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kotlin.html.markdown
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---
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language: kotlin
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contributors:
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- ["S Webber", "https://github.com/s-webber"]
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filename: LearnKotlin.kt
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---
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Kotlin is a Statically typed programming language for the JVM, Android and the
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browser. It is 100% interoperable with Java.
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[Read more here.](https://kotlinlang.org/)
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```kotlin
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// Single-line comments start with //
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/*
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Multi-line comments look like this.
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*/
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// The "package" keyword works in the same way as in Java.
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package com.learnxinyminutes.kotlin
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/*
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The entry point to a Kotlin program is a function named "main".
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The function is passed an array containing any command line arguments.
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*/
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fun main(args: Array<String>) {
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/*
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Declaring values is done using either "var" or "val".
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"val" declarations cannot be reassigned, whereas "vars" can.
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*/
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val fooVal = 10 // we cannot later reassign fooVal to something else
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var fooVar = 10
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fooVar = 20 // fooVar can be reassigned
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/*
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In most cases, Kotlin can determine what the type of a variable is,
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so we don't have to explicitly specify it every time.
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We can explicitly declare the type of a variable like so:
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*/
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val foo : Int = 7
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/*
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Strings can be represented in a similar way as in Java.
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Escaping is done with a backslash.
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*/
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val fooString = "My String Is Here!";
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val barString = "Printing on a new line?\nNo Problem!";
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val bazString = "Do you want to add a tab?\tNo Problem!";
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println(fooString);
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println(barString);
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println(bazString);
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/*
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A raw string is delimited by a triple quote (""").
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Raw strings can contain newlines and any other characters.
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*/
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val fooRawString = """
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fun helloWorld(val name : String) {
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println("Hello, world!")
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}
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"""
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println(fooRawString)
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/*
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Strings can contain template expressions.
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A template expression starts with a dollar sign ($).
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*/
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val fooTemplateString = "$fooString has ${fooString.length} characters"
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println(fooTemplateString)
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/*
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For a variable to hold null it must be explicitly specified as nullable.
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A variable can be specified as nullable by appending a ? to its type.
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We can access a nullable variable by using the ?. operator.
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We can use the ?: operator to specify an alternative value to use
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if a variable is null
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*/
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var fooNullable: String? = "abc"
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println(fooNullable?.length) // => 3
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println(fooNullable?.length ?: -1) // => 3
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fooNullable = null
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println(fooNullable?.length) // => null
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println(fooNullable?.length ?: -1) // => -1
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/*
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Functions can be declared using the "fun" keyword.
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Function arguments are specified in brackets after the function name.
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Function arguments can optionally have a default value.
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The function return type, if required, is specified after the arguments.
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*/
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fun hello(name: String = "world") : String {
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return "Hello, $name!"
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}
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println(hello("foo")) // => Hello, foo!
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println(hello(name = "bar")) // => Hello, bar!
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println(hello()) // => Hello, world!
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/*
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A function parameter may be marked with the "vararg" keyword
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to allow a variable number of arguments to be passed to the function.
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*/
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fun varargExample(vararg names: Int) {
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println("Argument has ${names.size} elements")
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}
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varargExample() // => Argument has 0 elements
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varargExample(1) // => Argument has 1 elements
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varargExample(1, 2, 3) // => Argument has 3 elements
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/*
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When a function consists of a single expression then the curly brackets can
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be omitted. The body is specified after a = symbol.
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*/
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fun odd(x: Int): Boolean = x % 2 == 1
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println(odd(6)) // => false
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println(odd(7)) // => true
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// If the return type can be inferred then we don't need to specify it.
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fun even(x: Int) = x % 2 == 0
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println(even(6)) // => true
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println(even(7)) // => false
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// Functions can take functions as arguments and return functions.
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fun not(f: (Int) -> Boolean) : (Int) -> Boolean {
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return {n -> !f.invoke(n)}
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}
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// Named functions can be specified as arguments using the :: operator.
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val notOdd = not(::odd)
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val notEven = not(::even)
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// Anonymous functions can be specified as arguments.
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val notZero = not {n -> n == 0}
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/*
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If an anonymous function has only one parameter
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then its declaration can be omitted (along with the ->).
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The name of the single parameter will be "it".
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*/
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val notPositive = not {it > 0}
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for (i in (0..4)) {
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println("${notOdd(i)} ${notEven(i)} ${notZero(i)} ${notPositive(i)}")
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}
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//The "class" keyword is used to declare classes.
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class ExampleClass(val x: Int) {
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fun memberFunction(y: Int) : Int {
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return x + y
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}
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infix fun infixMemberFunction(y: Int) : Int {
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return x * y
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}
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}
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/*
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To create a new instance we call the constructor.
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Note that Kotlin does not have a "new" keyword.
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*/
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val fooExampleClass = ExampleClass(7)
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// Member functions can be called using dot notation.
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println(fooExampleClass.memberFunction(4)) // => 11
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/*
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If a function has been marked with the "infix" keyword then it can be
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called using infix notation.
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*/
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println(fooExampleClass infixMemberFunction 4) // => 28
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/*
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Data classes are a concise way to create classes that just hold data.
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The "hashCode"/"equals" and "toString" methods are automatically generated.
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*/
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data class DataClassExample (val x: Int, val y: Int, val z: Int)
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val fooData = DataClassExample(1, 2, 4)
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println(fooData) // => DataClassExample(x=1, y=2, z=4)
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// Data classes have a "copy" function.
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val fooCopy = fooData.copy(y = 100)
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println(fooCopy) // => DataClassExample(x=1, y=100, z=4)
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// Objects can be destructured into multiple variables.
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val (a, b, c) = fooCopy
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println("$a $b $c") // => 1 100 4
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// The "with" function is similar to the JavaScript "with" statement.
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data class MutableDataClassExample (var x: Int, var y: Int, var z: Int)
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val fooMutableDate = MutableDataClassExample(7, 4, 9)
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with (fooMutableDate) {
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x -= 2
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y += 2
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z--
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}
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println(fooMutableDate) // => MutableDataClassExample(x=5, y=6, z=8)
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/*
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We can create a list using the "listOf" function.
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The list will be immutable - elements cannot be added or removed.
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*/
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val fooList = listOf("a", "b", "c")
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println(fooList.size) // => 3
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println(fooList.first()) // => a
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println(fooList.last()) // => c
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// elements can be accessed by index
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println(fooList[1]) // => b
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// A mutable list can be created using the "mutableListOf" function.
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val fooMutableList = mutableListOf("a", "b", "c")
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fooMutableList.add("d")
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println(fooMutableList.last()) // => d
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println(fooMutableList.size) // => 4
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// We can create a set using the "setOf" function.
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val fooSet = setOf("a", "b", "c")
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println(fooSet.contains("a")) // => true
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println(fooSet.contains("z")) // => false
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// We can create a map using the "mapOf" function.
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val fooMap = mapOf("a" to 8, "b" to 7, "c" to 9)
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// Map values can be accessed by their key.
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println(fooMap["a"]) // => 8
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// Kotlin provides higher-order functions for working with collections.
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val x = (1..9).map {it * 3}
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.filter {it < 20}
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.groupBy {it % 2 == 0}
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.mapKeys {if (it.key) "even" else "odd"}
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println(x) // => {odd=[3, 9, 15], even=[6, 12, 18]}
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// A "for" loop can be used with anything that provides an iterator.
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for (c in "hello") {
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println(c)
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}
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// "while" loops work in the same way as other languages.
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var ctr = 0
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while (ctr < 5) {
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println(ctr)
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ctr++
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}
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do {
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println(ctr)
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ctr++
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} while (ctr < 10)
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// "when" can be used as an alternative to "if-else if" chains.
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val i = 10
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when {
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i < 7 -> println("first block")
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fooString.startsWith("hello") -> println("second block")
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else -> println("else block")
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}
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// "when" can be used with an argument.
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when (i) {
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0, 21 -> println("0 or 21")
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in 1..20 -> println("in the range 1 to 20")
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else -> println("none of the above")
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}
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// "when" can be used as a function that returns a value.
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var result = when (i) {
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0, 21 -> "0 or 21"
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in 1..20 -> "in the range 1 to 20"
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else -> "none of the above"
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}
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println(result)
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/*
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We can check if an object is a particular type by using the "is" operator.
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If an object passes a type check then it can be used as that type without
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explicitly casting it.
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*/
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fun smartCastExample(x: Any) : Boolean {
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if (x is Boolean) {
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// x is automatically cast to Boolean
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return x
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} else if (x is Int) {
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// x is automatically cast to Int
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return x > 0
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} else if (x is String) {
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// x is automatically cast to String
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return x.isNotEmpty()
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} else {
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return false
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}
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}
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println(smartCastExample("Hello, world!")) // => true
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println(smartCastExample("")) // => false
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println(smartCastExample(5)) // => true
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println(smartCastExample(0)) // => false
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println(smartCastExample(true)) // => true
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/*
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Extensions are a way to add new functionality to a class.
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This is similar to C# extension methods.
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*/
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fun String.remove(c: Char): String {
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return this.filter {it != c}
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}
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println("Hello, world!".remove('l')) // => Heo, word!
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println(EnumExample.A) // => A
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println(ObjectExample.hello()) // => hello
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}
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// Enum classes are similar to Java enum types.
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enum class EnumExample {
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A, B, C
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}
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/*
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The "object" keyword can be used to create singleton objects.
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We cannot assign it to a variable, but we can refer to it by its name.
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This is similar to Scala singleton objects.
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*/
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object ObjectExample {
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fun hello() : String {
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return "hello"
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}
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}
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```
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### Further Reading
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A web-based mini-IDE for Kotlin:
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[http://try.kotlinlang.org/)
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