How to sort an array of object in kotlin with custom order?

Question

I have an array of object User(val name: String, val role: String). Role can be: leader, student, teacher, parent. And should be sorted in that order. I've read some article about Comparator but haven't figure out how they work. Can you please add some explanation too?

Answer 1

You can instantiate a HashMap for indicating roles priority:

private val roles: HashMap<String, Int> = hashMapOf(
    "Leader" to 0,
    "Student" to 1,
    "Teacher" to 2,
    "Parent" to 3
)

fun sortUsers(users: ArrayList<User>): ArrayList<User> {
    val comparator = Comparator { o1: User, o2: User ->
        return@Comparator roles[o1.role]!! - roles[o2.role]!!
    }
    val copy = arrayListOf<User>().apply { addAll(users) }
    copy.sortWith(comparator)
    return copy
}

In the comparator, if the result of the subtraction is negative, it will sort it in the order [o1, o2]. Otherwise, [o2, o1]. So in this case, it will sort your list in an ascending manner (since we have the highest priority as 0 - Leader as indicated in the HashMap).

For example, if we have o1 as Teacher and o2 as Leader, the result of the comparator will be: 2 - 0 = 2 which is a positive integer. Hence, it is sorted as [o2 (Leader), o1 (Teacher)]. Reversing the roles however yields the opposite result: 0 - 2 = -2 and hence it will be ordered [o1 (Teacher), o2 (Leader)].

You can verify this result with:

fun main(args: Array<String>) {
    val users = arrayListOf(
        User("john", "Student"),
        User("tim", "Teacher"),
        User("nice", "Student"),
        User("hey", "Leader"),
        User("you", "Parent")
    )
    println(sortUsers(users))
}

Which prints out: [User(name=hey, role=Leader), User(name=john, role=Student), User(name=nice, role=Student), User(name=tim, role=Teacher), User(name=you, role=Parent)]