Basic Usage

Creating collections is nice, but eventually, we need to get data in and out of them and otherwise manipulate them.

[] Syntax

To retrieve values from a collection (other than a Set), you can use []. For arrays and lists, the value passed into the [] operator is the 0-based index into the collection. For maps, the value passed into the [] operator is the key for which you wish to look up the corresponding value.

  val thingsArray = arrayOf("foo", "bar", "goo")

  println(thingsArray[0])

  val thingsIntArray = intArrayOf(1, 3, 3, 7)

  println(thingsIntArray[1])

  val thingsList = listOf("foo", "bar", "goo")

  println(thingsList[2])

  val thingsMap = mapOf("key" to "value", "other-key" to "other-value")

  println(thingsMap["other-key"])

Running this in the Klassbook will give you:

foo
3
goo
other-value

Things get a bit interesting if we start playing with that map of the lists of odd and even single-digit integers:

val oddEven = mapOf("odd" to listOf(1, 3, 5, 7, 9), "even" to listOf(2, 4, 6, 8))

println(oddEven["odd"].size)

This will fail to run, with a compile error:

error: only safe (?.) or non-null asserted (!!.) calls are allowed on a nullable receiver of type List<Int>?

A Map may return null for a given key, as there may not be a value for that key. As it turns out, our Map has a value for the odd key, and so at runtime, this would succeed. And in most programming languages, we could actually compile and run this sort of operation. This fails in Kotlin, because of how Kotlin handles the possibility of null as a value. We will explore this in much greater detail later in the book.

Typical Stuff

If you are used to working with collections in other programming languages, it will come as little surprise that Kotlin has a lot of the same basic functions for working with its collections.

In addition to the size property, you will have things like:

• isEmpty(), for a quick Boolean value based on whether the collection is empty
• contains(), which will return true if the collection contains the requested item (for maps, this checks the keys — use containsValue() to check the values)

For lists and arrays, you have things like:

• first(), to get the first element
• last(), to get the last element
• firstOrNull(), to get the first element or null if the collection is empty
• And so on

Conversion Functions

The one-dimensional collection types (arrays, lists, sets) have a rich set of conversion functions to migrate between types:

• Array offers toList() and toSet()
• List offers toSet() and toTypedArray()
• Set offers toList() and toTypedArray()