Kotlin s List missing add remove Map missing put etc

Question

In Java we could do the following  public class TempClass       List lt Integer gt  myList   null      void doSomething             myList   new ArrayList lt  gt             myList add 10           myList remove 10             But if we rewrite it to Kotlin directly as below  class TempClass       var myList  List lt Int gt     null     fun doSomething             myList   ArrayList lt Int gt            myList   add 10          myList   remove 10            I got the error of not finding add and remove function from my List  I work around casting it to ArrayList  but that is odd needing to cast it  while in Java casting is not required  And that defeats the purpose of having the abstract class List  class TempClass       var myList  List lt Int gt     null     fun doSomething             myList   ArrayList lt Int gt             myList   as ArrayList  add 10           myList   as ArrayList  remove 10            Is there a way for me to use List but not needing to cast it  like what could be done in Java

User · Answer

Confining Mutability to Builders

The top answers here correctly speak to the difference in Kotlin between read-only List (NOTE: it's read-only, not "immutable"), and MutableList.

In general, one should strive to use read-only lists, however, mutability is still often useful at construction time, especially when dealing with third-party libraries with non-functional interfaces. For cases in which alternate construction techniques are not available, such as using listOf directly, or applying a functional construct like fold or reduce, a simple "builder function" construct like the following nicely produces a read-only list from a temporary mutable one:

val readonlyList = mutableListOf<...>().apply {
  // manipulate your list here using whatever logic you need
  // the `apply` function sets `this` to the `MutableList`
  add(foo1)
  addAll(foos)
  // etc.
}.toList()

and this can be nicely encapsulated into a re-usable inline utility function:

inline fun <T> buildList(block: MutableList<T>.() -> Unit) = 
  mutableListOf<T>().apply(block).toList()

which can be called like this:

val readonlyList = buildList<String> {
  add("foo")
  add("bar")
}

Now, all of the mutability is isolated to one block scope used for construction of the read-only list, and the rest of your code uses the read-only list that is output from the builder.

UPDATE: As of Kotlin 1.3.70, the exact buildList function above is available in the standard library as an experimental function, along with its analogues buildSet and buildMap. See https://blog.jetbrains.com/kotlin/2020/03/kotlin-1-3-70-released/.

User · Answer

A list is immutable by Default  you can use ArrayList instead  like this     val orders   arrayListOf lt String gt      then you can add delete items from this like below    orders add  Item 1   orders add  Item 2        by default ArrayList is mutable so you can perform the operations on it

User · Answer

In concept of immutable data  maybe this is a better way   class TempClass       val list  List lt Int gt  by lazy           listOf lt Int gt              fun doSomething             list    10         list -  10

User · Answer

https   kotlinlang org docs reference collections html  According to above link List lt E  is immutable in Kotlin   However this would work   var list2   ArrayList lt String gt    list2 removeAt 1

User · Answer

Unlike many languages  Kotlin distinguishes between mutable and immutable collections  lists  sets  maps  etc   Precise control over exactly when collections can be edited is useful for eliminating bugs  and for designing good APIs    https   kotlinlang org docs reference collections html  You ll need to use a MutableList list   class TempClass       var myList  MutableList lt Int gt    mutableListOf lt Int gt        fun doSomething                myList   ArrayList lt Int gt       initializer is redundant         myList add 10          myList remove 10            MutableList lt Int gt    arrayListOf   should also work

User · Answer

You can do with create new one like this   var list1   ArrayList lt Int gt    var list2    list1 toMutableList   list2 add item    Now you can use list2  Thank you

User · Answer

Agree with all above answers of using MutableList but you can also add remove from List and get a new list as below   val newListWithElement   existingList   listOf element  val newListMinusElement   existingList - listOf element    Or  val newListWithElement   existingList plus element  val newListMinusElement   existingList minus element

User · Answer

In Kotlin you must use MutableList or ArrayList   Let s see how the methods of MutableList work   var listNumbers  MutableList lt Int gt    mutableListOf 10  15  20     Result  10  15  20  listNumbers add 1000     Result  10  15  20  1000  listNumbers add 1  250     Result  10  250  15  20  1000  listNumbers removeAt 0     Result  250  15  20  1000  listNumbers remove 20     Result  250  15  1000  for  i in listNumbers         println i       Let s see how the methods of ArrayList work   var arrayNumbers  ArrayList lt Int gt    arrayListOf 1  2  3  4  5     Result  1  2  3  4  5  arrayNumbers add 20     Result  1  2  3  4  5  20  arrayNumbers remove 1     Result  2  3  4  5  20  arrayNumbers clear      Result  Empty  for  j in arrayNumbers         println j

User · Answer

Defining a List collection in Kotlin in different ways    Immutable variable with immutable  read only  list   val users  List lt User gt    listOf  User  Tom   32   User  John   64       Immutable variable with mutable list   val users  MutableList lt User gt    mutableListOf  User  Tom   32   User  John   64      or without initial value - empty list and without explicit variable type   val users   mutableListOf lt User gt      or val users   ArrayList lt User gt       you can add items to list     users add anohterUser  or  users    anotherUser  under the hood it s users add anohterUser          Mutable variable with immutable list   var users  List lt User gt    listOf  User  Tom   32   User  John   64      or without initial value - empty list and without explicit variable type   var users   emptyList lt User gt       NOTE  you can add  items to list     users    anotherUser -  it creates new ArrayList and assigns it to users        Mutable variable with mutable list   var users  MutableList lt User gt    mutableListOf  User  Tom   32   User  John   64      or without initial value - empty list and without explicit variable type   var users   emptyList lt User gt    toMutableList     or var users   ArrayList lt User gt       NOTE  you can add items to list     users add anohterUser   but not using users    anotherUser      Error  Kotlin  Assignment operators ambiguity     public operator fun  Collection plus element  String   List defined in kotlin collections     InlineOnly public inline operator fun  MutableCollection plusAssign element  String   Unit defined in kotlin collections       see also  https   kotlinlang org docs reference collections html

User · Answer

Apparently  the default List of Kotlin is immutable  To have a List that could change  one should use MutableList as below  class TempClass       var myList  MutableList lt Int gt     null     fun doSomething             myList   ArrayList lt Int gt            myList   add 10          myList   remove 10            Updated Nonetheless  it is not recommended to use MutableList unless for a list that you really want to change  Refers to https   hackernoon com read-only-collection-in-kotlin-leads-to-better-coding-40cdfa4c6359 for how Read-only collection provides better coding

[collections] Kotlin's List missing "add", "remove", Map missing "put", etc?

Confining Mutability to Builders

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