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diff --git a/kotlin.html.markdown b/kotlin.html.markdown new file mode 100644 index 00000000..ca11ee3c --- /dev/null +++ b/kotlin.html.markdown @@ -0,0 +1,379 @@ +--- +language: kotlin +contributors: + - ["S Webber", "https://github.com/s-webber"] +filename: LearnKotlin.kt +--- + +Kotlin is a statically typed programming language for the JVM, Android and the +browser. It is 100% interoperable with Java. +[Read more here.](https://kotlinlang.org/) + +```kotlin +// Single-line comments start with // +/* +Multi-line comments look like this. +*/ + +// The "package" keyword works in the same way as in Java. +package com.learnxinyminutes.kotlin + +/* +The entry point to a Kotlin program is a function named "main". +The function is passed an array containing any command line arguments. +*/ +fun main(args: Array<String>) { + /* + Declaring values is done using either "var" or "val". + "val" declarations cannot be reassigned, whereas "vars" can. + */ + val fooVal = 10 // we cannot later reassign fooVal to something else + var fooVar = 10 + fooVar = 20 // fooVar can be reassigned + + /* + In most cases, Kotlin can determine what the type of a variable is, + so we don't have to explicitly specify it every time. + We can explicitly declare the type of a variable like so: + */ + val foo: Int = 7 + + /* + Strings can be represented in a similar way as in Java. + Escaping is done with a backslash. + */ + val fooString = "My String Is Here!" + val barString = "Printing on a new line?\nNo Problem!" + val bazString = "Do you want to add a tab?\tNo Problem!" + println(fooString) + println(barString) + println(bazString) + + /* + A raw string is delimited by a triple quote ("""). + Raw strings can contain newlines and any other characters. + */ + val fooRawString = """ +fun helloWorld(val name : String) { + println("Hello, world!") +} +""" + println(fooRawString) + + /* + Strings can contain template expressions. + A template expression starts with a dollar sign ($). + */ + val fooTemplateString = "$fooString has ${fooString.length} characters" + println(fooTemplateString) + + /* + For a variable to hold null it must be explicitly specified as nullable. + A variable can be specified as nullable by appending a ? to its type. + We can access a nullable variable by using the ?. operator. + We can use the ?: operator to specify an alternative value to use + if a variable is null. + */ + var fooNullable: String? = "abc" + println(fooNullable?.length) // => 3 + println(fooNullable?.length ?: -1) // => 3 + fooNullable = null + println(fooNullable?.length) // => null + println(fooNullable?.length ?: -1) // => -1 + + /* + Functions can be declared using the "fun" keyword. + Function arguments are specified in brackets after the function name. + Function arguments can optionally have a default value. + The function return type, if required, is specified after the arguments. + */ + fun hello(name: String = "world"): String { + return "Hello, $name!" + } + println(hello("foo")) // => Hello, foo! + println(hello(name = "bar")) // => Hello, bar! + println(hello()) // => Hello, world! + + /* + A function parameter may be marked with the "vararg" keyword + to allow a variable number of arguments to be passed to the function. + */ + fun varargExample(vararg names: Int) { + println("Argument has ${names.size} elements") + } + varargExample() // => Argument has 0 elements + varargExample(1) // => Argument has 1 elements + varargExample(1, 2, 3) // => Argument has 3 elements + + /* + When a function consists of a single expression then the curly brackets can + be omitted. The body is specified after a = symbol. + */ + fun odd(x: Int): Boolean = x % 2 == 1 + println(odd(6)) // => false + println(odd(7)) // => true + + // If the return type can be inferred then we don't need to specify it. + fun even(x: Int) = x % 2 == 0 + println(even(6)) // => true + println(even(7)) // => false + + // Functions can take functions as arguments and return functions. + fun not(f: (Int) -> Boolean): (Int) -> Boolean { + return {n -> !f.invoke(n)} + } + // Named functions can be specified as arguments using the :: operator. + val notOdd = not(::odd) + val notEven = not(::even) + // Lambda expressions can be specified as arguments. + val notZero = not {n -> n == 0} + /* + If a lambda has only one parameter + then its declaration can be omitted (along with the ->). + The name of the single parameter will be "it". + */ + val notPositive = not {it > 0} + for (i in 0..4) { + println("${notOdd(i)} ${notEven(i)} ${notZero(i)} ${notPositive(i)}") + } + + // The "class" keyword is used to declare classes. + class ExampleClass(val x: Int) { + fun memberFunction(y: Int): Int { + return x + y + } + + infix fun infixMemberFunction(y: Int): Int { + return x * y + } + } + /* + To create a new instance we call the constructor. + Note that Kotlin does not have a "new" keyword. + */ + val fooExampleClass = ExampleClass(7) + // Member functions can be called using dot notation. + println(fooExampleClass.memberFunction(4)) // => 11 + /* + If a function has been marked with the "infix" keyword then it can be + called using infix notation. + */ + println(fooExampleClass infixMemberFunction 4) // => 28 + + /* + Data classes are a concise way to create classes that just hold data. + The "hashCode"/"equals" and "toString" methods are automatically generated. + */ + data class DataClassExample (val x: Int, val y: Int, val z: Int) + val fooData = DataClassExample(1, 2, 4) + println(fooData) // => DataClassExample(x=1, y=2, z=4) + + // Data classes have a "copy" function. + val fooCopy = fooData.copy(y = 100) + println(fooCopy) // => DataClassExample(x=1, y=100, z=4) + + // Objects can be destructured into multiple variables. + val (a, b, c) = fooCopy + println("$a $b $c") // => 1 100 4 + + // destructuring in "for" loop + for ((a, b, c) in listOf(fooData)) { + println("$a $b $c") // => 1 100 4 + } + + val mapData = mapOf("a" to 1, "b" to 2) + // Map.Entry is destructurable as well + for ((key, value) in mapData) { + println("$key -> $value") + } + + // The "with" function is similar to the JavaScript "with" statement. + data class MutableDataClassExample (var x: Int, var y: Int, var z: Int) + val fooMutableData = MutableDataClassExample(7, 4, 9) + with (fooMutableData) { + x -= 2 + y += 2 + z-- + } + println(fooMutableData) // => MutableDataClassExample(x=5, y=6, z=8) + + /* + We can create a list using the "listOf" function. + The list will be immutable - elements cannot be added or removed. + */ + val fooList = listOf("a", "b", "c") + println(fooList.size) // => 3 + println(fooList.first()) // => a + println(fooList.last()) // => c + // Elements of a list can be accessed by their index. + println(fooList[1]) // => b + + // A mutable list can be created using the "mutableListOf" function. + val fooMutableList = mutableListOf("a", "b", "c") + fooMutableList.add("d") + println(fooMutableList.last()) // => d + println(fooMutableList.size) // => 4 + + // We can create a set using the "setOf" function. + val fooSet = setOf("a", "b", "c") + println(fooSet.contains("a")) // => true + println(fooSet.contains("z")) // => false + + // We can create a map using the "mapOf" function. + val fooMap = mapOf("a" to 8, "b" to 7, "c" to 9) + // Map values can be accessed by their key. + println(fooMap["a"]) // => 8 + + /* + Sequences represent lazily-evaluated collections. + We can create a sequence using the "generateSequence" function. + */ + val fooSequence = generateSequence(1, { it + 1 }) + val x = fooSequence.take(10).toList() + println(x) // => [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] + + // An example of using a sequence to generate Fibonacci numbers: + fun fibonacciSequence(): Sequence<Long> { + var a = 0L + var b = 1L + + fun next(): Long { + val result = a + b + a = b + b = result + return a + } + + return generateSequence(::next) + } + val y = fibonacciSequence().take(10).toList() + println(y) // => [1, 1, 2, 3, 5, 8, 13, 21, 34, 55] + + // Kotlin provides higher-order functions for working with collections. + val z = (1..9).map {it * 3} + .filter {it < 20} + .groupBy {it % 2 == 0} + .mapKeys {if (it.key) "even" else "odd"} + println(z) // => {odd=[3, 9, 15], even=[6, 12, 18]} + + // A "for" loop can be used with anything that provides an iterator. + for (c in "hello") { + println(c) + } + + // "while" loops work in the same way as other languages. + var ctr = 0 + while (ctr < 5) { + println(ctr) + ctr++ + } + do { + println(ctr) + ctr++ + } while (ctr < 10) + + /* + "if" can be used as an expression that returns a value. + For this reason the ternary ?: operator is not needed in Kotlin. + */ + val num = 5 + val message = if (num % 2 == 0) "even" else "odd" + println("$num is $message") // => 5 is odd + + // "when" can be used as an alternative to "if-else if" chains. + val i = 10 + when { + i < 7 -> println("first block") + fooString.startsWith("hello") -> println("second block") + else -> println("else block") + } + + // "when" can be used with an argument. + when (i) { + 0, 21 -> println("0 or 21") + in 1..20 -> println("in the range 1 to 20") + else -> println("none of the above") + } + + // "when" can be used as a function that returns a value. + var result = when (i) { + 0, 21 -> "0 or 21" + in 1..20 -> "in the range 1 to 20" + else -> "none of the above" + } + println(result) + + /* + We can check if an object is a particular type by using the "is" operator. + If an object passes a type check then it can be used as that type without + explicitly casting it. + */ + fun smartCastExample(x: Any) : Boolean { + if (x is Boolean) { + // x is automatically cast to Boolean + return x + } else if (x is Int) { + // x is automatically cast to Int + return x > 0 + } else if (x is String) { + // x is automatically cast to String + return x.isNotEmpty() + } else { + return false + } + } + println(smartCastExample("Hello, world!")) // => true + println(smartCastExample("")) // => false + println(smartCastExample(5)) // => true + println(smartCastExample(0)) // => false + println(smartCastExample(true)) // => true + + // Smartcast also works with when block + fun smartCastWhenExample(x: Any) = when (x) { + is Boolean -> x + is Int -> x > 0 + is String -> x.isNotEmpty() + else -> false + } + + /* + Extensions are a way to add new functionality to a class. + This is similar to C# extension methods. + */ + fun String.remove(c: Char): String { + return this.filter {it != c} + } + println("Hello, world!".remove('l')) // => Heo, word! + + println(EnumExample.A) // => A + println(ObjectExample.hello()) // => hello +} + +// Enum classes are similar to Java enum types. +enum class EnumExample { + A, B, C +} + +/* +The "object" keyword can be used to create singleton objects. +We cannot instantiate it but we can refer to its unique instance by its name. +This is similar to Scala singleton objects. +*/ +object ObjectExample { + fun hello(): String { + return "hello" + } +} + +fun useObject() { + ObjectExample.hello() + val someRef: Any = ObjectExample // we use objects name just as is +} + +``` + +### Further Reading + +* [Kotlin tutorials](https://kotlinlang.org/docs/tutorials/) +* [Try Kotlin in your browser](http://try.kotlinlang.org/) +* [A list of Kotlin resources](http://kotlin.link/) |