How to reduce the list of doubles to string?

I'm new to functional methods like reduce and fold in java 8 and Kotlin. I want to reduce -

List<List<List<Double>>> boundingPolygon = [[[-125.48845080566404,47.94508483691371],[-124.96110705566404,42.309040799653665],[-117.13884143066404,45.04173793121063],[-118.36931018066404,48.93624688577435],[-125.48845080566404,47.94508483691371]]]; 

to single string that represents one String concatenated by coordinates -

"-118.359053 33.931562,-118.372443 33.946939,-118.369053 33.951562,-118.337612 33.944342,-118.342012 33.944042,-118.359053 33.931562"

Trying to do -

val polygonCoordinates = boundingPolygon.first().reduce { acc, element ->
                    acc + "${element[0]} ${element[1]}"
                }

This is not working.

The acc in your reduce operation is of type List<Double>, not String. Look at the reduce function signature and you should understand why. Here's what I propose to do what you want:

coords.first().joinToString(", ") { (x, y) -> "$x $y" }

I used destructuring to extract the first and second values from the coordinate lists. So this will only work with 2D coordinates.

Instead of reducing it, you should just add those into a StringBuilder, which is efficient when you are doing multiple operations (like concatenating a ton of Strings):

val boundingPolygon = listOf(
    listOf(
        listOf(-125.48845080566404, 47.94508483691371),
        listOf(-124.96110705566404, 42.309040799653665),
        listOf(-117.13884143066404, 45.04173793121063),
        listOf(-118.36931018066404, 48.93624688577435),
        listOf(-125.48845080566404, 47.94508483691371)
    )
)

val sb = StringBuilder()
for (nestedList in boundingPolygon) {
    for (innerNestedList in nestedList) {
        sb.append(innerNestedList.joinToString(" "))
        sb.append(',')
    }
}
if (sb.isNotEmpty()) sb.deleteCharAt(sb.lastIndex)

println(sb)
// Output: -125.48845080566404 47.94508483691371,-124.96110705566404 42.309040799653665,-117.13884143066404 45.04173793121063,-118.36931018066404 48.93624688577435,-125.48845080566404 47.94508483691371

// val stringRepresentation = sb.toString()  // for further use as String data-type

Instead of Reduce, you can use flatMap. It will help you.

        List<List<List<Double>>> boundingPolygon = List.of(List.of(List.of(-124.96110705566404, 42.309040799653665)
            , List.of(-117.13884143066404, 45.04173793121063)
            , List.of(118.36931018066404, 48.93624688577435)
    ));
        var l = boundingPolygon.stream().flatMap(lists -> lists.stream().flatMap(doubles -> doubles.stream())).collect(Collectors.toList());
        System.out.println(l);

It will print an output as below.

[-124.96110705566404, 42.309040799653665, -117.13884143066404, 45.04173793121063, 118.36931018066404, 48.93624688577435]

Try above code, this will help you.