Learn the Basics of the Kotlin Language
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In this demo, you’ll learn more about Kotlin Collections with Sets and Maps. Start a new Kotlin Playground session to follow along with the demo. Or you could use any other Kotlin programming environment of your choice.
Set
A set stores only unique values. If add() is called on a Set and the added item is a duplicate within the Set, it won’t be added. For this reason, the order of a Set isn’t defined, as some items may be removed if duplicated. Sets in Kotlin are represented by the Set interface.
Read-only Sets
A set may also be mutable or immutable, just like a List. To initialize an immutable set, use setOf:
fun main() {
val oceans = setOf("Pacific Ocean", "Southern Ocean", "Arctic Ocean", "Atlantic Ocean", "Indian Ocean")
println(oceans)
}
Mutable Sets
Use mutableSetOf to initialize a mutable Set:
fun main() {
val oceans = mutableSetOf("Pacific Ocean", "Southern Ocean", "Arctic Ocean", "Atlantic Ocean", "Indian Ocean")
}
fun main() {
val oceans = mutableSetOf("Atlantic Ocean", "Indian Ocean")
oceans.add("Pacific Ocean")
oceans.addAll(listOf("Southern Ocean", "Arctic Ocean"))
println(oceans)
}
fun main() {
val oceans = mutableSetOf("Atlantic Ocean", "Indian Ocean")
oceans.add("Pacific Ocean")
oceans.addAll(listOf("Southern Ocean", "Arctic Ocean"))
oceans.add("Pacific Ocean")
println(oceans)
}
fun main() {
val oceans = mutableSetOf("Atlantic Ocean", "Indian Ocean")
println(oceans.elementAt(0))
}
fun main() {
val oceans = mutableSetOf("Atlantic Ocean", "Indian Ocean")
oceans.remove("Atlantic Ocean")
println(oceans)
}
fun main() {
val oceans = emptySet<String>() // You need to specify the data type for the items the set could contain.
}
fun main() {
val digits = setOf(1,2,3,4,5)
for (digit in digits){
println(digit)
}
}
Map
A Map differs from the List and Set in that they store key-value pairs instead of single items. A key in a Map is unique. If a new key duplicates an existing key, the existing key’s value is updated with the new one.
fun main() {
emptyMap<Int, String>()
}
fun main() {
val techBlogs = mapOf(1 to "TechCrunch", 2 to "Engadget", 3 to "The Verge", 4 to "Mashable")
println(techBlogs)
}
fun main() {
val techBlogs = mutableMapOf(1 to "TechCrunch", 2 to "Engadget", 3 to "The Verge", 4 to "Mashable")
println(techBlogs)
techBlogs.put(5, "VentureBeat")
println(techBlogs)
}
fun main() {
val techBlogs = mapOf(1 to "TechCrunch", 2 to "Engadget", 3 to "The Verge", 4 to "Mashable")
for (blog in techBlogs.entries.iterator()) {
println("${blog.key} : ${blog.value}")
}
}
Arrays
In Kotlin, arrays are used to store a group of items, like other collections you may have encountered before. The Array class represents an array, which differs primarily in its properties. Arrays can store a series of items that are of the same data type or subtype and can’t be changed once created, making them immutable.
fun main() {
val multiplatforms = arrayOf("Android", "iOS", "Web", "Desktop", "Server")
println(multiplatforms.toList())
}
fun main() {
emptyArray<String>()
}
fun main() {
val books = Array<Int>(3) { 0 }
println(books.toList())
}
fun main() {
val books = Array<Int>(3) { it * 2 }
println(books.toList())
}
fun main() {
val multiplatforms = arrayOf("Android", "iOS", "Web", "Desktop", "Server")
println(multiplatforms[2])
}
fun main() {
val multiplatforms = arrayOf("iOS", "Web", "Desktop", "Server")
multiplatforms[0] = "Android"
println(multiplatforms.toList())
}
Comparing Arrays
To compare arrays, use contentEquals(). The items will have to be the same in both arrays and at the same position for the two arrays to be equal:
fun main() {
val multiplatformsA = arrayOf("iOS", "Web", "Desktop", "Server")
val multiplatformsB = arrayOf("iOS", "Web", "Desktop", "Server")
println(multiplatformsA.contentEquals(multiplatformsB))
}
Collection Elements
The items in a collection are typically of the same type, but they can also be of different types or subtypes. Create a list made up of a string, a float, a boolean, and a map:
fun main() {
val mixedTypesList = listOf("Doughnuts", 200.0f, true, mapOf("color" to "Red"))
println(mixedTypesList)
}