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.

It'd be pretty easy to help you, honestly, if we had the code you were using that produced that error. — Louis Wasserman
– Louis Wasserman, Commented Dec 3, 2021 at 4:42
Make sure import types are matching, java.util.Arrays$ArrayList, vs java.util.ArrayList — Gayan Weerakutti
– Gayan Weerakutti, Commented Dec 3, 2021 at 4:45
@LouisWasserman i've added the line of code towards the very end (: — callMe_whatever
– callMe_whatever, Commented Dec 3, 2021 at 4:49
@GayanWeerakutti I don't actually import anything.. should I be? — callMe_whatever
– callMe_whatever, Commented Dec 3, 2021 at 4:55

Arpit Shukla · Accepted Answer · 2021-12-03 04:58:33Z

5

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())

edited Dec 3, 2021 at 4:58

answered Dec 3, 2021 at 4:51

Arpit Shukla

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Nhân Khuất Văn · Accepted Answer · 2021-12-03 04:52:55Z

0

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"}]

answered Dec 3, 2021 at 4:52

Nhân Khuất Văn

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Anshul · Accepted Answer · 2021-12-03 05:00:56Z

0

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

answered Dec 3, 2021 at 5:00

Anshul

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