mirror of
https://github.com/adambard/learnxinyminutes-docs.git
synced 2024-12-18 21:01:52 +03:00
ven
commit
eea20d626f
321
kotlin.html.markdown
Normal file
321
kotlin.html.markdown
Normal file
@ -0,0 +1,321 @@
|
||||
---
|
||||
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 ${names.size} elements")
|
||||
}
|
||||
varargExample() // => Argument has 0 elements
|
||||
varargExample(1) // => Argument has 1 elements
|
||||
varargExample(1, 2, 3) // => Argument has 3 elements
|
||||
|
||||
/*
|
||||
When a function consists of a single expression then the curly brackets can
|
||||
be omitted. The body is specified after a = symbol.
|
||||
*/
|
||||
fun odd(x: Int): Boolean = x % 2 == 1
|
||||
println(odd(6)) // => false
|
||||
println(odd(7)) // => true
|
||||
|
||||
// If the return type can be inferred then we don't need to specify it.
|
||||
fun even(x: Int) = x % 2 == 0
|
||||
println(even(6)) // => true
|
||||
println(even(7)) // => false
|
||||
|
||||
// Functions can take functions as arguments and return functions.
|
||||
fun not(f: (Int) -> Boolean) : (Int) -> Boolean {
|
||||
return {n -> !f.invoke(n)}
|
||||
}
|
||||
// Named functions can be specified as arguments using the :: operator.
|
||||
val notOdd = not(::odd)
|
||||
val notEven = not(::even)
|
||||
// Anonymous functions can be specified as arguments.
|
||||
val notZero = not {n -> n == 0}
|
||||
/*
|
||||
If an anonymous function has only one parameter
|
||||
then its declaration can be omitted (along with the ->).
|
||||
The name of the single parameter will be "it".
|
||||
*/
|
||||
val notPositive = not {it > 0}
|
||||
for (i in (0..4)) {
|
||||
println("${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 can be accessed by 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
|
||||
|
||||
// Kotlin provides higher-order functions for working with collections.
|
||||
val x = (1..9).map {it * 3}
|
||||
.filter {it < 20}
|
||||
.groupBy {it % 2 == 0}
|
||||
.mapKeys {if (it.key) "even" else "odd"}
|
||||
println(x) // => {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"
|
||||
}
|
||||
}
|
||||
|
||||
```
|
||||
|
||||
### Further Reading
|
||||
|
||||
A web-based mini-IDE for Kotlin:
|
||||
[http://try.kotlinlang.org/)
|
||||
Loading…
Reference in New Issue
Block a user