Kotlin anonymous function and lambda

@[TOC]( Anonymous functions And Lambda)

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Preface

Anonymous functions , seeing the name of a thing one thinks of its function , Is a function without a name

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1、 Anonymous functions

Anonymous functions

   val len = "tiger".count();    println(len);   val len2 = "tiger".count {// it  Equivalent to  t i g e r  The characters of  char        it == 'g'    }    println(len2);

2、 Function type and implicit return

Implicit return of function type

//  First step ： Declaration of function input and output     val methodAction: () -> String//  The second step ： The implementation of the above functions     methodAction = {        val inputValue = 666//  Anonymous functions , Do not write return,  The last line is the return value         "$inputValue tiger"    }//  The third step ： Call function     println(methodAction());

3、 Function parameter

//  You can directly put the above   First step ： Declaration of function input and output   and   The second step ： The implementation of the above functions   Merge     val methodAction: (Int, Int, Int) -> String = {        number1, number2, number3 ->        val inputValue = 666        "$inputValue tiger  Parameter one ：$number1  Parameter two ：$number2  Parameter 3 ：$number3"    }//  The third step ： Call function     println(methodAction(1, 2, 3))

4、it Keyword features

//  stay kotlin in   If there is only one parameter passed in ,kotlin  Will automatically generate a it  Implicit parameter     val methodAction: (String) -> String = {        "$it tiger"    }    println(methodAction("666"))    val methodAction2: (Double) -> String = {        "$it tiger"    }    println(methodAction2(666.0))    val methodAction3: (Int) -> String = {        "$it tiger"    }    println(methodAction3(666))

5、 Type inference of anonymous functions

//  Anonymous functions , The type is inferred as String （ Return type , Depending on the return parameters , Any type of parameter can be returned ）//  Method name  ：  Must specify   Parameter type   and   Return type //  Method name  =  Type inference return type     val methodValue = { num1: Double, num2: Float, num3: Int ->        "num1: $num1, num2: $num2: num3: $num3"    }    println(methodValue(32.2, 33.4f, 44))

6、Lambda

Anonymous functions Belong to Lambda

//  Anonymous functions  == Lambda  expression     val addResultMethod = {number1: Int, number2: Int ->        " Two sums  ${number1 + number2}"    }    println(addResultMethod(3, 6))    //  Anonymous functions   Enter the reference  Int  return Any  type  （Any  Type on Java  Medium  Object ）// Lambda  The argument to the expression  Int, Lambda  Result of expression Any  type     val  weakResultMethod = {number: Int ->        when(number){            1 -> " Monday "            2 -> " Tuesday "            3 -> " Wednesday "            4 -> " Thursday "            5 -> " Friday "            else -> -1        }    }     // weakResultMethod  function  （Int） -> Any    println(weakResultMethod(3))

7、 Function to define parameters Is a function of a function

//  Call function     loginAPI("tiger", "123456"){        msg: String, code: Int ->        println(" The final login is ：msg: $msg code: $code")    }//  Simulate the data of the server const val USER_NAME_SAVE_DB = "tiger"const val USER_PASS_SAVE_DB = "123456"//  Imitate front-end login fun loginAPI(userName: String, passWord: String, responseResult: (String, Int) -> Unit) {    if (userName == null || passWord == null){        //  Problems arise , To terminate the program   Throw an exception  Nothing（ Is an abstract concept at compile time , Only when you need to mark a function as incomplete , To be useful to Nothing Necessity of type ）        TODO(" Wrong user name or password ")    }    if (userName.length > 3 && passWord.length > 3){        if (wbeServerLoginApi(name = userName, pwd = passWord)){            responseResult("login success", 200)        }else{            responseResult("login error", 500)        }    }else{        TODO(" Wrong user name or password ")    }}//  Simulate login operation private fun wbeServerLoginApi(name: String, pwd: String): Boolean {    return name == USER_NAME_SAVE_DB && pwd == USER_PASS_SAVE_DB}

8、 Function inlining

If the function argument has Lambda , Use as much as possible inline keyword , In this way, internal optimization will be done , Reduce the number of functions , Object development , Loss of performance

Use inline , amount to C++ #define Macro definition , Macro replace , Will replace the code with the calling code , There is no development of any function , Object development , Cause loss of performance
If inline is not used , On the calling side , Multiple objects will be generated to complete Lambda Call to （ Cause loss of performance ）

//  Imitate front-end login  inline  Inline keyword inline fun loginAPI(userName: String, passWord: String, responseResult: (String, Int) -> Unit) {    if (userName == null || passWord == null){        //  Problems arise , To terminate the program   Throw an exception  Nothing（ Is an abstract concept at compile time , Only when you need to mark a function as incomplete , To be useful to Nothing Necessity of type ）        TODO(" Wrong user name or password ")    }    if (userName.length > 3 && passWord.length > 3){        if (wbeServerLoginApi(name = userName, pwd = passWord)){            responseResult("login success", 200)        }else{            responseResult("login error", 500)        }    }else{        TODO(" Wrong user name or password ")    }}

9、 Function reference

// Lambda  Objects of function type , Need to put methodResponseResult  Ordinary function   adopt  ::  Into a   Object of function type （ Function reference ）    loginAPI("tiger", "123456", ::methodResponseResult)// Define a function fun methodResponseResult(msg: String, code: Int){    println(" The final login is ：msg: $msg code: $code")}//  Simulate the data of the server const val USER_NAME_SAVE_DB = "tiger"const val USER_PASS_SAVE_DB = "123456"//  Imitate front-end login inline fun loginAPI(userName: String, passWord: String, responseResult: (String, Int) -> Unit) {    if (userName == null || passWord == null){        //  Problems arise , To terminate the program   Throw an exception  Nothing（ Is an abstract concept at compile time , Only when you need to mark a function as incomplete , To be useful to Nothing Necessity of type ）        TODO(" Wrong user name or password ")    }    if (userName.length > 3 && passWord.length > 3){        if (wbeServerLoginApi(name = userName, pwd = passWord)){            responseResult("login success", 200)        }else{            responseResult("login error", 500)        }    }else{        TODO(" Wrong user name or password ")    }}//  Simulate login operation fun wbeServerLoginApi(name: String, pwd: String): Boolean {    return name == USER_NAME_SAVE_DB && pwd == USER_PASS_SAVE_DB}

10、 Function type as return type

    val showMethod = show("tiger")// showMethod  yes  show  The return value of the function , Just this return value , It's a function // show ==  Anonymous functions     println(showMethod(" Wang Meili ", 18))fun show(info: String) :(String, Int) -> String {    println(" I am a show  function  info: $info")    return {name: String, age: Int ->        " Inside  name： $name, age: $age"    }}

11、 Anonymous and named functions

//  Anonymous functions     showPersonInfo("tiger", 18, ' male ', " Study kotlin"){        println(" Show results ： $it")    }//  Named functions  show    showPersonInfo("tiger", 18, ' male ', " Study kotlin", ::showResultImpl)fun showResultImpl(result: String){    println(" Display results of named functions ： $result")}inline fun showPersonInfo(name: String, age: Int, sex: Char, study: String, showResult: (String) -> Unit){    val str = "name: $name, age: $age, sex: $sex, study: $study"    showResult(str)}

summary

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