Initialize list

In Scala lists are immutable. This presents a problem when creating lists. We sometimes need complicated initialization logic.

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.

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.

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)

Lists are immutable in Scala, so initializing lists is critical. With List.newBuilder and List.empty we can begin initializing lists with ease.