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) {

            }
        })
    }
}

Solution 1

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 */ }

Solution 2

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.

Solution 3

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

Solution 4

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")

Solution 5

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

View more solutions

Comments

[Peppe Scab]

what JB Nizet said is true, but if my answer solved your question please mark it as correct

[Peppe Scab]

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

[SMBiggs]

Oh yeah, that's so obvious! Much easier than java. I'm so glad we switched.

[The incredible Jan]

@SMBiggs You mean that ironically, don't you?

[The incredible Jan]

Didn't you miss an assignment? toMutableList() creates a copy of itemsList and doesn't change it, right?

[SMBiggs]

@TheincredibleJan Yes. To quote TJ Miller, "The problem with satire is that it's so easily misinterpreted."

[jure]

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.

[Jay Whitsitt]

The one thing with .plus() is that it has to be a MutableList. Regular List doesn't implement that.

[Jonik]

@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.