How do I keep the iteration order of a List when using Collections.toMap() on a stream?

Question

I am creating a Map from a List as follows:

List strings = Arrays.asList(\"a\", \"bb\", \"ccc\");

Map

        

Answer 1

In Kotlin, toMap() is order-preserving.

fun  Iterable>.toMap(): Map

Returns a new map containing all key-value pairs from the given collection of pairs.

The returned map preserves the entry iteration order of the original collection. If any of two pairs would have the same key the last one gets added to the map.

Here's its implementation:

public fun  Iterable>.toMap(): Map {
    if (this is Collection) {
        return when (size) {
            0 -> emptyMap()
            1 -> mapOf(if (this is List) this[0] else iterator().next())
            else -> toMap(LinkedHashMap(mapCapacity(size)))
        }
    }
    return toMap(LinkedHashMap()).optimizeReadOnlyMap()
}

The usage is simply:

val strings = listOf("a", "bb", "ccc")
val map = strings.map { it to it.length }.toMap()

The underlying collection for map is a LinkedHashMap (which is insertion-ordered).