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# Kotlin Map collection – map() methods

loizenai
Software Engineer - Founder at https://loizenai.com

https://grokonez.com/kotlin/kotlin-map-collection-map-methods

Kotlin Map collection – map() methods

In the tutorial, JavaSampleApproach will show you how to use `map()` methods to transform Kotlin Map collection to Kotlin List collection or a new Map collection.

## I. Kotlin Map collection with map() methods

Kotlin Map collection supports a set of map() methods to transform the given map to a new map or a new list:

``````1. fun <K, V, R> Map<out K, V>.map(transform: (Map.Entry<K, V>) -> R): List<R>
2. fun <K, V, R : Any> Map<out K, V>.mapNotNull(transform: (Map.Entry<K, V>) -> R?): List<R>
3. fun <K, V, R, C : MutableCollection<in R>> Map<out K, V>.mapTo(destination: C, transform: (Map.Entry<K, V>) -> R): C
4. fun <K, V, R : Any, C : MutableCollection<in R>> Map<out K, V>.mapNotNullTo(destination: C, transform: (Map.Entry<K, V>) -> R?): C
5. fun <K, V, R> Map<out K, V>.mapKeys(transform: (Map.Entry<K, V>) -> R): Map<R, V>
6. fun <K, V, R> Map<out K, V>.mapKeys(transform: (Map.Entry<K, V>) -> R): Map<R, V>
7. fun <K, V, R> Map<out K, V>.mapValues(transform: (Map.Entry<K, V>) -> R): Map<K, R>
8. fun <K, V, R, M : MutableMap<in K, in R>> Map<out K, V>.mapValuesTo(destination: M, transform: (Map.Entry<K, V>) -> R): M ``````

Now practicing ->

### 0. Initial data for practicing

``````
val street: String,
val postcode: String
)

data class Customer(
val firstName: String,
val lastName: String,
val age: Int
)

data class Person(
val fullname: String,
val age: Int,
)

val customerMap = mapOf(Pair(Customer("Jack", "Davis", 25), Address("NANTERRE CT", "77471")),
Pair(Customer("Mary", "Taylor", 37), Address("W NORMA ST", "77009")),
Pair(Customer("Peter", "Thomas",17), Address("S NUGENT AVE", "77571")),
Pair(Customer("Amos", "Nelson",23), Address("E NAVAHO TRL", "77449")),
Pair(Customer("Laura", "Lewis", 32), Address("NANTERRE CT", "77471")),
Pair(Customer("Steven", "Harris", 39), Address("S NUGENT AVE", "77571")),
Pair(Customer("Paul", "Moore", 18), Address("E NAVAHO TRL", "77449")),
Pair(Customer("Mary", "Cook", 61), Address("S NUGENT AVE", "77571")),
Pair(null, null))

``````

### 1. Transform a given Kotlin Map to List

#### 1.1 map()

Method signature:

More at:

https://grokonez.com/kotlin/kotlin-map-collection-map-methods

Kotlin Map collection – map() methods