Kotlin basic grammar

Basic grammar ： Defined function 、 Defining variables 、 Constant 、 Use conditional expressions

import java.lang.Integer.parseInt

//① Defined function 
// One function accepts two  int  Type parameter , The return value is  int ：
fun sum1(a:Int ,b :Int):Int{
    
    return a+b
}
// This function has only one expression function body and a self deriving return value ：
fun sum2(a:Int,b:Int) = a+b

// Returns a meaningless value ：
fun printSum(a:Int,b:Int):Unit{
    
    println(a+b)
}
//Unit The return type of can be omitted 
fun printSum2(a:Int,b:Int){
    
    println(a+b)
}

//② Defining local variables 
fun declareVariables(): Unit {
    
    // declare constant ：
    val a: Int = 1
    val b = 1 // Deduce Int type 
    val c: Int // When there is no initialization value, you must declare   type 
    c = 1 //  assignment 

    // Declare variables 
    var x = 5 // Deduce Int  type 
    x += 1

}

//③ Use string templates 
fun main(args:Array<String>){
    
    if (args.size == 0) return
    println("First argument: ${
      args[0]}")
}
//④ Use conditional expressions 
fun max(a:Int,b:Int):Int{
    
    if(a>b)
        return a
    else
        return b
}
// Or the  if  When the conditional expression 
fun max2(a:Int,b:Int) = if(a>b)a else b

//⑤ Use nullable variables and null values to check 

// When a null value may appear, it should be clearly indicated that the reference can be null .
// The following function is when  str  Returns NULL if it does not contain an integer :
fun parseInt(str:String):Int?{
    
    //.....
}

// Use a function that returns nullable values ：
fun main(args:Array<String>){
    
    if(args.size<2){
    
        print("length 2 expected")
        return
    }
    val x = parseInt(args[0])
    val y = parseInt(args[1])
    // Use it directly  x*y  Errors will occur because there may be null values in them 
    if(x != null && y != null){
    
        //x  and  y  It will be automatically converted to non null value after null value detection 
        print(x*y)
    }

    // Or so 

    if (x == null) {
    
        print("Wrong number format in '${
      args[0]}' ")
        return
    }
    if (y == null) {
    
        print("Wrong number format in '${
      args[1]}' ")
        return
    }
    //x  and  y  It will be automatically converted to non null value after null value detection 
    print(x * y)
}

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