Generic function :

fun <T> List<T>.itemAt ( position : Int) : T {

return this[position]

}

Generic class :

class Library <T> ( private books: List<T>) {

    val last : T? get () = books.lastOrNull()

}

More examples here

Create and filter a read only list:

var comics = listOf(Comics("Akira", "Japan" ), Comics ("Nausicaa", "Japan" ), Comics ("Spiderman", "Usa" ))

var titles = comics.filter ( it == "Japan")

.map(m ->m.title)

//titles will contain Akira and Nausicaa

class Comics (title : String, nation : String )

For adding or removing elements, we must use mutableListOf

A nice page about collections

In Kotlin we can overload some operators

  class Set(val num : Int) {
      operator fun plus(s: Set) = Set(this.num+ s.num)
  }

And we can use it like this

Set s1 = Set(5)

Set s2 = Set(6)

Set s3 = s1 + s2

We have a new Set with 11 element.

In Kotlin, functions can be passed as arguments to other functions, returned from functions, stored in variables.

In the code below a function add is declared as a lambda and passed as argument to the function calc:

val add ={a: Int, b: Int ->a+ b}

fun calc(i1:Int, i2: Int, action : (p1: Int, p2: Int) -> Int) {

var result = action (i1, i2)

}

//usage

calc(add)

In the code below a function add is declared as an anonymous function and passed as argument to the function calc:

val add =fun(a: Int, b: Int) =a+ b

fun calc(i1:Int, i2: Int, action : (p1: Int, p2: Int) -> Int) {

var result = action (i1, i2)

}

//usage

calc(add)

Returning a function that accepts a string and returns nothing:

fun displaMsg(msg : String) : (String) -> Unit {

return { s -> println ("$msg is $s" ) }

}

val d = displayMsg("error")

d("401")

//output is "error is 401"

For classes that contain only data, Kotlin provides an implementation of

• equals
• getHashCode
• toString
• copy

Consider the following examples

class Person (name : String)

Person a = Person ("Tom")

Person b = Person ("Tom")

println(a == b)

this print false because equals compares the objects references.

If instead we use a data class:

data class Person (name : String)

Person a = Person ("Tom")

Person b = Person ("Tom")

println(a == b)

this print true because equals is called on each member, and in this case the two instances have the same name.

It is possible to copy an instance of a data class changing some of its properties:

class Person (name : String, age : Int)

Person a = Person ("Tom", 23)

Person b = a.copy(age = 35)