What does toSortedMap in Kotlin

Question

What does the method "toSortedMap"? According to the documentation we can read:

Converts this Map to a SortedMap. The resulting SortedMap determines the equality and order of keys according to their natural sorting order.

But what does natural sorting order mean ? Their example doesn't look sorted.

absolutely not, I don't know why someone devote it, I told in the documentation it doesn't look sorted — discCard, Feb 23 '22 at 20:50
Do you mean `[b, c, d]` doesn't look sorted or you mean some other example? — broot, Feb 23 '22 at 20:55
kotlin official documentation, thanks for help, karma will back to u — discCard, Feb 26 '22 at 17:10

Endzeit · Accepted Answer · 2022-02-23T21:12:08.080

The function signature of toSortedMap is defined as:

fun <K : Comparable<K>, V> Map<out K, V>.toSortedMap(): SortedMap<K, V>

The sort order thus is defined by the comparison logic defined while implementing Comparable by the type K resembling the keys.

For numeric and string values, this is most likely the intuitive ascending sort order. For custom types, e.g. User, a custom implementation to fulfill the Comparable interface must be provided.

Alternatively there is a second signature, which allows to provide a custom Comparator<in K> to make use of a custom sort order, if the keys of type K don't implement Comparable<K> or this implementation does not fullfil the expected ordering.

fun <K, V> Map<out K, V>.toSortedMap(
    comparator: Comparator<in K>
): SortedMap<K, V>

Additional information can be found in the documentation of SortedMap and Comparable<T>.

What does toSortedMap in Kotlin

1 Answers1