Kotlin introductory notes (II) a brief introduction to kotlin functions

Preface : This tutorial is best done with JAVA Study on the basis of

One 、main Use of functions

        (1) Did you learn java My friends should know ,java Of main The method is static , And written in class in .Kotlin It's different , In general, you can write main The function is written in class Outside

class Test{
	
}
/**  I am a main Entry function  **/
fun main(args: Array<String>) {
    var test=Test();
    println("main stay class External execution !")
}

Running results ：

        (2) Of course, there will be main The function is written in class In the method

（ This paragraph is reproduced from ：https://blog.csdn.net/qq_19306415/article/details/81902197）

       

class Test{
    companion object {
    /**  I am a main Entry function  **/
        @JvmStatic
        fun main(args: Array<String>) {
           var test=Test();
        }
    }
}

explain ：

companion object { // I'm a static block 
/**
 The static variables and static methods defined here , similar java Of ：static
**/
}

amount to java Medium

public static void main(String[] args){
}

Link to the original text ：https://blog.csdn.net/qq_19306415/article/details/81902197

Two 、 Ordinary function

Statement ： Use fun Keywords to declare (function Abbreviation )

Format ：

fun method(param1 : Int , param2 : Int) : Int {
    return 0
}

PS: Declare the return value type of the function , If it is Void Don't write ( Omit )

Above method refer to Method name ,  Parameter one And Parameter two   Refers to the parameter name , Method name And Parameter name You can modify it yourself . If you don't want to receive any parameters , Then you can write only a pair of empty parentheses .( This is related to java identical )

Two 、 Built in functions

Definition :  from Kotlin Provides a series of built-in functions

Some simple built-in functions ：

Such as ：Int max(Int num)

usage :（ To guide the package : import kotlin.math.max）

fun largerNumber(num1 : Int , num2 : Int) : Int {
    return max(num1,num2)
}

There are also some complex built-in functions , Such as

• let function
• also function
• with function
• run function
• apply function
• ...

Not today , But the above is biased Java A way of writing ,Kotlin Provides a more convenient syntax format ：

(1)  When there is only one line of code in a function ,Kotlin Allow us not to write the function body , You can use the only line of code "=" Connected at the tail

fun largerNumber(num1 : Int , num2 : Int) : Int = max(num1 , num2)

however , In fact, it can be further simplified ...

(2) because max() Function returns Int value ,largerNumber() The equals sign is used in the tail again , therefore Kotlin It can automatically deduce largerNumber() The return of the function must also be a Int value , So you don't have to show the declaration Int

fun largerNumber(num1 : Int , num2 : Int) = max(num1 , num2)

Kotlin The small white , I took notes while studying teacher Guolin's works . If there are any mistakes, please point out ！ Thank you for reading ！

Reference resources :

        Kotlin How to create in a class main function _ New ape blog -CSDN Blog _kotlin main function

        《 First line of code Android ( The third edition )》   --- Guo Lin