How to initialize List<T> in Kotlin?
ListKotlinList Problem Overview
I see Kotlin has a List<out E>
collection and I was wondering about different ways to initialize one. In Java, I could write:
List<String> geeks = Arrays.asList("Fowler", "Beck", "Evans");
How can I achieve the same in Kotlin?
List Solutions
Solution 1 - List
listOf
top-level function to the rescue:
val geeks = listOf("Fowler", "Beck", "Evans")
Solution 2 - List
Both the upvoted answers by Ilya and gmariotti are good and correct. Some alternatives are however spread out in comments, and some are not mentioned at all.
This answer includes a summary of the already given ones, along with clarifications and a couple of other alternatives.
List
)
Immutable lists (Immutable, or read-only lists, are lists which cannot have elements added or removed.
- As Ilya points out,
listOf()
often does what you want. This creates an immutable list, similar toArrays.asList
in Java. - As frogcoder states in a comment,
emptyList()
does the same, but naturally returns an empty list. listOfNotNull()
returns an immutable list excluding allnull
elements.
MutableList
)
Mutable lists (Mutable lists can have elements added or removed.
-
gmariotti suggests using
mutableListOf()
, which typically is what you want when you need to add or remove elements from the list. -
Greg T gives the alternative,
arrayListOf()
. This creates a mutableArrayList
. In case you really want anArrayList
implementation, use this overmutableListOf()
. -
For other
List
implementations, which have not got any convenience functions, they can be initialized as, for example,val list = LinkedList<String>()
. That is simply create the object by calling its constructor. Use this only if you really want, for example, aLinkedList
implementation.
Solution 3 - List
Just for adding more info, Kotlin offers both immutable List
and MutableList
that can be initialized with listOf
and mutableListOf
. If you're more interested in what Kotlin offers regarding Collections, you can go to the official reference docs at Collections.
Solution 4 - List
Let me explain some use-cases : let's create an immutable(non changeable) list with initializing items :
val myList = listOf("one" , "two" , "three")
let's create a Mutable (changeable) list with initializing fields :
val myList = mutableListOf("one" , "two" , "three")
Let's declare an immutable(non changeable) and then instantiate it :
lateinit var myList : List<String>
// and then in the code :
myList = listOf("one" , "two" , "three")
And finally add some extra items to each :
val myList = listOf("one" , "two" , "three")
myList.add() //Unresolved reference : add, no add method here as it is non mutable
val myMutableList = mutableListOf("one" , "two" , "three")
myMutableList.add("four") // it's ok
Solution 5 - List
In this way, you can initialize the List in Kotlin
val alphabates : List<String> = listOf("a", "b", "c")
Solution 6 - List
There is one more way to build a list in Kotlin that is as of this writing in the experimental state but hopefully should change soon.
inline fun <E> buildList(builderAction: MutableList<E>.() -> Unit): List<E>
> Builds a new read-only List by populating a MutableList using the given builderAction and returning a read-only list with the same elements.
Example:
val list = buildList {
testDataGenerator.fromJson("/src/test/resources/data.json").forEach {
add(dao.insert(it))
}
}
For further reading check the official doc.
Solution 7 - List
If you want to initialize without type:
var temp: ArrayList<String> = ArrayList()