Kotlin - Convert Map to List Examples

1. Overview

In this tutorial, We will learn how to convert the Map to List in kotlin programming. Map implementations such as HashMap or TreeMap can be converted into an ArrayList or List object.

Let us explore the different ways to do conversion from map to get the list of keys and values.

2. Converting HashMap to List in Kotlin Using ArrayList Constructor

This approach is the simple one and easy. First create the HashMap object with HashMap() constructor. 

Next, adding few key value pairs to the map. Use map.keys and map.values to get the all keys and values.

pass these values to ArrayList constructor to convert the map to list.

These properties will be invoking the below functions internally when you are using java.util.HashMap.

map.keys --> map.keySet()

map.values --> map.values()

package com.javaprogramto.kotlin.collections.map.list

import java.util.HashMap

fun main(array: Array<String>) {

    var map = HashMap<Int, String>();

    map.put(10, "Ten")
    map.put(20, "Twenty")
    map.put(30, "Thirty")
    map.put(40, "Fourty")
    map.put(50, "Fifty")

    var keysList = ArrayList(map.keys);
    var valuesList = ArrayList(map.values);

    println("Keys list : $keysList")
    println("Values list : $valuesList")

}

Output:

Keys list : [50, 20, 40, 10, 30]
Values list : [Fifty, Twenty, Fourty, Ten, Thirty]

3. Converting LinkedHashMap to List in Kotlin Using ArrayList Constructor

From the above program, the output is not in the order what is inserted into the map. To preserve the insertion order then need to use LinkedHashMap class instead of HashMap.

Look at the below program and observer the output order.

package com.javaprogramto.kotlin.collections.map.list

import java.util.HashMap

fun main(array: Array<String>) {

    // to holds the insertion order
    var map = LinkedHashMap<Int, String>();

    map.put(10, "Ten")
    map.put(20, "Twenty")
    map.put(30, "Thirty")
    map.put(40, "Fourty")
    map.put(50, "Fifty")

    // gets the keys and values as per the insertion
    var keysList = ArrayList(map.keys);
    var valuesList = ArrayList(map.values);

    // prints
    println("Keys list : $keysList")
    println("Values list : $valuesList")
}

Output:

Keys list : [10, 20, 30, 40, 50]
Values list : [Ten, Twenty, Thirty, Fourty, Fifty]

4. Kotlin Map to List using toList() method

Use toList() method on map object or keys, values properties.

package com.javaprogramto.kotlin.collections.map.list
import java.util.HashMap

fun main(array: Array<String>) {

    var map = HashMap<Int, String>();

    map[10] = "Ten"
    map[20] = "Twenty"
    map[30] = "Thirty"
    map[40] = "Fourty"
    map[50] = "Fifty"

    // example 1 using toList() method
    var keysList = map.keys.toList();
    var valuesList = map.values.toList();
    println("using toList()")
    println("Keys list : $keysList")
    println("Values list : $valuesList")

    // example 2 using toList() method

    var mutableMap: MutableMap<Int, String> = HashMap()
    mutableMap[10] = "Ten"
    mutableMap[20] = "Twenty"
    mutableMap[30] = "Thirty"
    mutableMap[40] = "Fourty"
    mutableMap[50] = "Fifty"

    var entries = mutableMap.toList().map { "(${it.first}, ${it.second})"}
    println("\nusing toList() on mutable map")
    entries.forEach{print("$it ")}
}

Output:

using toList()
Keys list : [50, 20, 40, 10, 30]
Values list : [Fifty, Twenty, Fourty, Ten, Thirty]

using toList() on mutable map
(50, Fifty) (20, Twenty) (40, Fourty) (10, Ten) (30, Thirty) 

5. Kotlin Map to List using entries properties

map.entries properties will returns the key-value pairs. Use map() method  on the entries object.  We can use the any pattern in between the key and value.

package com.javaprogramto.kotlin.collections.map.list

import java.util.HashMap

fun main(array: Array<String>) {

    var mutableMap: MutableMap<Int, String> = HashMap()

    mutableMap[10] = "Ten"
    mutableMap[20] = "Twenty"
    mutableMap[30] = "Thirty"
    mutableMap[40] = "Fourty"
    mutableMap[50] = "Fifty"

    var list : List<String> = mutableMap.entries.map { ("${it.key} -->  ${it.value}") }

    list.forEach{println(it)}
}

Output:

50 -->  Fifty
20 -->  Twenty
40 -->  Fourty
10 -->  Ten
30 -->  Thirty

6. Kotlin Map to List using keys and values

Use keys and values properties of map object to call map() method.

package com.javaprogramto.kotlin.collections.map.list

import java.util.HashMap

fun main(array: Array<String>) {

    var mutableMap: MutableMap<Int, String> = HashMap()

    mutableMap[10] = "Ten"
    mutableMap[20] = "Twenty"
    mutableMap[30] = "Thirty"
    mutableMap[40] = "Fourty"
    mutableMap[50] = "Fifty"

    var keysList : List<String> = mutableMap.keys.map { ("${it} ") }
    var valuesList : List<String> = mutableMap.values.map { ("${it} ") }

    println("---- keys list ------")
    keysList.forEach{println(it)}

    println("----- values list ------")
    valuesList.forEach{println(it)}
}

Output:

---- keys list ------
50 
20 
40 
10 
30 
----- values list ------
Fifty 
Twenty 
Fourty 
Ten 
Thirty 

7. Conclusion

in this article, We have seen how to convert Map to List in Kotlin programming and examples on different approaches.

GitHub

How to convert List to Map in Kotlin?