Kotlin.kt

    class CollateralArrangement(val trackingNumber: Int)

    class Facility(val collateralArrangement: List<CollateralArrangement>)

    class CollateralArrangementTuple(val collateralArrangement: CollateralArrangement, val facility: Facility)

    val facilities = listOf(Facility(collateralArrangement = listOf(CollateralArrangement(1), CollateralArrangement(2), CollateralArrangement(3))))

    // with Kotlin, results in: Map<Int, CollateralArrangementTuple> 
    val map = facilities.flatMap { facility -> facility.collateralArrangement.map { Pair(it.trackingNumber, CollateralArrangementTuple(it, facility)) } }.toMap()

    // with Java
    Map<Integer, CollateralArrangementTuple> map = facilities.stream().flatMap(facility -> facility.getCollateralArrangement().stream().map(ca -> new Pair<>(ca.getTrackingNumber(), new CollateralArrangementTuple(ca, facility)))).collect(Collectors.toMap(Pair::getKey, Pair::getValue));