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 : [e-mail]
 * 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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