Kotlin list : listOf()

A list is a generic ordered collection of elements. Kotlin has two types of lists, immutable lists (cannot be modified) and mutable lists (can be modified). 
Read-only lists are created with listOf() whose elements can not be modified and mutable lists created with mutableListOf() method where we alter or modify the elements of the list.
Kotlin program of list contains Integers –

Java

fun main(args: Array<String>) {
    val a = listOf('1', '2', '3')
    println(a.size)
    println(a.indexOf('2'))
    println(a[2])
}

Output: 

3
1
3

Kotlin program of list contains Strings –

Java

fun main(args: Array<String>) {
    //creating list of strings
    val a = listOf("Ram", "Shyam", "Raja", "Rani")
    println("The size of the list is: "+a.size)
    println("The index of the element Raja is: "+a.indexOf("Raja"))
    println("The element at index "+a[2])
    for(i in a.indices){
        println(a[i])
    }
}

Output:

The size of the list is: 4
The index of the element Raja is: 2
The element at index Raja
Ram
Shyam
Raja
Rani

Indexing of Elements of List in Kotlin –

Each element of a list has an index. The first element has an index of zero (0) and the last Element has index 
len – 1, where ‘len’ is the length of the list. 

Java

fun main(args: Array<String>)
{
    val numbers = listOf(1, 5, 7, 32, 0, 21, 1, 6, 10)

    val num1 = numbers.get(0)
    println(num1)

    val num2 = numbers[7]
    println(num2)

    val index1 = numbers.indexOf(1)
    println("The first index of number is $index1")

    val index2 = numbers.lastIndexOf(1)
    println("The last index of number is $index2")

    val index3 = numbers.lastIndex
    println("The last index of the list is $index3")
}

Output:

1
6
The first index of number is 0
The last index of number is 6
The last index of the list is 8

First and Last Elements –

We can retrieve the first and the last elements of the list without using the get() method. 
Considering the previous example, if we include the following code after line no. 17 

Java

fun main(args: Array<String>)
{
    val numbers = listOf(1, 5, 7, 32, 0, 21, 1, 6, 10)
    println(numbers.first())
    println(numbers.last())
}

Output: 

1
10

List Iteration methods –

It is process of accessing the elements of list one by one. 
There are several ways of doing this in Kotlin.

Java

fun main(args: Array<String>)
{
    val names = listOf("Gopal", "Asad", "Shubham", "Aditya",
        "Devarsh", "Nikhil", "Gagan")

    // method 1
    for (name in names) {
        print("$name, ")
    }
    println()

    // method 2
    for (i in 0 until names.size) {
        print("${names[i]} ")
    }
    println()

    // method 3
    names.forEachIndexed({i, j -> println("names[$i] = $j")})

    // method 4
    val it: ListIterator<String> = names.listIterator()
    while (it.hasNext()) {
        val i = it.next()
        print("$i ")
    }
    println()
}

Output:

Gopal, Asad, Shubham, Aditya, Devarsh, Nikhil, Gagan, 
Gopal Asad Shubham Aditya Devarsh Nikhil Gagan 
names[0] = Gopal
names[1] = Asad
names[2] = Shubham
names[3] = Aditya
names[4] = Devarsh
names[5] = Nikhil
names[6] = Gagan
Gopal Asad Shubham Aditya Devarsh Nikhil Gagan 

Explanation:

for (name in names) {

        print("$name, ")
    }

The for loop traverses the list. In each cycle, the variable ‘name’ points to the next element in the list. 

for (i in 0 until names.size) {

        print("${names[i]} ")
    }

This method uses the size of the list. The until keyword creates a range of the list indexes.

names.forEachIndexed({i, j -> println("namess[$i] = $j")})

With the forEachIndexed() method, we loop over the list having index and value available in each iteration. 

val it: ListIterator = names.listIterator()

    while (it.hasNext()) {

        val i = it.next()
        print("$i ")
    }

Here we use a ListIterator to iterate through the list.

Sorting the elements in list –

The following example show how to sort a list in ascending or descending order. 

Java

fun main(args: Array<String>)
{
    val list = listOf(8, 4, 7, 1, 2, 3, 0, 5, 6 )

    val asc = list.sorted()
    println(asc)

    val desc = list.sortedDescending()
    println(desc)
}

Output:

[0, 1, 2, 3, 4, 5, 6, 7, 8]
[8, 7, 6, 5, 4, 3, 2, 1, 0]

Explanation:

val asc = list.sorted()

The sorted() method sorts the list in ascending order. 

val desc = list.sortedDescending()

The sortedDescending() method sorts the list in descending order.

Contains() and containsAll() functions –

This method checks whether an element exists in the list or not. 

Java

fun main(args: Array<String>)
{
    val list = listOf(8, 4, 7, 1, 2, 3, 0, 5, 6 )

    val res = list.contains(0)

    if (res)
        println("The list contains 0")
    else
        println("The list does not contain 0")

    val result = list.containsAll(listOf(3, -1))

    if (result)
        println("The list contains 3 and -1")
    else
        println("The list does not contain 3 and -1")
}

Output:

The list contains 0
The list does not contain 3 and -1

Explanation:

val res = list.contains(0)

Checks whether the list contains 0 or not and returns true or false (her true) that is stored in res. 

 val result = list.containsAll(listOf(3, -1))

Checks whether the list contains 3 and -1 or not.