Convert Kotlin List to Map

In the tutorial, JavaSampleApproach will show you how to convert Kotlin List to Map.

Related posts:
– Convert Kotlin Map to List

A. Convert Kotlin List to Map

Initialize data:

val custList = listOf(Customer("Jack", 20, Address("NANTERRE CT", "77471")),
										Customer("Peter", 25, Address("W NORMA ST", "77009")))
println("custList = $custList") // custList = [Customer(name=Jack, age=20, address=Address(street=NANTERRE CT, postcode=77471)), Customer(name=Peter, age=25, address=Address(street=W NORMA ST, postcode=77009))]

I. Use toMap() method

Method signature: public fun Iterable>.toMap(): Map
-> Returns a new map containing all key-value pairs from the given collection.

1. Create a read-only map

val toMap = custList.map { it.name to it.address }.toMap()
println("toMap = $toMap") // toMap = {Jack=Address(street=NANTERRE CT, postcode=77471), Peter=Address(street=W NORMA ST, postcode=77009)}

2. Create a modifiable map

var modifiableMap = mutableMapOf()
modifiableMap.putAll(custList.map { it.name to it.address }.toMap())
modifiableMap.put("Mary", Address("E NAVAHO TRL", "77449"))
println("modifiableMap = $modifiableMap") // modifiableMap = {Jack=Address(street=NANTERRE CT, postcode=77471), Peter=Address(street=W NORMA ST, postcode=77009), Mary=Address(street=E NAVAHO TRL, postcode=77449)}

II. Use associate* methods

1. associate()

Method signature: public inline fun Iterable.associate(transform: (T) -> Pair): Map

val associateMap = custList.associate { it.name to it.address.street }
println("associateMap = $associateMap") // associateMap = {Jack=NANTERRE CT, Peter=W NORMA ST}

2. associateBy()

val associateByMap = custList.associateBy { it.name }
println("associateByMap = $associateByMap") // associateByMap = {Jack=Customer(name=Jack, age=20, address=Address(street=NANTERRE CT, postcode=77471)), Peter=Customer(name=Peter, age=25, address=Address(street=W NORMA ST, postcode=77009))}

val associateByMap2 = custList.associateBy({it.name}, {it.address})
println("associateByMap2 = $associateByMap2") // associateByMap2 = {Jack=Address(street=NANTERRE CT, postcode=77471), Peter=Address(street=W NORMA ST, postcode=77009)}

3. associateTo()

Method signature: public inline fun> Iterable.associateTo(destination: M, transform: (T) -> Pair): M

var associateToMap = mutableMapOf()
custList.associateTo(associateToMap, {it.name to it.address})
println("associateToMap = $associateToMap") // associateToMap = {Jack=Address(street=NANTERRE CT, postcode=77471), Peter=Address(street=W NORMA ST, postcode=77009)}

4. associateByTo()

var associateByToMap = mutableMapOf() 
custList.associateByTo(associateByToMap, {it.name})
println("associateByToMap = $associateByToMap") // associateByToMap = {Jack=Customer(name=Jack, age=20, address=Address(street=NANTERRE CT, postcode=77471)), Peter=Customer(name=Peter, age=25, address=Address(street=W NORMA ST, postcode=77009))}

var associateByToMap2 = mutableMapOf() 
custList.associateByTo(associateByToMap2, {it.name}, {it.address})
println("associateByToMap2 = $associateByToMap2") // associateByToMap2 = {Jack=Address(street=NANTERRE CT, postcode=77471), Peter=Address(street=W NORMA ST, postcode=77009)}

B. Practice

I. Implement

We create a Kotlin project as below:

1. Create data models

– Create Address model:

package com.javasampleapproach.list2map.model

data class Address(
	var street : String? = null,
	var postcode : String? = null
){}

– Create Customer model:

package com.javasampleapproach.list2map.model

data class Customer(
	var name: String? = null,
	var age: Int? = null,
	var address: Address = Address()) {
}

2. Kotlin Program

package com.javasampleapproach.list2map

import com.javasampleapproach.list2map.model.Address
import com.javasampleapproach.list2map.model.Customer

fun main(args : Array) {
	// initial list of Customers
	val custList = listOf(Customer("Jack", 20, Address("NANTERRE CT", "77471")),
											Customer("Peter", 25, Address("W NORMA ST", "77009")))
	println("custList = $custList") // custList = [Customer(name=Jack, age=20, address=Address(street=NANTERRE CT, postcode=77471)), Customer(name=Peter, age=25, address=Address(street=W NORMA ST, postcode=77009))]
	
	/*
     * I. use toMap() function to convert List -> Map
 	 */
	
	// 1. create a read-only map
	val toMap = custList.map { it.name to it.address }.toMap()
	println("toMap = $toMap") // toMap = {Jack=Address(street=NANTERRE CT, postcode=77471), Peter=Address(street=W NORMA ST, postcode=77009)}
	
	// 2. create a modifiable map
	var modifiableMap = mutableMapOf()
	modifiableMap.putAll(custList.map { it.name to it.address }.toMap())
	modifiableMap.put("Mary", Address("E NAVAHO TRL", "77449"))
	println("modifiableMap = $modifiableMap") // modifiableMap = {Jack=Address(street=NANTERRE CT, postcode=77471), Peter=Address(street=W NORMA ST, postcode=77009), Mary=Address(street=E NAVAHO TRL, postcode=77449)}

	/*
     * II. use associate* methods
     */
	
	// 1. use associate()
	// -> public inline fun  Iterable.associate(transform: (T) -> Pair): Map
	val associateMap = custList.associate { it.name to it.address.street }
	println("associateMap = $associateMap") // associateMap = {Jack=NANTERRE CT, Peter=W NORMA ST}
	
	// 2. use associateBy
	// -> 2.1 use public inline fun  Iterable.associateBy(keySelector: (T) -> K): Map 
	val associateByMap = custList.associateBy { it.name }
	println("associateByMap = $associateByMap") // associateByMap = {Jack=Customer(name=Jack, age=20, address=Address(street=NANTERRE CT, postcode=77471)), Peter=Customer(name=Peter, age=25, address=Address(street=W NORMA ST, postcode=77009))}

	// -> 2.2 use public inline fun  Iterable.associateBy(keySelector: (T) -> K, valueTransform: (T) -> V): Map
	val associateByMap2 = custList.associateBy({it.name}, {it.address})
	println("associateByMap2 = $associateByMap2") // associateByMap2 = {Jack=Address(street=NANTERRE CT, postcode=77471), Peter=Address(street=W NORMA ST, postcode=77009)}
	
	// 3. use associateTo
	// -> public inline fun > Iterable.associateTo(destination: M, transform: (T) -> Pair): M
	var associateToMap = mutableMapOf()
	custList.associateTo(associateToMap, {it.name to it.address})
	println("associateToMap = $associateToMap") // associateToMap = {Jack=Address(street=NANTERRE CT, postcode=77471), Peter=Address(street=W NORMA ST, postcode=77009)}

	// 4. use associateByTo
	// ->4.1 use public inline fun > Iterable.associateByTo(destination: M, keySelector: (T) -> K): M
	var associateByToMap = mutableMapOf() 
	custList.associateByTo(associateByToMap, {it.name})
	println("associateByToMap = $associateByToMap") // associateByToMap = {Jack=Customer(name=Jack, age=20, address=Address(street=NANTERRE CT, postcode=77471)), Peter=Customer(name=Peter, age=25, address=Address(street=W NORMA ST, postcode=77009))}

	// -> 4.2 use public inline fun > Iterable.associateByTo(destination: M, keySelector: (T) -> K, valueTransform: (T) -> V): M
	var associateByToMap2 = mutableMapOf() 
	custList.associateByTo(associateByToMap2, {it.name}, {it.address})
	println("associateByToMap2 = $associateByToMap2") // associateByToMap2 = {Jack=Address(street=NANTERRE CT, postcode=77471), Peter=Address(street=W NORMA ST, postcode=77009)}
}

II. Sourcecode

KotlinConvertListToMap

Last updated on February 6, 2020.