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<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()

}

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