Syntax. List.newBuilder can be used to add many values into a temporary collection and then convert that to a list. We can initialize lists by combining 2 lists or adding elements.
Builder example. Here we create a list with the List constructor syntax (a list literal). Then we use List.newBuilder to create the same list with a Builder.
Tip We need to specify the type of elements as part of the List.newBuilder function call. Here I use Int.
And We can add elements to the Builder's internal buffer by using the addition operator. With result() we get the final immutable list.
object Program {
def main(args: Array[String]): Unit = {
// Create a list with values.
val numbers = List(10, 11, 12)
println(numbers)
// Create a list with a Builder.
val builder = List.newBuilder[Int]
builder += 10
builder += 11
builder += 12
val numbers3 = builder.result()
println(numbers3)
}
}List(10, 11, 12)
List(10, 11, 12)
Combine two lists. Sometimes the easiest way to initialize a list is to combine two (or more than two) lists that already exist. Here we create two small lists and then merge them.
object Program {
def main(args: Array[String]): Unit = {
// Add two lists together.
val numbers1 = List(13, 14)
val numbers2 = List(15, 16)
val numbers3 = numbers1 ::: numbers2
println(numbers3)
}
}List(13, 14, 15, 16)
New element. Here we create a list by taking an existing list and adding another element to its start. This changes the head of the list. This only works with individual elements.
Warning If we use "::" with another list, that entire list (not its individual elements) will be placed in the head of the new list.
object Program {
def main(args: Array[String]): Unit = {
// Initialize a list by placing an element at its start.
val numbers1 = List(100, 200)
val numbers2 = 50 :: numbers1
println(numbers2)
}
}List(50, 100, 200)
List.empty. This can be used to initialize lists. We can place new elements alongside List.empty and initialize a new list. We must specify the type of elements (like String).
object Program {
def main(args: Array[String]): Unit = {
// Initialize an empty list and add an element to it.
val numbers1 = List.empty[String]
val numbers2 = "cat" :: numbers1
println(numbers2)
}
}List(cat)
A summary. Lists are immutable in Scala, so initializing lists is critical. With List.newBuilder and List.empty we can begin initializing lists with ease.
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This page was last updated on Oct 3, 2023 (simplify).