How to add an item to a list in Kotlin?

Question

I'm trying to add an element list to the list of string, but I found Kotlin does not have an add function like java so please help me out how to add the items to the list.

class RetrofitKotlin : AppCompatActivity() {

    var listofVechile:List<Message>?=null
    var listofVechileName:List<String>?=null
    var listview:ListView?=null
    var progressBar:ProgressBar?=null
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_retrofit_kotlin)

        listview=findViewById<ListView>(R.id.mlist)
        var apiInterfacee=ApiClass.client.create(ApiInterfacee::class.java)
        val call=apiInterfacee.getTaxiType()
        call.enqueue(object : Callback<TaxiTypeResponse> {

            override fun onResponse(call: Call<TaxiTypeResponse>, response: Response<TaxiTypeResponse>) {

                listofVechile=response.body()?.message!!
                println("Sixze is here listofVechile   ${listofVechile!!.size}")
                if (listofVechile!=null) {
                    for (i in 0..listofVechile!!.size-1) {

                        //how to add the name only listofVechileName list

                    }
                }
                //println("Sixze is here ${listofVechileName!!.size}")
                val arrayadapter=ArrayAdapter<String>(this@RetrofitKotlin,android.R.layout.simple_expandable_list_item_1,listofVechileName)
                listview!!.adapter=arrayadapter

            }
            override fun onFailure(call: Call<TaxiTypeResponse>, t: Throwable) {

            }
        })
    }
}

huh? what list are you talking about? Kotlin lists ARE Java lists, aren't they? I'm sorry if the foreign names are throwing me off. — CryptoFool, Apr 03 '19 at 06:25
@Steve I'm talking about var listofVechileName:List?=null how to add the item on it — Mohit Lakhanpal, Apr 03 '19 at 06:27
You need to take a step back, and read the documentation to learn the fundamentals of Kotlin, by simply reading the documentation: https://kotlinlang.org/docs/reference/collections.html — JB Nizet, Apr 03 '19 at 06:32
Ah yes...the immutable thing. I just didn't put 2 and 2 together on this one. Makes perfect, obvious sense. I don't get to do enough Kotlin coding yet. It's not about reading the docs. Rather, it's about doing your day job in Java, and having to pull your head out of that and think in Kotlin. — CryptoFool, Apr 03 '19 at 06:47
A simple answer, **you can't** directly add items to **List** without converting it to its mutable form. — iCantC, Feb 04 '20 at 07:08

score 72 · Answer 1 · answered Apr 03 '19 at 17:41

72

A more idiomatic approach would be to use MutableList instead of specifically ArrayList. You can declare:

val listOfVehicleNames: MutableList<String> = mutableListOf()

And add to it that way. Alternatively, you may wish to prefer immutability, and declare it as:

var listOfVehicleNames: List<String> = emptyList()

And in your completion block, simply reassign it:

listOfVehicleNames = response.body()?.message()?.orEmpty()
    .map { it.name() /* assumes name() function exists */ }

answered Apr 03 '19 at 17:41

Kevin Coppock

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Oh yeah, that's so obvious! Much easier than java. I'm so glad we switched. – SMBiggs Jan 18 '20 at 20:24
@SMBiggs You mean that ironically, don't you? – The incredible Jan Jul 20 '21 at 09:17
1

@TheincredibleJan Yes. To quote TJ Miller, "The problem with satire is that it's so easily misinterpreted." – SMBiggs Aug 12 '21 at 14:47

score 71 · Answer 2 · answered Apr 30 '20 at 13:07

Talking about an idiomatic approach...

When you can get away with only using immutable lists (which means usually in Kotlin), simply use + or plus. It returns a new list with all elements of the original list plus the newly added one:

val original = listOf("orange", "apple")
val modified = original + "lemon" // [orange, apple, lemon]

original.plus("lemon") yields the same result as original + "lemon". Slightly more verbose but might come in handy when combining several collection operations:

return getFruit()
       .plus("lemon")
       .distinct()

Besides adding a single element, you can use plus to concatenate a whole collection too:

val original = listOf("orange", "apple")
val other = listOf("banana", "strawberry")
val newList = original + other // [orange, apple, banana, strawberry]

Disclaimer: this doesn't directly answer OP's question, but I feel that in a question titled "How to add an item to a list in Kotlin?", which is a top Google hit for this topic, plus must be mentioned.

The one thing with `.plus()` is that it has to be a MutableList. Regular List doesn't implement that. — Jay Whitsitt, May 06 '22 at 21:35
@JayWhitsitt That is incorrect. Please try running the code examples in my answer. For example `listOf` returns a read-only list, yet you absolutely can call `plus()` on that. As I mentioned above, `plus` returns a new list; it never modifies the original list. — Jonik, May 17 '22 at 20:05

Radesh · Answer 3 · 2020-02-18T07:06:16.680

53

If you don't want or can't use array list directly use this code for add item

itemsList.toMutableList().add(item)

itemlist : list of your items

item : item you want to add

edited Feb 18 '20 at 07:06

answered Jun 16 '19 at 07:08

Radesh

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Didn't you miss an assignment? toMutableList() creates a copy of itemsList and doesn't change it, right? – The incredible Jan Jul 20 '21 at 09:21
2

I think you're correct, an assignment is missing and also, .add(item) returns a Boolean so this has to be done in multiple steps, if I'm not mistaken. – jure Nov 26 '21 at 13:26
1

@PhilipRego don't use it in initialize. use it like this => aggList.toMutableList().add(user) , in initialize => var aggList: List = emptyList – Radesh Jul 06 '22 at 06:49
2

It doesn't change anything about itemsList – c-an Sep 12 '22 at 04:02

Brent · Answer 4 · 2020-05-26T15:10:43.740

11

instead of using a regular list which is immutable just use an arrayListof which is mutable

so your regular list will become

var listofVehicleNames = arrayListOf("list items here")

then you can use the add function

listOfVehicleNames.add("what you want to add")

edited May 26 '20 at 15:10

answered Apr 03 '19 at 18:02

Brent

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score 8 · Answer 5 · answered Apr 03 '19 at 06:26

8

you should use a MutableList like ArrayList

var listofVechileName:List<String>?=null

becomes

 var listofVechileName:ArrayList<String>?=null

and with that you can use the method add

https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-mutable-list/add.html

answered Apr 03 '19 at 06:26

Peppe Scab

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what JB Nizet said is true, but if my answer solved your question please mark it as correct – Peppe Scab Apr 03 '19 at 06:47
I didnt want to change too much author, but as someone makes me notice you should avoid to init to null, it would be better using lazeinit – Peppe Scab Apr 03 '19 at 12:45

score 4 · Answer 6 · answered Jun 24 '20 at 14:16

For any specific class, the following may help

 var newSearchData = List<FIRListValuesFromServer>()

        for (i in 0 until this.singleton.firListFromServer.size) {

            if (searchText.equals(this.singleton.firListFromServer.get(i).FIR_SRNO)) {

                newSearchData.toMutableList().add(this.singleton.firListFromServer.get(i))

            }
        }

Antriksh Chhipa · Answer 7 · 2022-11-20T17:29:03.050

1

val listofVechile = mutableListOf<String>()

Declare mutable list like that and you will be able to add elements to list :

listofVechile.add("car")

https://kotlinlang.org/docs/collections-overview.html

edited Nov 20 '22 at 17:29

answered Nov 20 '22 at 17:28

Antriksh Chhipa

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How to add an item to a list in Kotlin?

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