Collections are a common concept for most programming languages. A collection usually contains a number of objects of the same type and Objects in a collection are called elements or items.

The Kotlin Standard Library provides a comprehensive set of tools for managing collections. The following collection types are relevant for Kotlin:

• Kotlin List – List is an ordered collection with access to elements by indices. Elements can occur more than once in a list.

• Kotlin Set – Set is a collection of unique elements which means a group of objects without repetitions.

• Kotlin Map – Map (or dictionary) is a set of key-value pairs. Keys are unique, and each of them maps to exactly one value.

Kotlin Collection Types

Kotlin provides the following types of collection:

• Collection or Immutable Collection

• Mutable Collection

Kotlin Immutable Collection

Immutable Collection or simply calling a Collection interface provides read-only methods which means once a collection is created, we can not change it because there is no method available to change the object created.

Example

fun main() {
    val numbers = listOf("one", "two", "three", "four")
    
    println(numbers)
}

When you run the above Kotlin program, it will generate the following output:

[one, two, three, four]

Kotlin Mutable Collection

Mutable collections provides both read and write methods.

Example

fun main() {
    val numbers = mutableListOf("one", "two", "three", "four")
    
    numbers.add("five")
    
    println(numbers)
}

When you run the above Kotlin program, it will generate the following output:

[one, two, three, four, five]

Note that altering a mutable collection doesn”t require it to be a var.

fun main() {
    val numbers = listOf("one", "two", "three", "four")
    
    numbers = listOf("five")
}