本文根据 Devhints / Kotlin 翻译。代码示例及版本范围沿用原文。原文标注更新日期：2018-12-06。
Kotlin 是一种用于现代多平台应用程序的静态类型编程语言。
变量
可变性
var mutableString: String = "Adam"
val immutableString: String = "Adam"
val inferredString = "Adam"字符串
val name = "Adam"
val greeting = "Hello, " + name
val greetingTemplate = "Hello, $name"
val interpolated = "Hello, ${name.toUpperCase()}"数字
val intNum = 10
val doubleNum = 10.0
val longNum = 10L
val floatNum = 10.0F布尔值
val trueBoolean = true
val falseBoolean = false
val andCondition = trueBoolean && falseBoolean
val orCondition = trueBoolean || falseBoolean静态字段
class Person {
companion object {
val NAME_KEY = "name_key"
}
}
val key = Person.NAME_KEY空安全
可空属性
val cannotBeNull: String = null // Invalid
val canBeNull: String? = null // Valid
val cannotBeNull: Int = null // Invalid
val canBeNull: Int? = null // Valid空值检查
val name: String? = "Adam"
if (name != null && name.length > 0) {
print("String length is ${name.length}")
} else {
print("String is empty.")
}安全调用操作符
val nullableStringLength: Int? = nullableString?.length
val nullableDepartmentHead: String? = person?.department?.head?.nameElvis 操作符
val nonNullStringLength: Int = nullableString?.length ?: 0
val nonNullDepartmentHead: String = person?.department?.head?.name ?: ""
val nonNullDepartmentHead: String = person?.department?.head?.name.orEmpty()安全类型转换
// Will not throw ClassCastException
val nullableCar: Car? = (input as? Car)集合（Collections）
创建
val numArray = arrayOf(1, 2, 3)
val numList = listOf(1, 2, 3)
val mutableNumList = mutableListOf(1, 2, 3)访问
val firstItem = numList[0]
val firstItem = numList.first()
val firstItem = numList.firstOrNull()映射 (Maps)
val faceCards = mutableMapOf("Jack" to 11, "Queen" to 12, "King" to 13)
val jackValue = faceCards["Jack"] // 11
faceCards["Ace"] = 1可变性
val immutableList = listOf(1, 2, 3)
val mutableList = immutableList.toMutableList()
val immutableMap = mapOf("Jack" to 11, "Queen" to 12, "King" to 13)
val mutableMap = immutableMap.toMutableMap()迭代
for (item in myList) {
print(item)
}
myList.forEach {
print(it)
}
myList.forEachIndexed { index, item ->
print("Item at $index is: $item")
}过滤与搜索
val evenNumbers = numList.filter { it % 2 == 0 }
val containsEven = numList.any { it % 2 == 0 }
val containsNoEvens = numList.none { it % 2 == 0 }
val containsNoEvens = numList.all { it % 2 == 1 }
val firstEvenNumber: Int = numList.first { it % 2 == 0 }
val firstEvenOrNull: Int? = numList.firstOrNull { it % 2 == 0 }
val fullMenu = objList.map { "${it.name} - $${it.detail}" }注意： it 是 单参数的隐式名称.
函数
参数与返回类型
fun printName() {
print("Adam")
}
fun printName(person: Person) {
print(person.name)
}
fun getGreeting(person: Person): String {
return "Hello, ${person.name}"
}
fun getGreeting(person: Person): String = "Hello, ${person.name}"
fun getGreeting(person: Person) = "Hello, ${person.name}"高阶函数
fun callbackIfTrue(condition: Boolean, callback: () -> Unit) {
if (condition) {
callback()
}
}
callbackIfTrue(someBoolean) {
print("Condition was true")
}扩展函数
fun Int.timesTwo(): Int {
return this * 2
}
val four = 2.timesTwo()默认参数
fun getGreeting(person: Person, intro: String = "Hello,"): String {
return "$intro ${person.name}"
}
// Returns "Hello, Adam"
val hello = getGreeting(Person("Adam"))
// Returns "Welcome, Adam"
val welcome = getGreeting(Person("Adam"), "Welcome,")命名参数
class Person(val name: String = "", age: Int = 0)
// All valid
val person = Person()
val person = Person("Adam", 100)
val person = Person(name = "Adam", age = 100)
val person = Person(age = 100)
val person = Person(age = 100, name = "Adam")静态函数
class Fragment(val args: Bundle) {
companion object {
fun newInstance(args: Bundle): Fragment {
return Fragment(args)
}
}
}
val fragment = Fragment.newInstance(args)类
主构造函数
class Person(val name: String, val age: Int)
val adam = Person("Adam", 100)次构造函数
class Person(val name: String) {
private var age: Int? = null
constructor(name: String, age: Int) : this(name) {
this.age = age
}
}
// Above can be replaced with default params
class Person(val name: String, val age: Int? = null)继承与实现
open class Vehicle
class Car : Vehicle()
interface Runner {
fun run()
}
class Machine : Runner {
override fun run() {
// ...
}
}控制流
If 语句
if (someBoolean) {
doThing()
} else {
doOtherThing()
}For 循环
for (i in 0..10) { } // 1 - 10
for (i in 0 until 10) // 1 - 9
(0..10).forEach { }
for (i in 0 until 10 step 2) // 0, 2, 4, 6, 8When 语句
when (direction) {
NORTH -> {
print("North")
}
SOUTH -> print("South")
EAST, WEST -> print("East or West")
"N/A" -> print("Unavailable")
else -> print("Invalid Direction")
}While 循环
while (x > 0) {
x--
}
do {
x--
} while (x > 0)解构声明
对象与列表
val person = Person("Adam", 100)
val (name, age) = person
val pair = Pair(1, 2)
val (first, second) = pair
val coordinates = arrayOf(1, 2, 3)
val (x, y, z) = coordinatesComponentN 函数
class Person(val name: String, val age: Int) {
operator fun component1(): String {
return name
}
operator fun component2(): Int {
return age
}
}