Kotlin基本语法（译）

# 基本语法

##定义包

软件包规范应位于源文件的顶部：

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。

## 定义功能

全Kotlin参考
编辑页面
基本语法

定义包

软件包规范应位于源文件的顶部：

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。

参见包。

定义功能

具有返回类型的两个Int参数的Int函数：

//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))
}

函数与表达体和推断返回类型：

//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)}")
}

函数返回无意义值：

//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 返回类型可以省略：

//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)
}

## 定义局部变量

分配一次（只读）局部变量：

fun main(args: Array<String>) {
//sampleStart
val a: Int = 1  // immediate assignment
val b = 2   // `Int` type is inferred
val c: Int  // Type required when no initializer is provided
c = 3       // deferred assignment
//sampleEnd
println("a = $a, b = $b, c = $c")
}

可变变量：

fun main(args: Array<String>) {
//sampleStart
var x = 5 // `Int` type is inferred
x += 1
//sampleEnd
println("x = $x")
}

## 注释

就像Java和JavaScript一样，Kotlin支持行尾和//注释。

// This is an end-of-line comment

/* This is a block comment
on multiple lines. */

与Java不同，Kotlin中的块注释可以嵌套。

## 使用字符串模板

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)
}

## 使用条件表达式

//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)}")
}

## 使用可空值并检查null

当空值可能时，引用必须被明确地标记为可空。

如果不持有整数则返回nullstr：

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")
}

## 使用类型检查和自动转换

这是操作者检查是否一个表达式是一个类型的一个实例。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()))
}

甚至

//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)
}

## 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
}

## 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
}

##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"))
}

##使用范围

检查一个数字是使用范围内操作

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

检查一个数字是否超出范围：

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
}

迭代一个循环：

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
}

## 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
}

检查如果集合包含对象使用in{关键词} 操作:

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
}

原文Kotlin基本语法