SortedArrayList.kt

class SortedArrayList<T>(
        vararg elements: T,
        private val comp: Comparator<T>
) : SortedMutableList<T> {

    private val list = ArrayList(elements.toList())

    override val size: Int
        get() = list.size

    override fun add(element: T) = findIndex(element)
            .let { index -> list.add(if (index < 0) -(index + 1) else index, element) }

    override fun remove(element: T) = findIndex(element)
            .let { index -> if (index >= 0) list.removeAt(index) }

    override fun get(index: Int): T = list[index]

    override fun contains(element: T) = findIndex(element).let { index ->
        index >= 0 && element == list[index] || (findEquals(index + 1, element, 1) || findEquals(index - 1, element, -1))
    }

    override fun iterator(): Iterator<T> = list.iterator()

    private fun findIndex(element: T): Int = list.binarySearch(element, comp)

    private tailrec fun findEquals(index: Int, element: T, step: Int): Boolean = when {
        index !in 0..(size - 1) -> false
        comp.compare(element, list[index]) != 0 -> false
        list[index] == element -> true
        else -> findEquals(index + step, element, step)
    }
}

fun <T : Comparable<T>> sortedMutableListOf(vararg elements: T): SortedMutableList<T> =
        SortedArrayList(*elements, comp = compareBy { it })

fun <T> sortedMutableListOf(comparator: Comparator<T>, vararg elements: T): SortedMutableList<T> =
        SortedArrayList(elements = *elements, comp = comparator)

... many years later, but this doesn't pass the tests. I had to change

private val list = ArrayList(elements.toList()) 

to sort the list being inserted to get all tests to pass, otherwise the list starts in wrong order and isn't sorted.

i.e.

private val list = ArrayList(elements.toList().sortedWith(comp))

Illustrated simplest with the following tests:

    @Test
    fun `When we define elements using sortedMutableListOf, their order is corrected`() {
        assertCollectionEquals(listOf(1, 2, 5, 6), sortedMutableListOf(2, 6, 1, 5))
        assertCollectionEquals(listOf(1, 2, 5, 6), sortedMutableListOf(compareBy { it }, 2, 6, 1, 5))
        assertCollectionEquals(listOf(6, 5, 2, 1), sortedMutableListOf(compareBy { -it }, 2, 6, 1, 5))
    }