| language | contributors | filename | |||
|---|---|---|---|---|---|
kotlin |
|
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.
// 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 ${names.size} elements")
}
varargExample() // => Argument has 0 elements
varargExample(1) // => Argument has 1 elements
varargExample(1, 2, 3) // => Argument has 3 elements
/*
When a function consists of a single expression then the curly brackets can
be omitted. The body is specified after a = symbol.
*/
fun odd(x: Int): Boolean = x % 2 == 1
println(odd(6)) // => false
println(odd(7)) // => true
// If the return type can be inferred then we don't need to specify it.
fun even(x: Int) = x % 2 == 0
println(even(6)) // => true
println(even(7)) // => false
// Functions can take functions as arguments and return functions.
fun not(f: (Int) -> Boolean) : (Int) -> Boolean {
return {n -> !f.invoke(n)}
}
// Named functions can be specified as arguments using the :: operator.
val notOdd = not(::odd)
val notEven = not(::even)
// Anonymous functions can be specified as arguments.
val notZero = not {n -> n == 0}
/*
If an anonymous function has only one parameter
then its declaration can be omitted (along with the ->).
The name of the single parameter will be "it".
*/
val notPositive = not {it > 0}
for (i in 0..4) {
println("${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
// The "with" function is similar to the JavaScript "with" statement.
data class MutableDataClassExample (var x: Int, var y: Int, var z: Int)
val fooMutableDate = MutableDataClassExample(7, 4, 9)
with (fooMutableDate) {
x -= 2
y += 2
z--
}
println(fooMutableDate) // => 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)
// "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
/*
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 assign it to a variable, but we can refer to it by its name.
This is similar to Scala singleton objects.
*/
object ObjectExample {
fun hello() : String {
return "hello"
}
}