Kotlin forEach & forEachIndexed method on Array, List, Map collection example

In the tutorial, Grokonez will show you how to use Kotlin forEach and forEachIndexed methods to loop through Kotlin Array, List, Map collections.

I. forEach method

forEach method is used to performs the given action on each element.

1. with Array

Method signature:

public inline fun  Array.forEach(action: (T) -> Unit): Unit

Practice:

val simpleArray = arrayOf(1, 2, 3,  4)
simpleArray.forEach { println(it) }
// print on console
// ->
/*
	1
	2
	3
	4		
*/

val customerArray = arrayOf(Customer("Craig", 45),
								Customer("Amos", 23),
								Customer("Jack", 20))

customerArray.forEach { println(it) }
// println on console
// ->
/*
	Customer(name=Craig, age=45)
	Customer(name=Amos, age=23)
	Customer(name=Jack, age=20) 
*/

2. with List

Method signature:

public inline fun  Iterable.forEach(action: (T) -> Unit): Unit

Practice:

val simpleList = listOf(5, 6, 7, 8)
simpleList.forEach{println(it)}
// print on console
// ->
/*
	5
	6
	7
	8
*/

val customerList = listOf(Customer("Smith", 26),
								Customer("Peter", 43),
								Customer("Mary", 27))
customerList.forEach { println(it) }
// print on console
// ->
/*
	Customer(name=Smith, age=26)
	Customer(name=Peter, age=43)
	Customer(name=Mary, age=27) 
*/

3. with Map

Method Signature:

public inline fun  Map.forEach(action: (Map.Entry) -> Unit): Unit

Practice:

val simpleHashMap = hashMapOf(1 to "one", 2 to "two", 3 to "three" )
simpleHashMap.forEach{ println(it) }
// println on console
// ->
/*
	1=one
	2=two
	3=three
*/

val customerHashMap = hashMapOf("Smith" to Customer("Smith", 26),
								"Peter" to Customer("Peter", 43),
								"Mary" to Customer("Mary", 27))
customerHashMap.forEach{ println(it) }
// println on console
// ->
/*
	Smith=Customer(name=Smith, age=26)
	Peter=Customer(name=Peter, age=43)
	Mary=Customer(name=Mary, age=27)
*/

II. forEachIndexed method

forEachIndexed method performs the given action on each element, providing sequential index with the element.

1. with Array

Method Signature:

public inline fun  Array.forEachIndexed(action: (index: Int, T) -> Unit): Unit

Practice:

simpleArray.forEachIndexed{index, element -> println("index = $index, element = $element")}
// print on console
// ->
/*
	index = 0, element = 1
	index = 1, element = 2
	index = 2, element = 3
	index = 3, element = 4 
*/
customerArray.forEachIndexed{index, customer -> println("index = $index, customer = $customer")}
// print on console
// ->
/*
	index = 0, customer = Customer(name=Craig, age=45)
	index = 1, customer = Customer(name=Amos, age=23)
	index = 2, customer = Customer(name=Jack, age=20)
*/

2. with List

Method signature:

public inline fun  Iterable.forEachIndexed(action: (index: Int, T) -> Unit): Unit

Practice:

simpleList.forEachIndexed{index, element -> println("index = $index, element = $element")}
// print on console
// ->
/*
	index = 0, element = 5
	index = 1, element = 6
	index = 2, element = 7
	index = 3, element = 8
*/

customerList.forEachIndexed{index, customer -> println("index = $index, customer = $customer")}
// print on console
// ->
/*
	index = 0, customer = Customer(name=Smith, age=26)
	index = 1, customer = Customer(name=Peter, age=43)
	index = 2, customer = Customer(name=Mary, age=27)
*/

III. Full sourcecode

package com.javasampleapproach.kotlinforeach

data class Customer(
	val name: String,
	val age: Int
)

fun main(args : Array) {
	
	println("#############################################################")
	println("---------------------I. forEach() method---------------------")
	println("#############################################################")
	
	println("-------------1. work with Array-------------")
	val simpleArray = arrayOf(1, 2, 3,  4)
	simpleArray.forEach { println(it) }
	// print on console
	// ->
	/*
		1
		2
		3
		4		
 	*/
	
	val customerArray = arrayOf(Customer("Craig", 45),
									Customer("Amos", 23),
									Customer("Jack", 20))
	
	customerArray.forEach { println(it) }
	// println on console
	// ->
	/*
		Customer(name=Craig, age=45)
		Customer(name=Amos, age=23)
		Customer(name=Jack, age=20) 
 	*/
	
	println("-------------2. work with List-------------")
	val simpleList = listOf(5, 6, 7, 8)
	simpleList.forEach{println(it)}
	// print on console
	// ->
	/*
		5
		6
		7
		8
 	*/
	
	val customerList = listOf(Customer("Smith", 26),
									Customer("Peter", 43),
									Customer("Mary", 27))
	customerList.forEach { println(it) }
	// print on console
	// ->
	/*
		Customer(name=Smith, age=26)
		Customer(name=Peter, age=43)
		Customer(name=Mary, age=27) 
 	*/
	
	println("-------------3. work with HashMap -------------")
	val simpleHashMap = hashMapOf(1 to "one", 2 to "two", 3 to "three" )
	simpleHashMap.forEach{ println(it) }
	// println on console
	// ->
	/*
		1=one
		2=two
		3=three
 	*/
	
	val customerHashMap = hashMapOf("Smith" to Customer("Smith", 26),
									"Peter" to Customer("Peter", 43),
									"Mary" to Customer("Mary", 27))
	customerHashMap.forEach{ println(it) }
	// println on console
	// ->
	/*
		Smith=Customer(name=Smith, age=26)
		Peter=Customer(name=Peter, age=43)
		Mary=Customer(name=Mary, age=27)
 	*/
	
	// II. forEachIndexed() method
	println("#############################################################")
	println("-----------------II. forEachIndexed() method-----------------")
	println("#############################################################")
	// 1. work with Array
	println("-------------1. work with Array-------------")
	simpleArray.forEachIndexed{index, element -> println("index = $index, element = $element")}
	// print on console
	// ->
	/*
		index = 0, element = 1
		index = 1, element = 2
		index = 2, element = 3
		index = 3, element = 4 
 	*/
	customerArray.forEachIndexed{index, customer -> println("index = $index, customer = $customer")}
	// print on console
	// ->
	/*
		index = 0, customer = Customer(name=Craig, age=45)
		index = 1, customer = Customer(name=Amos, age=23)
		index = 2, customer = Customer(name=Jack, age=20)
 	*/
	
	// 2. work with List
	println("-------------2. work with List-------------")
	simpleList.forEachIndexed{index, element -> println("index = $index, element = $element")}
	// print on console
	// ->
	/*
		index = 0, element = 5
		index = 1, element = 6
		index = 2, element = 7
		index = 3, element = 8
 	*/
	
	customerList.forEachIndexed{index, customer -> println("index = $index, customer = $customer")}
	// print on console
	// ->
	/*
		index = 0, customer = Customer(name=Smith, age=26)
		index = 1, customer = Customer(name=Peter, age=43)
		index = 2, customer = Customer(name=Mary, age=27)
 	*/
}