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开源软件名称（OpenSource Name）：HugoMatilla/KotlinCheatSheet开源软件地址(OpenSource Url)：https://github.com/HugoMatilla/KotlinCheatSheet开源编程语言(OpenSource Language)：开源软件介绍(OpenSource Introduction)：Hello KotlinHello WorldWe declare a package-level function main which returns Unit and takes an Array of strings as a parameter. Note that semicolons are optional. fun main(args: Array<String>) {
println("Hello, world!")
}Reading a name from the command lineLine 13 demonstrates string templates and array access. See this pages for details: http://kotlinlang.org/docs/reference/basic-types.html#strings http://kotlinlang.org/docs/reference/basic-types.html#arrays fun main(args: Array<String>) {
if (args.size == 0) {
println("Please provide a name as a command-line argument")
return
}
println("Hello, ${args[0]}!")
}Reading many names from the command lineLine 2 demonstrates the for-loop, that would have been called "enhanced" if there were any other for-loop in Kotlin. See http://kotlinlang.org/docs/reference/basic-syntax.html#using-a-for-loop fun main(args: Array<String>) {
for (name in args)
println("Hello, $name!")
for (i in args.indices)
print(args[i])
}WhenIn this example, fun main(args: Array<String>) {
val language = if (args.size == 0) "EN" else args[0]
println(when (language) {
"EN" -> "Hello!"
"FR" -> "Salut!"
"IT" -> "Ciao!"
else -> "Sorry, I can't greet you in $language yet"
})
}OOP Simple ClassHere we have a class with a primary constructor and a member function.
Note that there's no class Greeter(val name: String) {
fun greet() {
println("Hello, ${name}");
}
}
fun main(args: Array<String>) {
Greeter(args[0]).greet()
}Basicsif
fun main(args: Array<String>) {
println(max(Integer.parseInt(args[0]), Integer.parseInt(args[1])))
}
fun max(a: Int, b: Int) = if (a > b) a else b
NullablesA reference must be explicitly marked as nullable to be able hold a null. See http://kotlinlang.org/docs/reference/null-safety.html#null-safety package multiplier
// Return null if str does not hold a number
fun parseInt(str: String): Int? {
try {
return Integer.parseInt(str)
} catch (e: NumberFormatException) {
println("One of the arguments isn't Int")
}
return null
}
fun main(args: Array<String>) {
if (args.size < 2) {
println("No number supplied");
} else {
val x = parseInt(args[0])
val y = parseInt(args[1])
// We cannot say 'xy' now because they may hold nulls
if (x != null && y != null) {
print(xy) // Now we can
} else {
println("One of the arguments is null")
}
}
}isThe fun main(args: Array<String>) {
println(getStringLength("aaa"))
println(getStringLength(1))
}
fun getStringLength(obj: Any): Int? {
if (obj is String)
return obj.length // no cast to String is needed
return null
}while and do..whileSee http://kotlinlang.org/docs/reference/control-flow.html#while-loops fun main(args: Array<String>) {
var i = 0
while (i < args.size)
println(args[i++])
}RangesCheck if a number lies within a range. Check if a number is out of range. Check if a collection contains an object. See http://kotlinlang.org/docs/reference/ranges.html#ranges fun main(args: Array<String>) {
val x = Integer.parseInt(args[0])
//Check if a number lies within a range:
val y = 10
if (x in 1..y - 1)
println("OK")
//Iterate over a range:
for (a in 1..5)
print("${a} ")
//Check if a number is out of range:
println()
val array = arrayListOf<String>()
array.add("aaa")
array.add("bbb")
array.add("ccc")
if (x !in 0..array.size - 1)
println("Out: array has only ${array.size} elements. x = ${x}")
//Check if a collection contains an object:
if ("aaa" in array) // collection.contains(obj) is called
println("Yes: array contains aaa")
if ("ddd" in array) // collection.contains(obj) is called
println("Yes: array contains ddd")
else
println("No: array doesn't contains ddd")
}Control flow whenSee http://kotlinlang.org/docs/reference/control-flow.html#when-expression fun main(args: Array<String>) {
cases("Hello")
cases(1)
cases(System.currentTimeMillis())
cases(MyClass())
cases("hello")
}
fun cases(obj: Any) {
when (obj) {
1 -> println("One")
"Hello" -> println("Greeting")
is Long -> println("Long")
!is String -> println("Not a string")
else -> println("Unknown")
}
}
class MyClass() {
}Multi-declarations and Data classesMulti-declarationsThis example introduces a concept that we call mutli-declarations. It creates multiple variable at once. Anything can be on the right-hand side of a mutli-declaration, as long as the required number of component functions can be called on it. See http://kotlinlang.org/docs/reference/multi-declarations.html#multi-declarations fun main(args: Array<String>) {
val pair = Pair(1, "one")
val (num, name) = pair
println("num = $num, name = $name")
}
class Pair<K, V>(val first: K, val second: V) {
operator fun component1(): K {
return first
}
operator fun component2(): V {
return second
}
}Data classesData class gets component functions, one for each property declared in the primary constructor, generated automatically, same for all the other goodies common for data toString(), equals(), hashCode() and copy(). See http://kotlinlang.org/docs/reference/data-classes.html#data-classes data class User(val name: String, val id: Int)
fun getUser(): User {
return User("Alex", 1)
}
fun main(args: Array<String>) {
val user = getUser()
println("name = ${user.name}, id = ${user.id}")
// or
val (name, id) = getUser()
println("name = $name, id = $id")
// or
println("name = ${getUser().component1()}, id = ${getUser().component2()}")
}Data MapsKotlin Standart Library provide component functions for Map.Entry fun main(args: Array<String>) {
val map = hashMapOf<String, Int>()
map.put("one", 1)
map.put("two", 2)
for ((key, value) in map) {
println("key = $key, value = $value")
}
}Autogenerated FunctionsData class gets next functions, generated automatically: component functions, toString(), equals(), hashCode() and copy(). See http://kotlinlang.org/docs/reference/data-classes.html#data-classes data class User(val name: String, val id: Int)
fun main(args: Array<String>) {
val user = User("Alex", 1)
println(user) // toString()
val secondUser = User("Alex", 1)
val thirdUser = User("Max", 2)
println("user == secondUser: ${user == secondUser}")
println("user == thirdUser: ${user == thirdUser}")
// copy() function
println(user.copy())
println(user.copy("Max"))
println(user.copy(id = 2))
println(user.copy("Max", 2))
}Delegated propertiesCustom DelegateThere's some new syntax: you can say import kotlin.reflect.KProperty
class Example {
var p: String by Delegate()
override fun toString() = "Example Class"
}
class Delegate() {
): String {
return "$thisRef, thank you for delegating '${prop.name}' to me!"
}
, value: String) {
println("$value has been assigned to ${prop.name} in $thisRef")
}
}
fun main(args: Array<String>) {
val e = Example()
println(e.p)
e.p = "NEW"
}Lazy propertyDelegates.lazy() is a function that returns a delegate that implements a lazy property: the first call to get() executes the lambda expression passed to lazy() as an argument and remembers the result, subsequent calls to get() simply return the remembered result. If you want thread safety, use blockingLazy() instead: it guarantees that the values will be computed only in one thread, and that all threads will see the same value. class LazySample {
val lazy: String by lazy {
println("computed!")
"my lazy"
}
}
fun main(args: Array<String>) {
val sample = LazySample()
println("lazy = ${sample.lazy}")
println("lazy = ${sample.lazy}")
}Observable PropertyThe observable() function takes two arguments: initial value and a handler for modifications.
The handler gets called every time we assign to import kotlin.properties.Delegates
class User {
var name: String by Delegates.observable("no name") {
d, old, new ->
println("$old - $new")
}
}
fun main(args: Array<String>) {
val user = User()
user.name = "Carl"
}NotNull propertyUsers frequently ask what to do when you have a non-null var, but you don't have an appropriate value to assign to it in constructor (i.e. it must be assigned later)? You can't have an uninitialized non-abstract property in Kotlin. You could initialize it with null, bit then you'd have to check every time you access it. Now you have a delegate to handle this. If you read from this property before writing to it, it throws an exception, after the first assignment it works as expected. import kotlin.properties.Delegates
class User {
var name: String by Delegates.notNull()
fun init(name: String) {
this.name = name
}
}
fun main(args: Array<String>) {
val user = User()
// user.name -> IllegalStateException
user.init("Carl")
println(user.name)
}Properties in mapProperties stored in a map. This comes up a lot in applications like parsing JSON or doing other "dynamic" stuff. Delegates take values from this map (by the string keys - names of properties). Of course, you can have var's as well (add import kotlin.properties.setValue), that will modify the map upon assignment (note that you'd need MutableMap instead of read-only Map). import kotlin.properties.getValue
class User(val map: Map<String, Any?>) {
val name: String by map
val age: Int by map
}
fun main(args: Array<String>) {
val user = User(mapOf(
"name" to "John Doe",
"age" to 25
))
println("name = ${user.name}, age = ${user.age}")
}Callable referencesReference to a function"Callable References" or "Feature Literals", i.e. an ability to pass
named functions or properties as values. Users often ask
"I have a foo() function, how do I pass it as an argument?".
The answer is: "you prefix it with a fun main(args: Array<String>) {
val numbers = listOf(1, 2, 3)
println(numbers.filter(::isOdd))
}
fun isOdd(x: Int) = x % 2 != 0Composition of functionsThe composition function return a composition of two functions passed to it: compose(f, g) = f(g). Now, you can apply it to callable references. fun main(args: Array<String>) {
val oddLength = compose(::isOdd, ::length)
val strings = listOf("a", "ab", "abc")
println(strings.filter(oddLength))
}
fun isOdd(x: Int) = x % 2 != 0
fun length(s: String) = s.length
fun <A, B, C> compose(f: (B) -> C, g: (A) -> B): (A) -> C {
return { x -> f(g(x)) }
} |
