fun main(args: Array<String>) {
//sampleStart
val list = listOf("A", "aa", "b", "bb", "a")

val lengthComparator = compareBy<String> { it.length }
println(list.sortedWith(lengthComparator)) // [A, b, a, aa, bb]

val lengthThenCaseInsensitive = lengthComparator
    .thenBy(String.CASE_INSENSITIVE_ORDER) { it }
println(list.sortedWith(lengthThenCaseInsensitive)) // [A, a, b, aa, bb]
//sampleEnd
}