Kotlin 基本语法实例详解

Kotlin 基本语法实例详解

基本语法示例

实例代码：

package com.stone.basic.syntax

/**

* desc :

* author: stone

* email : aa86799@163.com

* time : 27/05/2017 11 01

*/

class BasicSyntax {

//Function having two Int parameters with Int return type:

public fun sum(a: Int, b: Int): Int {//访问修饰符 省略时，默认为 public

return a + b

}

//Function having three Int parameters with Int return type:

fun sum(a: Int, b: Int, c: Int) = a + b + c

//Function returning no meaningful value:

fun printSum(a: Int, b: Int): Unit {//Unit为无类型，类似java中的void，可以省略

println("sum of " + a + " and " + b + " is ${a + b}")

println("sum of $a and $b is ${a + b}") //在双引号中 直接用 $符操作变量 与上句等价

}

fun assignVarible() {

val a: Int = 1 // immediate assignment val = 本地只读变量 即不可变 immutable

val b = 2 // `Int` type is inferred 自动类型推断

val c: Int // Type required when no initializer is provided

c = 3 // deferred assignment

var x = 1 // Mutable variable:

x++

val s1 = "x is $x" // simple name in template:

val s2 = "${s1.replace("is", "was")}, but now is $x" // arbitrary expression in template:

println(s2)

}

fun maxOf(a: Int, b: Int): Int {

// return a > b ? a : b; //原java中的三目运算符 不可用

if (a > b) return a

else return b

}

//fun maxOf(a:Int, b: Int):Int

fun minOf(a: Int, b: Int): Int = if (a < b) a else b

//字符串转int

private fun parseInt(str: String): Int? {// ? 表示可以为空

return str.toIntOrNull(8)//参数为 进制数(radix), 不传默认为10 转换错误 返回null

}

fun getBaseSyntax(name: String?): BasicSyntax? { // ? 表示可以为空

// checkNotNull(name) // 参数不能为空的 检测函数

return BasicSyntax()

}

fun printProduct(arg1: String, arg2: String) {

val x1 = parseInt(arg1)

val x2 = parseInt(arg2)

if (x1 == null) return

if (x2 == null) return

println(x1 * x2)

}

//is operator

fun getStringLength1(obj: Any): Int? { //Any 是任何Kotlin类的超类

if (obj is String) {// 类似java中的 instanceof

// `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

}

// !is

fun getStringLength2(obj: Any): Int? {

if (obj !is String) return null

return obj.length

}

fun getStringLength3(obj: Any): Int? {

if (obj is String && obj.length > 0)

return obj.length

return null

}

//Using a for loop

fun foreachItems() {

// val items = listOf("apple", "banana", "kiwi")

val items = listOf("apple", "banana", "kiwi")

for (item in items) {//in operator

println("item is $item")

}

for (index in items.indices) {//indices 索引 type: Collection

// println("item at $index is ${items.get(index)}")

println("item at $index is ${items[index]}") //使用[index] 而不用 .get(index)

}

}

//Using when expression

fun describe(obj: Any): String =

when (obj) {//when 中 必须 有一个else

1 -> "One"

"Hello" -> "Greeting"

is Long -> "Long"

!is String -> "not a string"

else -> "Unknown"

}

//Using ranges 如果在if中 check的是一个数值，且使用了 in operator

fun range() {

val x = 10; val y = 9 //同一行中使用 ; 来分隔

if (x in 1..y + 1) {//使用 .. 来表示范围 最后转换成 x in 1..10

// if (x in (1..(y + 1))) {//如此解释 执行顺序 没问题 最后转换成 x in 1..10

// if (x in ((1..y) + 1)) {如此解释 执行顺序 不行 最后转换成 x in 10

println("fits in range")

}

for (x in 1..5) {//include 5

}

for (x in 1..10 step 2) {//x+=2 x is in {1, 3, 5, 7, 9}

println("rang 1..10 step 2: $x")

}

for (x in 9 downTo 0 step 3) {//x=9, x>=0 x-=3

println("x in 9 downTohttp:// 0 step 3: $x")

}

for (x in 0 until 10 step 2) {//until 10 ： not include 10

println("x in 1 until 10: $x")

}

}

//Checking if a collection contains an object usiOMlgQwcFbng in operator:

fun contains() {

val list = listOf("a1", "a2", "a3") //不可变list

when {// 匹配到一个条件 其它 就不再匹配

"a4" in list -> println("壹")

"a5" in list -> println(list.size)

"a3" in list -> println("the index is ${list.indexOf("a3")}")

}

}

//Using lambda expressions to filter and map collections:

fun collectionsLambda() {

// val list = mutableListOf() //可变list

// for (i in 1 ..10) {

// list.add(i)

//

// }

val list = (1..10).toList() //上面的 简写

list.filter { it % 2 == 0 }.map { it * 3 }.forEach(::println)

// list.filter { it % 2 == 0 }.map { it * 3 }.forEach{ println("item is $it")}

}

}

fun main(args: Array) {

var base = BasicSyntax()

base.printSum(10, 20)

base.assignVarible()

var min = base.minOf(10, 20)

println("min number is $min")

base.getBaseSyntax(null)

base.printProduct("1", "kk")

base.printProduct("33", "66")

println(null) //直接输出了 null 字符串

base.foreachItems()

println(base.describe(2))

base.range()

base.contains()

base.collectionsLambda()

}

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