How to declare empty list and then add string in scala

Question

I have code like this   val dm    List String    val dk   List Map String Object             dm add  text   dk add Map  1  - gt   ok      but it throws runtime java lang UnsupportedOperationException   I need to declare empty list or empty maps and some where later in the code need to fill them

User · Answer

Per default collections in scala are immutable  so you have a   method which returns a new list with the element added to it  If you really need something like an add method you need a mutable collection  e g  http   www scala-lang org api current scala collection mutable MutableList html which has a    method

User · Answer

In your case I use  val dm   ListBuffer String    and val dk   ListBuffer Map String anyRef

User · Answer

If you need to mutate stuff  use ArrayBuffer or LinkedBuffer instead  However  it would be better to address this statement      I need to declare empty list or empty   maps and some where later in the code   need to fill them    Instead of doing that  fill the list with code that returns the elements  There are many ways of doing that  and I ll give some examples      Fill a list with the results of calls to a method val l   List fill 50  scala util Random nextInt      Fill a list with the results of calls to a method until you get something different val l   Stream continually scala util Random nextInt  takeWhile x   gt  x  gt  0  toList     Fill a list based on its index val l   List tabulate 5  x   gt  x   2      Fill a list of 10 elements based on computations made on the previous element val l   List iterate 1  10  x   gt  x   2      Fill a list based on computations made on previous element  until you get something val l   Stream iterate 0  x   gt  x   2   1  takeWhile x   gt  x  lt  1000  toList     Fill list based on input from a file val l    for  line  lt - scala io Source fromFile  filename txt   getLines  yield line length  toList

User · Answer

Maybe you can use ListBuffers in scala to create empty list and add strings later because ListBuffers are mutable  Also all the List functions are available for the ListBuffers in scala   import scala collection mutable ListBuffer   val dm   ListBuffer String    dm  scala collection mutable ListBuffer String    ListBuffer   dm     text1  dm     text2  dm   ListBuffer text1  text2    if you want you can convert this to a list by using  toList

User · Answer

As everyone already mentioned  this is not the best way of using lists in Scala     scala gt  val list   scala collection mutable MutableList String    list  scala collection mutable MutableList String    MutableList    scala gt  list     hello  res0  list type   MutableList hello   scala gt  list     world  res1  list type   MutableList hello  world   scala gt  list mkString     res2  String   hello world

User · Answer

Scala lists are immutable by default  You cannot  add  an element  but you can form a new list by appending the new element in front  Since it is a new list  you need to reassign the reference  so you can t use a val    var dm    List String    var dk   List Map String AnyRef             dm    text     dm dk   Map 1 - gt   ok      dk   The operator    creates the new list  You can also use the shorter syntax   dm      text   dk     Map 1 - gt   ok     NB  In scala don t use the type Object but Any  AnyRef or AnyVal

User · Answer

As mentioned in an above answer  the Scala List is an immutable collection  You can create an empty list with  empty A   Then you can use a method         or    in order to add element to the list   scala gt  val strList   List empty String  strList  List String    List    scala gt  strList    Text  res3  List String    List Text   scala gt  val mapList   List empty Map String  Any   mapList  List Map String Any     List    scala gt  mapList    Map  1  - gt   ok   res4  List Map String Any     List Map 1 - gt  ok

[scala] How to declare empty list and then add string in scala?

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