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基本语法

定义包

包应该在源文件的头部声明：

package my.demo

import java.util.*

// ...

不需要匹配目录和包：源文件可以任意放在文件系统中。

查看 Packages.

定义函数

有两个 Int 参数 和 Int 返回值的函数定义：

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

fun main(args: Array<String>) {
    print("sum of 3 and 5 is ")
    println(sum(3, 5))
}

表达式作为函数体，返回类型为推断所得的函数：

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

fun main(args: Array<String>) {
    println("sum of 19 and 23 is ${sum(19, 23)}")
}

返回无意义值的函数:

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

fun main(args: Array<String>) {
    printSum(-1, 8)
}

Unit 返回类型可以被忽略不写 :

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

fun main(args: Array<String>) {
    printSum(-1, 8)
}

查看 Functions.

定义局部变量

一次分配（只读）的局部变量:

fun main(args: Array<String>) {
//sampleStart
    val a: Int = 1  // 立即赋值
    val b = 2   // `Int` 类型是推断得出ed
    val c: Int  // 没有初始化时需要指明类型
    c = 3       // 之后赋值
//sampleEnd
    println("a = $a, b = $b, c = $c")
}

可变变量:

fun main(args: Array<String>) {
//sampleStart
    var x = 5 // `Int` 类型是推断的
    x += 1
//sampleEnd
    println("x = $x")
}

查看 Properties And Fields.

注释

和 Java 与 JavaScript 类似，Kotlin 支持 行注释 和 块注释。

// 这是行注释

/* 块
   注释
 */

不同于Java，Kotlin 中 块注释可以嵌套。

查看 Documenting Kotlin Code 有关文档注释语法的信息。

使用 String 模板

fun main(args: Array<String>) {
//sampleStart
    var a = 1
    // simple name in template:
    val s1 = "a is $a" 
    
    a = 2
    // arbitrary expression in template:
    val s2 = "${s1.replace("is", "was")}, but now is $a"
//sampleEnd
    println(s2)
}

查看 String templates.

使用条件语句

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

fun main(args: Array<String>) {
    println("max of 0 and 42 is ${maxOf(0, 42)}")
}

使用 if 作为表达式:

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

fun main(args: Array<String>) {
    println("max of 0 and 42 is ${maxOf(0, 42)}")
}

查看 if-expressions.

使用 nullable 值 并且检查 null

当空值可能时，引用必须被明确地标记为可空。
如果str不包含整数，则返回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)

    // Using `x * y` yields error because they may hold nulls.
    if (x != null && y != null) {
        // x and y are automatically cast to non-nullable after null check
        println(x * y)
    }
    else {
        println("either '$arg1' or '$arg2' is not a number")
    }    
}
//sampleEnd

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

    // x and y are automatically cast to non-nullable after null check
    println(x * y)
//sampleEnd
}

fun main(args: Array<String>) {
    printProduct("6", "7")
    printProduct("a", "7")
    printProduct("99", "b")
}

查看 Null-safety.

使用类型检查和自动转换

is 运算符检查表达式是否是类型的实例。如果为特定类型检查不可变的局部变量或属性，则不需要显式转换：

//sampleStart
fun getStringLength(obj: Any): Int? {
    if (obj is String) {
        // `obj` is automatically cast to `String` in this branch
        return obj.length
    }

    // `obj` is still of type `Any` outside of the type-checked branch
    return null
}
//sampleEnd


fun main(args: Array<String>) {
    fun printLength(obj: Any) {
        println("'$obj' string length is ${getStringLength(obj) ?: "... err, 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` is automatically cast to `String` in this branch
    return obj.length
}
//sampleEnd

fun main(args: Array<String>) {
    fun printLength(obj: Any) {
        println("'$obj' string length is ${getStringLength(obj) ?: "... err, not a string"} ")
    }
    printLength("Incomprehensibilities")
    printLength(1000)
    printLength(listOf(Any()))
}

甚或

//sampleStart
fun getStringLength(obj: Any): Int? {
    // `obj` is automatically cast to `String` on the right-hand side of `&&`
    if (obj is String && obj.length > 0) {
        return obj.length
    }

    return null
}
//sampleEnd


fun main(args: Array<String>) {
    fun printLength(obj: Any) {
        println("'$obj' string length is ${getStringLength(obj) ?: "... err, is empty or not a string at all"} ")
    }
    printLength("Incomprehensibilities")
    printLength("")
    printLength(1000)
}

查看 Classes 和 Type casts.

使用 for 循环

fun main(args: Array<String>) {
//sampleStart
    val items = listOf("apple", "banana", "kiwi")
    for (item in items) {
        println(item)
    }
//sampleEnd
}

或者

fun main(args: Array<String>) {
//sampleStart
    val items = listOf("apple", "banana", "kiwi")
    for (index in items.indices) {
        println("item at $index is ${items[index]}")
    }
//sampleEnd
}

查看 for loop.

使用 while 循环

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

查看 while loop.

使用 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(args: Array<String>) {
    println(describe(1))
    println(describe("Hello"))
    println(describe(1000L))
    println(describe(2))
    println(describe("other"))
}

查看 when expression.

使用范围

使用操作符检查数字是否在范围内：

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

检查数字是否超出范围：

fun main(args: Array<String>) {
//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
}

迭代范围：

fun main(args: Array<String>) {
//sampleStart
    for (x in 1..5) {
        print(x)
    }
//sampleEnd
}

或过程：

fun main(args: Array<String>) {
//sampleStart
    for (x in 1..10 step 2) {
        print(x)
    }
    for (x in 9 downTo 0 step 3) {
        print(x)
    }
//sampleEnd
}

查看 Ranges.

使用集合

迭代集合:

fun main(args: Array<String>) {
    val items = listOf("apple", "banana", "kiwi")
//sampleStart
    for (item in items) {
        println(item)
    }
//sampleEnd
}

使用运算符 is 检查集合是否包含对象：

fun main(args: Array<String>) {
    val items = setOf("apple", "banana", "kiwi")
//sampleStart
    when {
        "orange" in items -> println("juicy")
        "apple" in items -> println("apple is fine too")
    }
//sampleEnd
}

使用lambda表达式过滤和映射集合：

fun main(args: Array<String>) {
    val fruits = listOf("banana", "avocado", "apple", "kiwi")
//sampleStart
    fruits
        .filter { it.startsWith("a") }
        .sortedBy { it }
        .map { it.toUpperCase() }
        .forEach { println(it) }
//sampleEnd
}

查看 Higher-order functions and Lambdas.