Kotlin introduction section 2

Keywords: kotlin

Characteristics of Unit function

It is similar to void in java, but java is a key frame, not a type, which is very contradictory
Unit is not written. It also exists by default. Unit represents the ignored type returned without parameters

package com.fyx.s1


fun main() {
}


fun convert() {

}
//Equivalent to the above
fun convert1() : Unit{

}

The role of backquotes

You can use Chinese characters or numbers to name functions or functions that name some keywords

package com.fyx.s1


fun main() {
    `This is a Chinese function name`()
}

//Function naming
fun `This is a Chinese function name`() {
    
}

//Is is a keyword in kotlin. It cannot be a function, but it can be replaced by backquotes
fun `is`() {
    println("ceshi")
}

lambda expressions

The {} after count is the lambda expression to filter out unqualified values

package com.fyx.s1

fun main() {
  //{} will traverse each character to filter the value of the composite parameter
    var datas = "Jeecg".count {
        it == 'e'
    }
    println(datas)
}

Function variable declaration and implicit return

package com.fyx.s1


fun main() {
    //Variable declaration function
    var testFuncation: () -> String

    //Function implementation
    testFuncation = {
        var count = 100
        //Implicit return means that you don't need to write return. The last line returns by default
        count.toString()
    }
    println(testFuncation())
}

Function variable argument declaration

package com.fyx.s1


fun main() {
    var testfunc: (Int, Int, Int) -> String = { num1, num2, num3 ->
        var prifix = "Digital set $num1 +number $num2 + number $num3";
        prifix
    }
    println(testfunc(1,2,3))
}

The role of it and invoke

it can be used when the function has only one parameter
invoke equivalence and direct use of function fun1(1,2,3)

package com.fyx.s1


fun main() {
    var fun1: (Int, Int, Int) -> String = { n1, n2, n3 ->
        var name = "1234"
        "This is the number $n1 $n2 $n3 "
    }
    println(fun1.invoke(1, 2, 3))

    var fun2: (String) -> String = {
        "This is the number $it "
    }
    println(fun2.invoke("12"))
}

Type inference of anonymous functions

Method name: return type must be specified
Method name = data type can be automatically inferred according to the last row

package com.fyx.s1


fun main() {
    var fun1 = {
        var name = "1234"
        "This is a string"
    }
}

Anonymous function lambda expression and return type Any

package com.fyx.s1


fun main() {
    //=lambda writing method of anonymous function
    var fun1 = { age1: Int, age2: Int ->
        "$age1 $age2"
    }
    println(fun1(12, 12))
    //Any type because it is uncertain whether the last line returns a number or a string
    var fun2 = { age: Int ->
        when (age) {
            in 10..20 -> "Like flowers"
            else -> -1
        }
    }
    println(fun2.invoke(21))
}

Trailing closure

If the last argument of the function is a function type, if you intend to pass in a lambda expression as an argument, you are allowed to specify the lambda expression outside the parentheses

package com.fyx.s1


fun main() {
    //Define function
    var loginResult = { mes: String, code: Int ->
        println("Login results $mes Status code $code")
    }
    login("root", "1234", loginResult)

}

const val USER_NAME = "root"
const val PASS_WORD = "1234"

//Define a login method
fun login(username: String, Passowrd: String, loginResult: (mes: String, code: Int) -> Unit) {
    if (username == USER_NAME && Passowrd == PASS_WORD) {
        loginResult("Login succeeded", 200)
    } else {
        loginResult("Login failed", -200)

    }
}

inline

If the function parameter has lambda, try to use inline key frames, which will be optimized internally to reduce the loss of function development and object development

package com.fyx.s1


fun main() {
    //Define function
    var loginResult = { mes: String, code: Int ->
        println("Login results $mes Status code $code")
    }
    login("root", "1234", loginResult)

}

const val USER_NAME = "root"
const val PASS_WORD = "1234"

//Define a login method
inline fun login(username: String, Passowrd: String, loginResult: (mes: String, code: Int) -> Unit) {
    if (username == USER_NAME && Passowrd == PASS_WORD) {
        loginResult("Login succeeded", 200)
    } else {
        loginResult("Login failed", -200)

    }
}

Precautions for inlining

Since the JVM can intelligently judge whether to inline optimize ordinary functions according to the actual situation, we do not need to use Kotlin's inline syntax, which will only make the bytecode more complex.
Try to avoid inlining functions with a large number of function bodies, which will lead to too many bytecodes.
Once a function is defined as an inline function, you cannot get the private members of the closure class unless you declare them internal.

Function to function object

Here we find that the direct reference function reports an error because it is a function type object. How to convert a function into a function object? We can use::

package com.fyx.s1

const val USER_NAME = "root"
const val PASS_WORD = "1234"

fun main() {

    login("root", "1234", ::loginResult)

}

//Function type
fun loginResult(mes: String, code: Int): Unit {
    println("Login results $mes Status code $code")
}

//Define a login method
inline fun login(username: String, Passowrd: String, loginResult: (mes: String, code: Int) -> Unit) {
    if (username == USER_NAME && Passowrd == PASS_WORD) {
        loginResult("Login succeeded", 200)
    } else {
        loginResult("Login failed", -200)

    }
}

Return function parameters

package com.fyx.s1


fun main() {
    var method2 = method1("details")
    println(method2.invoke("trouble", 18))
}

fun method1(info: String): (String, Int) -> String {
    println(info)
    return { name: String, age: Int ->
        "$name $age "
    }
}

Anonymous and named functions

Anonymous functions are declared inside methods

package com.fyx.s1

const val USER_NAME = "root"
const val PASS_WORD = "1234"

fun main() {
    login("root", "1234", { name, age ->
        println("$name + $age ")
    })
}


inline fun login(username: String, Passowrd: String, loginResult: (mes: String, code: Int) -> Unit) {
    if (username == USER_NAME && Passowrd == PASS_WORD) {
        loginResult("Login succeeded", 200)
    } else {
        loginResult("Login failed", -200)

    }
}

A named function converts a function into an object function through:: function

package com.fyx.s1

const val USER_NAME = "root"
const val PASS_WORD = "1234"

fun main() {
    login("root", "1234",::loginResult)
}

fun loginResult(mes: String, code: Int): Unit {
    println("Login results $mes Status code $code")
}
 
inline fun login(username: String, Passowrd: String, loginResult: (mes: String, code: Int) -> Unit) {
    if (username == USER_NAME && Passowrd == PASS_WORD) {
        loginResult("Login succeeded", 200)
    } else {
        loginResult("Login failed", -200)

    }
}

Nullability of Kotlin

Many null pointer exceptions can be encountered in traditional java projects, but not in kotlin projects

package com.fyx.s1
fun main() {
    //We found that the parameter cannot be set to nullable
    var name = "test"
    name = null
    //Can you add one if necessary? Setting this variable can be assigned to null
    var names: String?
    names=null
}

Safe call operator

If the variable is set to be nullable, remedial measures are required for all operation functions of the variable, that is, when it is empty, it will not be executed
The way to use this method is to add "yes" at the end of the variable when calling the function?

package com.fyx.s1

fun main() {
  var name:String? ="Model"
    name?.capitalize()
}

let security call

When the value is empty, you cannot enter the function of let, and you can perform security verification on variables

package com.fyx.s1

fun main() {
    var name: String? = null
    name?.let {
        println("Not empty")
    }
}

!! predicated operations

This means that if the name is empty, the next step will be carried out. If it is empty, the package null pointer will be abnormal

package com.fyx.s1

fun main() {
    var name: String? = null
    name!!.toBigDecimal();
}

Posted by thesmartone on Fri, 03 Dec 2021 06:26:47 -0800

Programmer Group

Kotlin introduction section 2

Characteristics of Unit function

The role of backquotes

lambda expressions

Function variable declaration and implicit return

Function variable argument declaration

The role of it and invoke

Type inference of anonymous functions

Anonymous function lambda expression and return type Any

Trailing closure

inline

Function to function object

Return function parameters

Anonymous and named functions

Nullability of Kotlin

Safe call operator

let security call

!! predicated operations

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