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 < 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<String>("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<Int>() //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<String>) {  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()}