Custom Components Sorting based on required Ordering

Issue

Trying to sort the custom components on defined order, and other subjects should be after that desired order: Like if there is any other subject except the defined List ie "GK" then it should be on last position etc. However I am getting Null Pointer Exception due to subject is not defined in the requireList if scheduleCommandList have the subject which is not in requiredList. Can anyone help me out to how to overcome this. Thanks.

Desired Order List is Below:

private val requiredList: HashMap<String, Int> = hashMapOf(
        "Maths" to 0,
        "Physics" to 1,
        "Science" to 2,
           )

Sorting function to sort the List:

private fun sortCommandList(scheduleCommandList: ArrayList<BaseComponent>): ArrayList<BaseComponent> {
              val comparator = Comparator { o1: BaseComponent, o2: BaseComponent ->
                return@Comparator requiredList[o1.name]!! - requiredList[o2.name]!!
            }
             val copy = arrayListOf<BaseComponent>().apply { addAll(scheduleCommandList) }
        copy.sortWith(comparator)
        return copy
    }

Solution

It seems you understand the problem correctly. If an item is not present in requiredList then you still try to compare their required positions and this causes NullPointerException. Remember that you should use !! only in cases when you are sure there can't be a null. In this case null is possible and we have to handle it somehow. The easiest is to replace it with Int.MAX_VALUE which places the item at the end. Also, this code can be really much simpler:

private fun sortCommandList(scheduleCommandList: List<BaseComponent>): List<BaseComponent> {
    return scheduleCommandList.sortedBy { requiredList[it.name] ?: Int.MAX_VALUE }
}

It can be even better to create this utility as extension function:

private fun List<BaseComponent>.mySort(): List<BaseComponent> {
    return sortedBy { requiredList[it.name] ?: Int.MAX_VALUE }
}

Then we can simplify the name of the function, because it is implicit that it is used to sort BaseComponent objects.

Answered By - broot