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Scala Initialize List: List.newBuilder, List.empty

Initialize Lists with operators and constructors. Use List.empty and newBuilder.
Initialize list. In Scala lists are immutable. This presents a problem when creating lists. We sometimes need complicated initialization logic.List
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.

Scala program that uses List.newBuilder // 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) Output 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.
Scala program that combines two lists // Add two lists together. val numbers1 = List(13, 14) val numbers2 = List(15, 16) val numbers3 = numbers1 ::: numbers2 println(numbers3) Output 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.

Scala program that initializes list with new element // Initialize a list by placing an element at its start. val numbers1 = List(100, 200) val numbers2 = 50 :: numbers1 println(numbers2) Output 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).String
Scala program that uses List.empty // Initialize an empty list and add an element to it. val numbers1 = List.empty[String] val numbers2 = "cat" :: numbers1 println(numbers2) Output 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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