When creating lists, Kotlin can infer the type of items stored. To declare the type explicitly, add the type within angled brackets <> after the list declaration:
fun main() {
//sampleStart
// Read only list
val readOnlyShapes = listOf("triangle", "square", "circle")
println(readOnlyShapes)
// [triangle, square, circle]
// Mutable list with explicit type declaration
val shapes: MutableList<String> = mutableListOf("triangle", "square", "circle")
println(shapes)
// [triangle, square, circle]
//sampleEnd
}
fun main() {
//sampleStart
val readOnlyShapes = listOf("triangle", "square", "circle")
println("The first item in the list is: ${readOnlyShapes[0]}")
// The first item in the list is: triangle
//sampleEnd
}
To get the first or last item in a list, use .first() and .last() functions respectively:
fun main() {
//sampleStart
val readOnlyShapes = listOf("triangle", "square", "circle")
println("The first item in the list is: ${readOnlyShapes.first()}")
// The first item in the list is: triangle
//sampleEnd
}
To get the number of items in a list, use the .count() function:
fun main() {
//sampleStart
val readOnlyShapes = listOf("triangle", "square", "circle")
println("This list has ${readOnlyShapes.count()} items")
// This list has 3 items
//sampleEnd
}
To check that an item is in a list, use the in operator:
fun main() {
//sampleStart
val readOnlyShapes = listOf("triangle", "square", "circle")
println("circle" in readOnlyShapes)
// true
//sampleEnd
}
To add or remove items from a mutable list, use .add() and .remove() functions respectively:
fun main() {
//sampleStart
val shapes: MutableList<String> = mutableListOf("triangle", "square", "circle")
// Add "pentagon" to the list
shapes.add("pentagon")
println(shapes)
// [triangle, square, circle, pentagon]
// Remove the first "pentagon" from the list
shapes.remove("pentagon")
println(shapes)
// [triangle, square, circle]
//sampleEnd
}
Set
Whereas lists are ordered and allow duplicate items, sets are unordered and only store unique items.
To create a read-only set (Set), use the setOf() function.
When creating sets, Kotlin can infer the type of items stored. To declare the type explicitly, add the type within angled brackets <> after the set declaration:
fun main() {
//sampleStart
// Read-only set
val readOnlyFruit = setOf("apple", "banana", "cherry", "cherry")
// Mutable set with explicit type declaration
val fruit: MutableSet<String> = mutableSetOf("apple", "banana", "cherry", "cherry")
println(readOnlyFruit)
// [apple, banana, cherry]
//sampleEnd
}
You can see in the previous example that because sets only contain unique elements, the duplicate "cherry" item is dropped.
To get the number of items in a set, use the .count() function:
fun main() {
//sampleStart
val readOnlyFruit = setOf("apple", "banana", "cherry", "cherry")
println("This set has ${readOnlyFruit.count()} items")
// This set has 3 items
//sampleEnd
}
To check that an item is in a set, use the in operator:
fun main() {
//sampleStart
val readOnlyFruit = setOf("apple", "banana", "cherry", "cherry")
println("banana" in readOnlyFruit)
// true
//sampleEnd
}
To add or remove items from a mutable set, use .add() and .remove() functions respectively:
fun main() {
//sampleStart
val fruit: MutableSet<String> = mutableSetOf("apple", "banana", "cherry", "cherry")
fruit.add("dragonfruit") // Add "dragonfruit" to the set
println(fruit) // [apple, banana, cherry, dragonfruit]
fruit.remove("dragonfruit") // Remove "dragonfruit" from the set
println(fruit) // [apple, banana, cherry]
//sampleEnd
}
Map
Maps store items as key-value pairs. You access the value by referencing the key. You can imagine a map like a food menu. You can find the price (value), by finding the food (key) you want to eat. Maps are useful if you want to look up a value without using a numbered index, like in a list.
To create a read-only map (Map), use the mapOf() function.
When creating maps, Kotlin can infer the type of items stored. To declare the type explicitly, add the types of the keys and values within angled brackets <> after the map declaration. For example: MutableMap<String, Int>. The keys have type String and the values have type Int.
The easiest way to create maps is to use to between each key and its related value:
fun main() {
//sampleStart
// Read-only map
val readOnlyJuiceMenu = mapOf("apple" to 100, "kiwi" to 190, "orange" to 100)
println(readOnlyJuiceMenu)
// {apple=100, kiwi=190, orange=100}
// Mutable map with explicit type declaration
val juiceMenu: MutableMap<String, Int> = mutableMapOf("apple" to 100, "kiwi" to 190, "orange" to 100)
println(juiceMenu)
// {apple=100, kiwi=190, orange=100}
//sampleEnd
}
fun main() {
//sampleStart
// Read-only map
val readOnlyJuiceMenu = mapOf("apple" to 100, "kiwi" to 190, "orange" to 100)
println("The value of apple juice is: ${readOnlyJuiceMenu["apple"]}")
// The value of apple juice is: 100
//sampleEnd
}
To get the number of items in a map, use the .count() function:
fun main() {
//sampleStart
// Read-only map
val readOnlyJuiceMenu = mapOf("apple" to 100, "kiwi" to 190, "orange" to 100)
println("This map has ${readOnlyJuiceMenu.count()} key-value pairs")
// This map has 3 key-value pairs
//sampleEnd
}
To add or remove items from a mutable map, use .put() and .remove() functions respectively:
fun main() {
//sampleStart
val juiceMenu: MutableMap<String, Int> = mutableMapOf("apple" to 100, "kiwi" to 190, "orange" to 100)
juiceMenu.put("coconut", 150) // Add key "coconut" with value 150 to the map
println(juiceMenu)
// {apple=100, kiwi=190, orange=100, coconut=150}
juiceMenu.remove("orange") // Remove key "orange" from the map
println(juiceMenu)
// {apple=100, kiwi=190, coconut=150}
//sampleEnd
}
To check if a specific key is already included in a map, use the .containsKey() function:
fun main() {
//sampleStart
val readOnlyJuiceMenu = mapOf("apple" to 100, "kiwi" to 190, "orange" to 100)
println(readOnlyJuiceMenu.containsKey("kiwi"))
// true
//sampleEnd
}
To obtain a collection of the keys or values of a map, use the keys and values properties respectively:
fun main() {
//sampleStart
val readOnlyJuiceMenu = mapOf("apple" to 100, "kiwi" to 190, "orange" to 100)
println(readOnlyJuiceMenu.keys)
// [apple, kiwi, orange]
println(readOnlyJuiceMenu.values)
// [100, 190, 100]
//sampleEnd
}
To check that a key or value is in a map, use the in operator:
fun main() {
//sampleStart
val readOnlyJuiceMenu = mapOf("apple" to 100, "kiwi" to 190, "orange" to 100)
println("orange" in readOnlyJuiceMenu.keys)
// true
println(200 in readOnlyJuiceMenu.values)
// false
//sampleEnd
}
For more information on what you can do with collections, see Collections.
Now that you know about basic types and how to manage collections, it's time to explore the control flow that you can use in your programs.
Practice
Exercise 1
You have a list of “green” numbers and a list of “red” numbers. Complete the code to print how many numbers there are in total.
fun main() {
val greenNumbers = listOf(1, 4, 23)
val redNumbers = listOf(17, 2)
// Write your code here
}
fun main() {
val greenNumbers = listOf(1, 4, 23)
val redNumbers = listOf(17, 2)
val totalCount = greenNumbers.count() + redNumbers.count()
println(totalCount)
}
Exercise 2
You have a set of protocols supported by your server. A user requests to use a particular protocol. Complete the program to check whether the requested protocol is supported or not (isSupported must be a Boolean value).
fun main() {
val SUPPORTED = setOf("HTTP", "HTTPS", "FTP")
val requested = "smtp"
val isSupported = // Write your code here
println("Support for $requested: $isSupported")
}
Hint
Make sure that you check the requested protocol in upper case. You can use the .uppercase() function to help you with this.
fun main() {
val SUPPORTED = setOf("HTTP", "HTTPS", "FTP")
val requested = "smtp"
val isSupported = requested.uppercase() in SUPPORTED
println("Support for $requested: $isSupported")
}
Exercise 3
Define a map that relates integer numbers from 1 to 3 to their corresponding spelling. Use this map to spell the given number.
fun main() {
val number2word = // Write your code here
val n = 2
println("$n is spelt as '${<Write your code here >}'")
}
fun main() {
val number2word = mapOf(1 to "one", 2 to "two", 3 to "three")
val n = 2
println("$n is spelt as '${number2word[n]}'")
}