Java/Kotlin - convert Set to Map<Long, Set<Long>>

Issue

I have Set of Long values

Set<Long> ids = {1,2,3,4}

What I'd like to achieve is

Set<Map<Long, Set<Long>>

and from this Set of ids I need to have Set with 4 elements like:

Set: {
Map -> key: 1, values: 2,3,4
Map -> key: 2, values: 1,3,4
Map -> key: 3, values: 1,2,4
Map -> key: 4, values: 1,2,3
}

How can i get it by stream or maybe kotlin's groupBy ?

Was anyone going to have a map like this? (Solution without a for or while loop)

Solution

Opinions on kotlin groupBy

Notice that groupBy can just split the original set into severial sets without intersection. So it's impossible to construct the mentioned map directly with groupBy function.

The solution below take advantage of groupBy when getting result, but result2 is much more clear to read and meets intuition:

fun main() {
    val set = setOf(1, 2, 3, 4)

    val result = set
        .groupBy { it }
        .mapValues { (_, values) -> set.filter { it !in values } }

    println(result) // {1=[2, 3, 4], 2=[1, 3, 4], 3=[1, 2, 4], 4=[1, 2, 3]}


    val result2 = HashMap<Int, List<Int>>().apply {
        set.forEach { this[it] = (set - it).toList() }
    }
    println(result2) // {1=[2, 3, 4], 2=[1, 3, 4], 3=[1, 2, 4], 4=[1, 2, 3]}
}

Answered By - SMSQO