Make a new list from two other lists of different types by comparing values of each type - list

Question

I have two Lists of objects that both implement an interface, but are otherwise unrelated. How can I create a new collection of objects containing only the objects of one of the lists that match a value in the other list?
Obviously I could use a for loop & do this manually, but I'd like to know how I can do this using Kotlin's standard library collection filtering functions.
So here's an example:
interface Ids
{
val id: Int
}
data class A(override val id: Int, val name: String) : Ids
data class B(override val id: Int, val timestamp: Long) : Ids
fun main(args: Array<String>) {
val a1 = A(1, "Steve")
val a2 = A(2, "Ed")
val aCol = listOf(a1, a2)
val b2 = B(2, 12345)
val b3 = B(3, 67890)
val bCol = listOf(b2, b3)
val matches = mutableListOf<B>()
// This is where I'm stuck.
// I want to filter bCol using objects from aCol as a filter.
// The result should be that matches contains only a single object: b2
// because bCol[0].id == aCol[1].id
// I'm guessing I need to start with something like this:
bCol.filterTo(matches) { ??? }
}

Answer 1

A straightforward approach would be to search aCol for an object with the same id for each b in bCol:
bCol.filter { b -> aCol.any { a -> a.id == b.id } }
However that may become too slow if your lists are big enough.
To make it more scalable you can first build a set of all ids in aCol:
val aColIds = aCol.map { it.id }.toSet()
And then use Set.contains method to determine whether b.id is in aColIds:
bCol.filter { it.id in aColIds }
// or equivalent
bCol.filter { aColIds.contains(it.id) }