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Kotlin miniguide #19 standard functions
“Mastering Kotlin standard functions: run, with, let, also and apply” by Elye https://link.medium.com/8WxL2najrU
Kotlin miniguide #18 Resources
Kotlin miniguide #17 Generics
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
Kotlin miniguide #16 Null safety
Unless explicitly declared, a reference in kotlin cannot hold the value null
When a reference is nullable, a check on the reference must be done before accessing the object, otherwise we will get a compiler error:
var s : String = "a" s = null //<strong>does not compile </strong> var p: String? = "a" p = null //<strong>ok</strong> p.lenght //<strong>does not compile </strong> p?.length //<strong>ok</strong>
Kotlin miniguide #15 Collections
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
Kotlin miniguide #9 operator overloading
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.
Kotlin miniguide #14 High order functions
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"
Kotlin miniguide #13 Data classes
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)
Kotlin miniguide #12 Sealed classes
When used in combination with the when expression, sealed classes are like enum for classes:
sealed class Port
val name : String
get() = when (this) { //custom getter
is USB ->"this is USB"
is COM ->"this is COM"
}
class USB : Port()
class COM : Port()
USB u = USB
println (u.name) //this prints "this is USB"
Sealed classes are abstract, can have abstract members and define the classes that can inherit from them (USB and COM can inherit from Port)
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