基本语法

这是一组基本语法元素及示例。在每段的末尾都有一个指向相关主题详述的链接。

也可以通过 JetBrains 学院的免费 Kotlin 核心课程学习所有 Kotlin 要领。

包的定义与导入

包的声明应处于源文件顶部:

package my.demo

import kotlin.text.*

// ……

目录与包的结构无需匹配:源代码可以在文件系统的任意位置。

参见

程序入口点

Kotlin 应用程序的入口点是 main 函数:

fun main() {
    println("Hello world!")
}

main 的另一种形式接受可变数量的 String 参数:

fun main(args: Array<String>) {
    println(args.contentToString())
}

输出打到标准输出

print 将其参数打到标准输出:

fun main() {
//sampleStart
    print("Hello ")
    print("world!")
//sampleEnd
}

println 输出其参数并添加换行符,以便接下来输出的内容出现在下一行:

fun main() {
//sampleStart
    println("Hello world!")
    println(42)
//sampleEnd
}

函数

带有两个 Int 参数、返回 Int 的函数:

//sampleStart
fun sum(a: Int, b: Int): Int {
    return a + b
}
//sampleEnd

fun main() {
    print("sum of 3 and 5 is ")
    println(sum(3, 5))
}

函数体可以是表达式。其返回类型可以推断出来:

//sampleStart
fun sum(a: Int, b: Int) = a + b
//sampleEnd

fun main() {
    println("sum of 19 and 23 is ${sum(19, 23)}")
}

返回无意义的值的函数:

//sampleStart
fun printSum(a: Int, b: Int): Unit {
    println("sum of $a and $b is ${a + b}")
}
//sampleEnd

fun main() {
    printSum(-1, 8)
}

Unit 返回类型可以省略:

//sampleStart
fun printSum(a: Int, b: Int) {
    println("sum of $a and $b is ${a + b}")
}
//sampleEnd

fun main() {
    printSum(-1, 8)
}

参见函数

变量

In Kotlin, you declare a variable starting with a keyword, val or var, followed by the name of the variable.

Use the val keyword to declare variables that are assigned a value only once. These are immutable, read-only local variables that can’t be reassigned a different value after initialization:

fun main() {
//sampleStart
    // Declares the variable x and initializes it with the value of 5
    val x: Int = 5
    // 5
//sampleEnd
    println(x)
}

Use the var keyword to declare variables that can be reassigned. These are mutable variables, and you can change their values after initialization:

fun main() {
//sampleStart
    // Declares the variable x and initializes it with the value of 5
    var x: Int = 5
    // Reassigns a new value of 6 to the variable x
    x += 1
    // 6
//sampleEnd
    println(x)
}

Kotlin supports type inference and automatically identifies the data type of a declared variable. When declaring a variable, you can omit the type after the variable name:

fun main() {
//sampleStart
    // Declares the variable x with the value of 5;`Int` type is inferred
    val x = 5
    // 5
//sampleEnd
    println(x)
}

You can use variables only after initializing them. You can either initialize a variable at the moment of declaration or declare a variable first and initialize it later. In the second case, you must specify the data type:

fun main() {
//sampleStart
    // Initializes the variable x at the moment of declaration; type is not required
    val x = 5
    // Declares the variable c without initialization; type is required
    val c: Int
    // Initializes the variable c after declaration 
    c = 3
    // 5 
    // 3
//sampleEnd
    println(x)
    println(c)
}

可以在顶层声明变量:

//sampleStart
val PI = 3.14
var x = 0

fun incrementX() {
    x += 1
}
// x = 0; PI = 3.14
// incrementX()
// x = 1; PI = 3.14
//sampleEnd

fun main() {
    println("x = $x; PI = $PI")
    incrementX()
    println("incrementX()")
    println("x = $x; PI = $PI")
}

For information about declaring properties, see Properties.

创建类与实例

使用 class 关键字定义类:

class Shape

类的属性可以在其声明或主体中列出:

class Rectangle(val height: Double, val length: Double) {
    val perimeter = (height + length) * 2 
}

具有类声明中所列参数的默认构造函数会自动可用:

class Rectangle(val height: Double, val length: Double) {
    val perimeter = (height + length) * 2 
}
fun main() {
    val rectangle = Rectangle(5.0, 2.0)
    println("The perimeter is ${rectangle.perimeter}")
}

类之间继承由冒号(:)声明。默认情况下类都是 final 的;如需让一个类可继承, 请将其标记为 open

open class Shape

class Rectangle(val height: Double, val length: Double): Shape() {
    val perimeter = (height + length) * 2 
}

For more information about constructors and inheritance, see Classes and Objects and instances.

注释

与大多数现代语言一样,Kotlin 支持单行(或行末)与多行()注释:

// 这是一个行注释

/* 这是一个多行的
   块注释。 */

Kotlin 中的块注释可以嵌套:

/* 注释从这里开始
/* 包含嵌套的注释 */     
并且在这里结束。 */

参见编写 Kotlin 代码文档 查看关于文档注释语法的信息。

字符串模板

fun main() {
//sampleStart
    var a = 1
    // 模板中的简单名称:
    val s1 = "a is $a" 

    a = 2
    // 模板中的任意表达式:
    val s2 = "${s1.replace("is", "was")}, but now is $a"
//sampleEnd
    println(s2)
}

参见字符串模板

条件表达式

//sampleStart
fun maxOf(a: Int, b: Int): Int {
    if (a > b) {
        return a
    } else {
        return b
    }
}
//sampleEnd

fun main() {
    println("max of 0 and 42 is ${maxOf(0, 42)}")
}

在 Kotlin 中,if 也可以用作表达式:

//sampleStart
fun maxOf(a: Int, b: Int) = if (a > b) a else b
//sampleEnd

fun main() {
    println("max of 0 and 42 is ${maxOf(0, 42)}")
}

参见if 表达式

for 循环

fun main() {
//sampleStart
    val items = listOf("apple", "banana", "kiwifruit")
    for (item in items) {
        println(item)
    }
//sampleEnd
}

或者:

fun main() {
//sampleStart
    val items = listOf("apple", "banana", "kiwifruit")
    for (index in items.indices) {
        println("item at $index is ${items[index]}")
    }
//sampleEnd
}

参见 for 循环

while 循环

fun main() {
//sampleStart
    val items = listOf("apple", "banana", "kiwifruit")
    var index = 0
    while (index < items.size) {
        println("item at $index is ${items[index]}")
        index++
    }
//sampleEnd
}

参见 while 循环

when 表达式

//sampleStart
fun describe(obj: Any): String =
    when (obj) {
        1          -> "One"
        "Hello"    -> "Greeting"
        is Long    -> "Long"
        !is String -> "Not a string"
        else       -> "Unknown"
    }
//sampleEnd

fun main() {
    println(describe(1))
    println(describe("Hello"))
    println(describe(1000L))
    println(describe(2))
    println(describe("other"))
}

参见 when 表达式

使用区间(range)

使用 in 操作符来检测某个数字是否在指定区间内:

fun main() {
//sampleStart
    val x = 10
    val y = 9
    if (x in 1..y+1) {
        println("fits in range")
    }
//sampleEnd
}

检测某个数字是否在指定区间外:

fun main() {
//sampleStart
    val list = listOf("a", "b", "c")

    if (-1 !in 0..list.lastIndex) {
        println("-1 is out of range")
    }
    if (list.size !in list.indices) {
        println("list size is out of valid list indices range, too")
    }
//sampleEnd
}

区间迭代:

fun main() {
//sampleStart
    for (x in 1..5) {
        print(x)
    }
//sampleEnd
}

或数列迭代:

fun main() {
//sampleStart
    for (x in 1..10 step 2) {
        print(x)
    }
    println()
    for (x in 9 downTo 0 step 3) {
        print(x)
    }
//sampleEnd
}

参见区间与数列

集合

对集合进行迭代:

fun main() {
    val items = listOf("apple", "banana", "kiwifruit")
//sampleStart
    for (item in items) {
        println(item)
    }
//sampleEnd
}

使用 in 操作符来判断集合内是否包含某实例:

fun main() {
    val items = setOf("apple", "banana", "kiwifruit")
//sampleStart
    when {
        "orange" in items -> println("juicy")
        "apple" in items -> println("apple is fine too")
    }
//sampleEnd
}

使用 lambda 表达式来过滤(filter)与映射(map)集合:

fun main() {
//sampleStart
    val fruits = listOf("banana", "avocado", "apple", "kiwifruit")
    fruits
      .filter { it.startsWith("a") }
      .sortedBy { it }
      .map { it.uppercase() }
      .forEach { println(it) }
//sampleEnd
}

参见集合概述

空值与空检测

当可能用 null 值时,必须将引用显式标记为可空。可空类型名称以问号(?)结尾。

如果 str 的内容不是数字返回 null

fun parseInt(str: String): Int? {
    // ……
}

使用返回可空值的函数:

fun parseInt(str: String): Int? {
    return str.toIntOrNull()
}

//sampleStart
fun printProduct(arg1: String, arg2: String) {
    val x = parseInt(arg1)
    val y = parseInt(arg2)

    // 直接使用 `x * y` 会导致编译错误,因为它们可能为 null
    if (x != null && y != null) {
        // 在空检测后,x 与 y 会自动转换为非空值(non-nullable)
        println(x * y)
    }
    else {
        println("'$arg1' or '$arg2' is not a number")
    }
}
//sampleEnd

fun main() {
    printProduct("6", "7")
    printProduct("a", "7")
    printProduct("a", "b")
}

或者:

fun parseInt(str: String): Int? {
    return str.toIntOrNull()
}

fun printProduct(arg1: String, arg2: String) {
    val x = parseInt(arg1)
    val y = parseInt(arg2)

//sampleStart
    // ……
    if (x == null) {
        println("Wrong number format in arg1: '$arg1'")
        return
    }
    if (y == null) {
        println("Wrong number format in arg2: '$arg2'")
        return
    }

    // 在空检测后,x 与 y 会自动转换为非空值
    println(x * y)
//sampleEnd
}

fun main() {
    printProduct("6", "7")
    printProduct("a", "7")
    printProduct("99", "b")
}

参见空安全

类型检测与自动类型转换

is 操作符检测一个表达式是否某类型的一个实例。 如果一个不可变的局部变量或属性已经判断出为某类型,那么检测后的分支中可以直接当作该类型使用,无需显式转换:

//sampleStart
fun getStringLength(obj: Any): Int? {
    if (obj is String) {
        // `obj` 在该条件分支内自动转换成 `String`
        return obj.length
    }

    // 在离开类型检测分支后,`obj` 仍然是 `Any` 类型
    return null
}
//sampleEnd

fun main() {
    fun printLength(obj: Any) {
        println("Getting the length of '$obj'. Result: ${getStringLength(obj) ?: "Error: The object is not a string"} ")
    }
    printLength("Incomprehensibilities")
    printLength(1000)
    printLength(listOf(Any()))
}

或者:

//sampleStart
fun getStringLength(obj: Any): Int? {
    if (obj !is String) return null

    // `obj` 在这一分支自动转换为 `String`
    return obj.length
}
//sampleEnd

fun main() {
    fun printLength(obj: Any) {
        println("Getting the length of '$obj'. Result: ${getStringLength(obj) ?: "Error: The object is not a string"} ")
    }
    printLength("Incomprehensibilities")
    printLength(1000)
    printLength(listOf(Any()))
}

甚至:

//sampleStart
fun getStringLength(obj: Any): Int? {
    // `obj` 在 `&&` 右边自动转换成 `String` 类型
    if (obj is String && obj.length > 0) {
        return obj.length
    }

    return null
}
//sampleEnd

fun main() {
    fun printLength(obj: Any) {
        println("Getting the length of '$obj'. Result: ${getStringLength(obj) ?: "Error: The object is not a string"} ")
    }
    printLength("Incomprehensibilities")
    printLength("")
    printLength(1000)
}

参见以及类型转换