Kotlin Map : mapOf()

Kotlin map is a collection that contains pairs of objects. Map holds the data in the form of pairs which consists of a key and a value. Map keys are unique and the map holds only one value for each key. 

Kotlin distinguishes between immutable and mutable maps. Immutable maps created with mapOf() means these are read-only and mutable maps created with mutableMapOf() means we can perform read and write both.

Syntax:

fun <K, V> mapOf(vararg pairs: Pair<K, V>): Map<K, V> 

• The first value of the pair is the key and the second is the value of the corresponding key.
• If multiple pair have same key then map will return the last pair value.
• The map entries is traversed in the specified order.

Kotlin program of mapOf() 

Kotlin

fun main(args : Array<String>)
{
    // declaring a map of integer to string
    val map = mapOf(1 to "Geeks", 2 to "for", 3 to "Geeks")
        // printing the map
        println(map)
}

Output:

{1=Geeks, 2=for, 3=Geeks}

Map keys, values and entries:

Kotlin

fun main(args: Array<String>)
{
    //declaring a map of integer to string
    val map = mapOf(1 to "One", 2 to "Two" , 3 to "Three", 4 to "Four")

    println("Map Entries : "+map)

    println("Map Keys: "+map.keys )

    println("Map Values: "+map.values )
}

Output:

Map Entries : {1=One, 2=Two, 3=Three, 4=Four}
Map Keys: [1, 2, 3, 4]
Map Values: [One, Two, Three, Four] 

Map Size –

We can determine the size of map using two methods. By using the size property of the map and by using the count() method. 

Kotlin

fun main() {

    val ranks = mapOf(1 to "India",2 to "Australia",3 to "England",4 to "Africa")
    //method 1
    println("The size of the map is: "+ranks.size)
    //method 2
    println("The size of the map is: "+ranks.count())
}

Output:

The size of the map is: 4
The size of the map is: 4 

Empty Map –

We can create an empty serialize-able map using mapOf()

Example 2: mapOf()

Kotlin

fun main(args: Array<String>)
{
    //here we have created an empty map using mapOf()
    val map1 = mapOf<String , Int>()

    println("Entries: " + map1.entries)  //entries of map

    println("Keys:" + map1.keys)  //keys of map

    println("Values:" + map1.values)  //values of map

}

Output:

Entries: []
Keys:[]
Values:[] 

Get values of Map –

We can retrieve values from a map using different methods discussed in the below program. 

Kotlin

fun main() {

    val ranks = mapOf(1 to "India",2 to "Australia",3 to "England",4 to "Africa")

    //method 1
    println("Team having rank #1 is: "+ranks[1])
    //method 2
    println("Team having rank #3 is: "+ranks.getValue(3))
    //method 3
    println("Team having rank #4 is: "+ranks.getOrDefault(4, 0))
    // method  4
    val team = ranks.getOrElse(2 ,{ 0 })
    println(team)
}

Output:

Team having rank #1 is: India
Team having rank #3 is: England
Team having rank #4 is: Africa
Australia

Map Contains Key or Values –

We can determine that a map contains a key or value using the containsKey() and containsValue() methods respectively. 

Kotlin

fun main() {
    val colorsTopToBottom = mapOf("red" to 1, "orange" to 2, "yellow" to 3,
        "green" to 4 , "blue" to 5, "indigo" to 6, "violet" to 7)
    var color = "yellow"
    if (colorsTopToBottom.containsKey(color)) {
        println("Yes, it contains color $color")
    } else {
        println("No, it does not contain color $color")
    }
    val value = 8
    if (colorsTopToBottom.containsValue(value)) {
        println("Yes, it contains value $value")
    } else {
        println("No, it does not contain value $value")
    }
}

Output:

Yes, it contains color yellow
No, it does not contain value 8 

Two values and same key:

If two values have same key value , then the map will contain the last value of the those numbers.

Example 3: mapOf()

Kotlin

fun main(args: Array<String>)
{
    //lets make two values with same key
    val map = mapOf(1 to "geeks1",2 to "for" , 1 to "geeks2")
    // return the map entries
    println("Entries of map : " + map.entries)
}

Output:

Entries of map : [1=geeks2, 2=for]

Explanation:

Here key value 1 has been initialized with two values: geeks1 and geeks2, but as we know that mapOf() can have only one value for one key item, therefore the last value is only stored by the map, and geeks1 is eliminated.