Kotlin语法--基础入门

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什么是Kotlin?

一种Android官方推荐的开发语言，可与Java兼容，适用于多平台开发。

用Kotlin输出"Hello world!"

fun main() {println("Hello world!")
}
//这是单行注释
/* 这是
多行注释*/

Kotlin 应用程序的入口点是main函数。

函数也可以接受参数：

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

可以同时使用println(),readln(),print()来打印请求和显示用户输入的消息：

// Prints a message to request input
println("Enter any word: ")// Reads and stores the user input. For example: Happiness
val yourWord = readln()// Prints a message with the input
print("You entered the word: ")
print(yourWord)
// You entered the word: Happiness//println打印参数并添加换行符
//print将参数打印到标准输出
//readln()读取用户输入的整行作为字符串

变量

声明变量使用var或val（二者区别在于：val初始化后无法改变值，var可重新赋值）

var x: Int = 5
x += 1 //Kotlin中用换行来分隔语句
val x: Int = 5 //变量的类型声明写在冒号之后
val x = 5 //初始化时也可以不写类型
val y: Int
y =3 //先声明再初始化，就需要事先指定数据类型

可以在顶层声明变量

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

变量类型

• String: 文本
• Int: 整数
• Double：小数
• Float: 小数
• Boolean: 布尔

文档中更详细的分类
关于字符串模版

函数

如何用函数实现两个整数的相加？

fun sum(a: Int, b: Int): Int{return a+b
}
//使用fun关键字定义函数，fun后是函数名，小括号内是形式参数，参数用逗号分隔
//第三个Int是对函数返回类型的推断
//不返回任何有意义的值:使用Unit;也可以直接省略这个部分
fun printSum(a: Int, b: Int): Unit{println("sum of $a and $b is ${a+b}")
}

再看一个例子：

fun birthdayGreeting(name: String, age: Int): String {val nameGreeting = "Happy Birthday, $name!"val ageGreeting = "You are now $age years old!"return "$nameGreeting\n$ageGreeting"
}
fun main() {println(birthdayGreeting("Rover", 5)) //调用函数，为形式参数传值println(birthdayGreeting("Rex", 2))
}
//Happy Birthday, Rover!
//You are now 5 years old!
//Happy Birthday, Rex!
//You are now 2 years old!
//

也可传递具名实参，这样就不受传参顺序的影响:

println(birthdayGreeting(name = "Rex", age = 2))
println(birthdayGreeting(age = 2, name = "Rex"))

还可以指定默认实参:

fun birthdayGreeting(name: String = "Rover", age: Int): String {return "Happy Birthday, $name! You are now $age years old!"
}

类

定义一个矩形类

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}")
}
//类的属性可以在其声明或正文中列出
//类声明中列出的参数的默认构造函数将自动可用

类之间的继承由冒号声明，想使类可继承，要将其标记为open

open class Shape //open--可继承class Rectangle(val height: Double, val length: Double): Shape() {val perimeter = (height + length) * 2
}
\\shape类被继承

关于类的更多内容
关于对象和实例

条件表达式与循环

if else

fun maxOf(a: Int, b: Int): Int {if (a > b) {return a} else {return b}
}
//也可以这样写
fun maxOf(a: Int, b: Int) = if (a > b) a else b

for loop

val items = listOf("apple", "banana", "kiwifruit")
for (item in items) {println(item)
}
//或者这么写
val items = listOf("apple", "banana", "kiwifruit")
for (index in items.indices) {//indices返回下标索引println("item at $index is ${items[index]}")
}
//item at 0 is apple
//item at 1 is banana
//item at 2 is kiwifruit

when loop

val items = listOf("apple", "banana", "kiwifruit")
var index = 0
while (index < items.size) {println("item at $index is ${items[index]}")index++
}
//item at 0 is apple
//item at 1 is banana
//item at 2 is kiwifruit

when expression

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

Ranges

检查是否在范围内
可规定步长

val x = 10
val y = 9
if (x in 1..y+1) {//..是封闭范围println("fits in range")
}
//fits in rangeval 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")
}
//-1 is out of range
//list size is out of valid list indices range, toofor (x in 1..5) {print(x)
}
//12345for (x in 1..10 step 2) {print(x)
}
println()
for (x in 9 downTo 0 step 3) {print(x)
}
//13579
//9630

set

遍历集合，可使用in，也可以用in检查是否在范围内

for (item in items) {println(item)
}when {"orange" in items -> println("juicy")"apple" in items -> println("apple is fine too")
}val fruits = listOf("banana", "avocado", "apple", "kiwifruit")
fruits.filter { it.startsWith("a") }.sortedBy { it }.map { it.uppercase() }.forEach { println(it) }
//APPLE
AVOCADO

null

当值可能时，必须将引用显式标记为可null。

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 checkprintln(x * y)}else {println("'$arg1' or '$arg2' is not a number")}
}
//42
//'a' or '7' is not a number
//'a' or 'b' is not a number

可为 null 的类型和不可为 null 的类型