Java – How to combine two object lists by id and select non-null values in kotlin

I want to merge two object lists. Each object has a specific ID. If the same object exists in both lists, I want to merge them. If a value is null, I want to get a non-null value. For example:

val list1 = listOf(
    Object(id = 1, hello, 100),
    Object(id = 2, null, 40)
)

val list2 = listOf(
    Object(id = 1, null, 100),
    Object(id = 2, test, 40),
    Object(id = 3, hi, 13)
)

The result I want to achieve is as follows:

val result = listOf(
    Object(id = 1, hello, 100),
    Object(id = 2, test, 40),
    Object(id = 3, hi, 13)
)

Note: If a value is not empty, I know it’s no different.

fun main() {
    val list1 = listOf(
        Obj(id = 1, "hello", 100),
        Obj(id = 2, null, 40)
    )

val list2 = listOf(
        Obj(id = 1, null, 100),
        Obj(id = 2, "test", 40),
        Obj(id = 3, "hi", 13)
    )

val result = (list1.asSequence() + list2)
        .groupingBy { it.id }
        .reduce { _, accumulator: Obj, element: Obj ->
            accumulator.copy(second = accumulator.second ?: element.second)
        }
        .values.toList()

println(result)
}

data class Obj(val id: Int, val second: String?, val third: Int)