Kotlin开发知识（二）

高阶函数

传入或者返回函数的函数

1.常见高阶函数

forEach

   val list = listOf(1,2,3)
   list.forEach(::println)

map

    val list = listOf(1,2,3)
    val newList = list.map {
        it*2+1
    }
    newList.forEach(::println)

flatMap

    val mlist = listOf(
            1..20,
            2..5
    )
    val flatList = mlist.flatMap{
            it.map {
                "No. $it"
            }
    }
    flatList.forEach(::println)

reduce

累积数据处理

  val mlist = listOf(
        1..20,
        2..5
   )
   val flatList = mlist.flatMap{ it }
   println(flatList.reduce{ acc,i -> acc + i })

fold

添加初始值

  (0..6).map(::factorial).fold(5){acc, i ->  acc+i}

  //n的阶乘
  fun factorial(n:Int):Int{
      if (n==0) return 1
      return (1..n).reduce{acc, i -> acc*i }
  }

filter

  println((0..6).map(::factorial).filter { it%2 == 1 })

takeWhile

当遇到不符合条件的退出

  println((0..6).map(::factorial).takeWhile { it%2 == 1 })

let/apply

  data class Person(val name:String, val age:Int){
      fun work(){
          println("$name is working!!")
      }
  }
  fun main(args: Array<String>) {
      findPerson()?.let { (name, age)->
          println(name)
          println(age)
      }
      findPerson()?.apply { 
          work()
          println(age)
      }
  }
  fun findPerson():Person?{
      return null
  }

with

    val br = BufferedReader(FileReader("hello.txt"))
    with(br){
        var line:String?
        while (true){
            line = readLine()?:break
            println(line)
        }
        close()
    }

2.尾递归优化

• 递归的一种特殊形式
• 调用自身后物其他操作
• tailrec关键字提示编译器尾递归优化
• 尾递归与迭代的关系

//尾递归
tailrec fun findListNode(head:ListNode?, value:Int):ListNode?{
    head?:return null
    if (head.value == value) return head
    return findListNode(head.next, value)
}
//非尾递归
fun factorial(n:Long):Long{
    return n* factorial(n-1)
}

3.闭包

• 函数运行的环境
• 持有函数运行状态
• 函数内部可以定义函数，也可以定义类

//fun add(x:Int) = fun(y:Int) = x+y

fun add(x:Int):(Int)->Int{
    return fun(y:Int):Int{
        return x+y
    }
}

4.函数复合

val add5 = {i:Int -> i+5}
val multiplyBy2 = {i:Int -> i*2}

infix fun <P1, P2, R> Function1<P1, P2>.andThen(function:Function1<P2,R>):Function1<P1,R>{
    return fun(p1:P1):R{
        return function.invoke(this.invoke(p1))
    }
}

fun main(args: Array<String>) {
    println(multiplyBy2(add5(8)))

    val infixFun = add5 andThen multiplyBy2
    println(infixFun(8))
}

5.颗粒化

• 简单说就是多元函数变换成一元函数调用链

fun log(tag:String, target:OutputStream, message:Any?){
    target.write("[$tag]$message\n".toByteArray())
}

//颗粒化
fun log(tag:String)
    = fun(target:OutputStream)
    = fun(message:Any?)
    = target.write("[$tag]$message\n".toByteArray())


fun main(args: Array<String>) {
    log("benny", System.out, "helloWorld")
    log("benny")(System.out)("HelloWorld Again!")
}

6.偏函数

• 传入部分参数得到的新函数

fun log(tag:String, target:OutputStream, message:Any?){
    target.write("[$tag]$message\n".toByteArray())
}

fun main(args: Array<String>) {
    log("benny", System.out, "helloWorld")
    
    val consoleWithTag = (::log.curried())("benny")(System.out)
    consoleWithTag("Hello World Again Again!")
    consoleWithTag("Hello World Again Again Again!")
}

fun <P1,P2,P3,R> Function3<P1,P2,P3,R>.curried()
= fun(p1:P1) = fun(p2:P2) = fun(p3:P3) = this(p1, p2, p3)