Y的Kotlin开发手册——Kotlin语法

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Kotlin语法

1. 程序入口

跟Java里面main函数一样，Koltin的入口也是main函数

fun main() {
    println("Hello world!")
}
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另外一个接收参数的main函数

fun main() {
    prinln("Hi kotlin")
}
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2. 函数声明

跟Java 函数参数里面 先声明类型，再声明变量不一样，Kotlin的函数里面参数是先声明变量，再指定类型 例如，一个求和函数声明如下：

fun sum(a:Int, b:Int):Int{
    return a+b
}
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另外，返回值类型也是跟着函数名字后面，用：分隔。还有一个细节不知道注意没有，Kotlin语法里结尾是不需要分号的。爱惜键盘按键。

甚至，可以直接写成

fun sum(a:Int, b:Int) = a+b
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3. 没有返回值的函数声明 Unit

fun printSum(a:Int, b:Int):Unit {
    println("sum of $a and $b is ${a+b}")
}
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4.变量声明

常量声明，一旦用了val，就无法修改改变量，类似Java中final

val a : Int = 1;
val b = 2
val c : Int
    c = 3
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变量声明 var

var x = 4
x = 5
x += 1
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5.类和实例

类声明 class 例如:

    class Rectangle(var height:Double, var width:Double) {
        var perimeter = (height + width) * 2
    }
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6.继承

一个类可以被继承需要用关键字open 修饰

open class Shape
class Rectangle(var height:Double, var width:Double):Shape(){
    var perimeter = (height + width) * 2
}
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7.字符串

fun main() {
    var a = 1
    val S1 = "a is $a"

    a = 2
    val s2 = "${s1.replace("is", "was")}, but now is $a"
    println(s2)
}
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结果：a was 1, but now is 2

8.条件语句

fun maxOf(a:Int, b:Int) {
    if(a > b) {
        return a
    } else {
        return b
    }
}
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或者是

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

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

val items = listOf("apple", "banana", "kiwifruit")
for (item in items) {
    println(item)
}

val items = listOf("apple", "banana", "kiwifruit")
for (index in items.indices) {
    println("item at $index is ${items[index]}")
}
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10，while 循环

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

类似于Java里面switch

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

val x = 10
val y = 9
if (x in 1..y+1) {
    println("fits in range")
}
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13.集合

fun main() {
    val items = listOf("apple", "banana", "kiwifruit")
    for (item in items) {
        println(item)
    }
}
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14.Nullable 和空值检测

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

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("'$arg1' or '$arg2' is not a number")
    }    
}
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15.类型检测 is

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
}
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或者

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

    // `obj` is automatically cast to `String` in this branch
    return obj.length
}
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又或者

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
}
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More

Kotlin官网

Kotlin GitHub地址

The Kotlin Blog