Kotlin gives the programmer the ability to add more functionality to the existing classes, without inheriting them.

This is achieved through a feature known as extensions.

When a function is added to an existing class it is known as Extension Function.

To add an extension function to a class, define a new function appended to the class name.

For Example :

fun main() {
 val list = mutableListOf(1,2,3,4)
 list.swap(0,2)
 print(list)
}
fun MutableList<Int>.swap(index1: Int, index2: Int) {
    val tmp = this[index1] // 'this' corresponds to the list
    this[index1] = this[index2]
    this[index2] = tmp

}

Output : [3,2,1,4]