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The specific format of the declaration is ： Visibility modifier fun Function name ( Parameter name : Parameter type ) : return type { The body of the function }

When using ： No return value can Function name ( Parameters use )

Those with return values can var Variable name = Function name ( Parameters use )

Examples are as follows ：

// Declared at the top Int Array summation function 
public fun getSum(args:Array<Int>):Int{
    var sum=0
    for (i in args){
        sum+=i
    }
    return sum
} //public Omission , Different files can be accessed 

private fun sub_arr(args1: Array<Int>, args2: Array<Int>): Array<Int> {
// private Decorated top-level function , It can only be accessed in this document 
//  This function realizes the subtraction of the corresponding elements of the array , Generally, two arrays are required , If unequal length , Then long - short 
    var res: Array<Int>
    if (args1.size>args2.size){
        res=args1
        for (i in args2.indices){
            res[i]=args1[i]-args2[i]
        }
    }else{
        res=args2
        for (i in args1.indices){
            res[i]=args2[i]-args1[i]
        }
    }
    return res
}

internal fun no_return(args:Int=5){
// No return value and default parameters 
    println(args)
}

fun main(args:Array<String>){
//  Declared at the top main function , The visibility modifier defaults to public, The return value is Unit type 
//  Parameter is String Array , The parameter name is args
    var arr1= arrayOf(1,3,5,7,9)
    var arr2= arrayOf(2,4,6,8)
    val sum=getSum(arr1)
    println(sum)// 25
    for (i in sub_arr(arr2,arr1)){
        print("$i ")// -1 -1 -1 -1 9
    }
    println()
    no_return() //5 
    no_return(8) //8
}

Visibility modifier link

public

public The modifier is Kotlin The default modifier in , Omit not to write , On the top floor 、 class 、 Can be used locally , Maximum visibility , Generally, all sub regions under the parent region are visible

internal

internal yes Kotlin Peculiar , It is visible in the module , It can be on the top floor , Class
Kotlin The module of refers to a set of compiled Kotlin Source file ： It could be a IntelliJ IDEA modular ; One Maven project ; One Gradle Source set ; A set of calls Ant Task compiled file

protected

protected Modifiers are similar to those encountered in previous languages , Can only be used in a class , It is generally accessible in this class and inherited classes

private

private The modifier is also consistent with the previous language , It can be on the top floor 、 Use in class , Generally, it can be accessed in this class or in this document

summary

Only division can be used at the top layer protected External 3 A modifier
Within class 4 Modifiers can be used
Local declarations within blocks can only be used public Modifying variables

// learn.kt
package kotlin_learn

private fun f() {  } //  stay  learn.kt  See inside 

public var a: Int = 5 //  Variable a It can also be seen in other documents （ You can make it accessible by importing packages ）
private var b:Int = 0   //  Variable b Only in learn.kt  See inside 
    
internal val c= 6    //  Variable c Visible within the same module 

open class A{
    private val a = 1 // Visible in the current class 
    protected open val b = 2 // Visible within the current class and its inherited classes 
    internal val c = 3 // Visible in the current class and the same module class 
    val d = 4  //  Default  public , With class A The visibility of 
    
    protected class Nested { // Inner class and protected
        public val e: Int = 5 // attribute e Access rights follow Nested Class visibility 
    }
}

class Subclass : A() { // Inherit A
    // a  invisible 
    // b、c、d  so 
    // Nested  and  e  so 

    override val b = 5  //  Rewrote b
}

class B(o:A) {  // Not an inherited relationship , primary constructor ,B Only access A Of public and interna attribute 、 Method 
    // o.a、o.b  invisible 
    // o.c  and  o.d  so （ Same module ）
    // A.Nested  invisible ,Nested::e  Nor visible 
}

The return value of the function

Kotlin The return value types of functions in can be roughly divided into two types Unit And have return value types
Generally, if a function has no return value , Then the function declaration may not write the return value and return sentence （return If the sentence is written , Can be directly to write return）
When a function has a specific return value , You need to specify the type of the return value in the declaration , It's written in "： return type "
And when the return value is nullable , Nullable type must also be written in the return value type
fun printArr(array: Array<Int?>){
    for (i in array){
        print("$i ")
    }
    println()
    return //return Omission 
}
fun getSum(array: Array<Int?>):Int?{
    var a=0
    var sign=true
    for (i in array){
        if (i!=null){
            a+=i
            sign=false
        }
    }
    return if (sign) null else a // Ternary expression 
}
fun main(args:Array<String>){
    var arr:Array<Int?> = arrayOf(1,3,5,7,9)
    printArr(arr) // 1 3 5 7 9
    println(getSum(arr))// 25
    arr= arrayOf(null)
    printArr(arr)// null
    println(getSum(arr)) // null
}

The parameters of the function

When a function is declared , The declaration of general parameters is based on Parameter variable name : Parameter type The format of

Default parameters when declaring

The meaning of default parameters will not be repeated , Here we mainly talk about the use of default parameters
Generally, it still needs to follow Set... From right to left Logic of transferring parameters in a convenient location
Use format ： Parameter variable name : Parameter type = value
internal fun no_return(args:Int=5){
// No return value and default parameters 
    println(args)
}

When using keywords to pass parameters

Pass the key words to , When the function is called , Functional () Inside , Must take Parameter variable name = Value or variable The formal transfer parameters
Use this parameter transfer method , It is not necessary to follow the default parameter setting sequence from right to left
fun add_arr(addition:Int=1,array: Array<Int?>):Array<Int?>{

    for (i in array.indices){
        array[i] = array[i]?.plus(addition)
    }
    return array
}
fun printArr(array: Array<Int?>){
    for (i in array){
        print("$i ")
    }
    println()
    return //return Omission 
}
fun main(args:Array<String>){
    var arr:Array<Int?> = arrayOf(1,3,5,7,9)
    add_arr(array = arr)
    printArr(arr)
    add_arr(-2,arr)
    printArr(arr)
}

Variable parameters

When the length of a parameter variable that needs to be used in a function is not fixed , You can use variable modifiers vararg modification , By vararg The decorated parameter variable is equivalent to an array , The length of the array is determined by the parameters
Declaration format ： Visibility modifier fun Function name ( Parameter name : type , vararg Parameter name : type )
Generally speaking , Put variable parameters at the end of function parameter declaration , In order to better conform to the logic of position parameter transmission , But when using keywords to pass parameters , The declaration position of variable parameters can be arbitrary
fun myPrint(vararg args:Int){
    for (i in args){
        print("$i ")
    }
    println()
}

fun main(args:Array<String>){
    myPrint(1,2,3,4,5)
    myPrint(args = IntArray(3) { index -> (index * 3) }) 
    //lambda  Use of functions , This array is declared in the previous Kotlin Data types are described in the blog 
}

Special functions use

Member functions

Functions declared in a class are called member functions
Use of member functions ： Class object . Function name ( Parameters )

Single expression functions

When a function has a return value , And the function body can only be expressed as return Some expression In the form of , At this point, the { } Omission , use = Direct connection return The expression after , This representation is called a single expression function
fun myMultiply(a:Double ,b:Double)=a*b

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