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ad6e1f09bc
lambda functions can not defined by `fun` directly, use `val` (or `var` if mutability is required) instead
465 lines
14 KiB
Markdown
465 lines
14 KiB
Markdown
---
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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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Since Kotlin 1.3 the "main" function can also be defined without
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any parameters.
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*/
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fun main(args: Array<String>) {
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/*
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Declaring variables 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) // => My String Is Here! has 18 characters
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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 the = 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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/*
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You can also use lambda functions, with the '->' operator seperating
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the parameters from the function body.
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*/
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val fooLambda: (Int) -> Int = {n -> n + 1}
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println(fooLambda(1)) // => 2
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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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/*
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Lambda expressions can be specified as arguments.
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If it's the only argument parentheses can be omitted.
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*/
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val notZero = not {n -> n == 0}
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/*
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If a lambda 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 $c") // => 1 100 4
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// destructuring in "for" loop
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for ((a, b, c) in listOf(fooData)) {
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println("$a $b $c") // => 1 2 4
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}
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val mapData = mapOf("a" to 1, "b" to 2)
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// Map.Entry is destructurable as well
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for ((key, value) in mapData) {
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println("$key -> $value")
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}
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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 fooMutableData = MutableDataClassExample(7, 4, 9)
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with (fooMutableData) {
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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(fooMutableData) // => 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 of a list can be accessed by their 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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/*
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Sequences represent lazily-evaluated collections.
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We can create a sequence using the "generateSequence" function.
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*/
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val fooSequence = generateSequence(1, { it + 1 })
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val x = fooSequence.take(10).toList()
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println(x) // => [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
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// An example of using a sequence to generate Fibonacci numbers:
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fun fibonacciSequence(): Sequence<Long> {
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var a = 0L
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var b = 1L
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fun next(): Long {
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val result = a + b
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a = b
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b = result
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return a
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}
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return generateSequence(::next)
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}
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val y = fibonacciSequence().take(10).toList()
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println(y) // => [1, 1, 2, 3, 5, 8, 13, 21, 34, 55]
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// Kotlin provides higher-order functions for working with collections.
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val z = (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(z) // => {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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/*
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"if" can be used as an expression that returns a value.
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For this reason the ternary ?: operator is not needed in Kotlin.
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*/
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val num = 5
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val message = if (num % 2 == 0) "even" else "odd"
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println("$num is $message") // => 5 is odd
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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 an expression 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 of 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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// Smartcast also works with when block
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fun smartCastWhenExample(x: Any) = when (x) {
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is Boolean -> x
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is Int -> x > 0
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is String -> x.isNotEmpty()
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else -> false
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}
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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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}
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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 // Enum constants are separated with commas.
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}
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fun printEnum() = println(EnumExample.A) // => A
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// Since each enum is an instance of the enum class, they can be initialized as:
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enum class EnumExample(val value: Int) {
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A(value = 1),
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B(value = 2),
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C(value = 3)
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}
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fun printProperty() = println(EnumExample.A.value) // => 1
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// Every enum has properties to obtain its name and ordinal(position) in the enum class declaration:
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fun printName() = println(EnumExample.A.name) // => A
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fun printPosition() = println(EnumExample.A.ordinal) // => 0
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/*
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The "object" keyword can be used to create singleton objects.
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We cannot instantiate it but we can refer to its unique instance 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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override fun toString(): String {
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return "Hello, it's me, ${ObjectExample::class.simpleName}"
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}
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}
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fun useSingletonObject() {
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println(ObjectExample.hello()) // => hello
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// In Kotlin, "Any" is the root of the class hierarchy, just like "Object" is in Java
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val someRef: Any = ObjectExample
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println(someRef) // => Hello, it's me, ObjectExample
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}
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/* The not-null assertion operator (!!) converts any value to a non-null type and
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throws an exception if the value is null.
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*/
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var b: String? = "abc"
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val l = b!!.length
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data class Counter(var value: Int) {
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// overload Counter += Int
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operator fun plusAssign(increment: Int) {
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this.value += increment
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}
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// overload Counter++ and ++Counter
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operator fun inc() = Counter(value + 1)
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// overload Counter + Counter
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operator fun plus(other: Counter) = Counter(this.value + other.value)
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// overload Counter * Counter
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operator fun times(other: Counter) = Counter(this.value * other.value)
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// overload Counter * Int
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operator fun times(value: Int) = Counter(this.value * value)
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// overload Counter in Counter
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operator fun contains(other: Counter) = other.value == this.value
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// overload Counter[Int] = Int
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operator fun set(index: Int, value: Int) {
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this.value = index + value
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}
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// overload Counter instance invocation
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operator fun invoke() = println("The value of the counter is $value")
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}
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/* You can also overload operators through extension methods */
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// overload -Counter
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operator fun Counter.unaryMinus() = Counter(-this.value)
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fun operatorOverloadingDemo() {
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var counter1 = Counter(0)
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var counter2 = Counter(5)
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counter1 += 7
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println(counter1) // => Counter(value=7)
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println(counter1 + counter2) // => Counter(value=12)
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println(counter1 * counter2) // => Counter(value=35)
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println(counter2 * 2) // => Counter(value=10)
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println(counter1 in Counter(5)) // => false
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println(counter1 in Counter(7)) // => true
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counter1[26] = 10
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println(counter1) // => Counter(value=36)
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counter1() // => The value of the counter is 36
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println(-counter2) // => Counter(value=-5)
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}
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```
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### Further Reading
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* [Kotlin tutorials](https://kotlinlang.org/docs/tutorials/)
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* [Try Kotlin in your browser](https://play.kotlinlang.org/)
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* [A list of Kotlin resources](http://kotlin.link/)
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