Sorting arraylist and returning an arraylist (not a list) - Kotlin

Question

I'm trying to display sorted information using a RecyclerView . To do so, I've got an ArrayList of custom datatype. I want to sort the data in the ArrayList by descending order, using one of the fields in the ArrayList . I'm familiar with the methods that are already available ( sortWith , sortBy , sortByDescending , etc) but unfortunately, they all change from ArrayList to List and this stops the recycler view from working. Ideally, I'd like to preserve the ArrayList because I need it for other processes that I carry out later in the project.

WHAT I'M TRYING TO ACHIEVE:

INPUT: arrayListName = ArrayList<CustomType>

PROCESS: [sort by decending order of arrayListName.firstName ]

OUTPUT: return ArrayList<CustomType>

I've tried casting the List (shown below) to ArrayList , but that throws the below error:

java.lang.ClassCastException: java.util.Arrays$ArrayList cannot be cast to java.util.ArrayList 

Line of code that throws the error (I've tried various methods, but they all throw an exception on that line):

val sortedModel = model.sortedWith(compareBy { it.name}) as ArrayList<Model>

Any help is appreciated(:

I'm happy with a java solution too because I understand it and can easily implement from java to kotlin.

Answer 1

You can not cast a List to an ArrayList directly. If you want to convert a List to ArrayList , you can do it this way:

val sortedModel = model.sortedByDescending { it.name }.toCollection(ArrayList())

Answer 2

This will help you

val list = arrayList.sortedByDescending { it.firstName.length }
        result.addAll(list)

Full code:

val arrayList: ArrayList<CustomType> = arrayListOf()
val result: ArrayList<CustomType> = arrayListOf()
arrayList.add(CustomType("Aadddd"))
arrayList.add(CustomType("Ccd"))
arrayList.add(CustomType("Bbdd"))
arrayList.add(CustomType("Ffddd"))
arrayList.add(CustomType("Dd"))

val list = arrayList.sortedByDescending { it.firstName.length }
result.addAll(list)

Output:

SORT [{"firstName":"Aadddd"},{"firstName":"Ffddd"},{"firstName":"Bbdd"},{"firstName":"Ccd"},{"firstName":"Dd"}]

Answer 3

Use sortWith instead it does inplace, try this

data class A(
    val x : Int
)

fun main(args: Array<String>) {
    var a = arrayListOf( A(1) ,A(2) ,A(3) ,A(4) )
    a.sortWith(compareByDescending { it.x })
    assert( a is ArrayList )
    println(a)
}

try yourself