Kotlin 基本语法实例详解

 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  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 downTo 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 using 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() }