Null security and exception

One 、null Security

• Kotlin Put the running time that may appear null problem , In the wrong way at compile time , In advance at compile time , Instead of waiting for an error to occur , Nip in the bud , Improved program robustness .
• Emptiness
  stay Kotlin in , Unless otherwise specified , Variable cannot be null null value , That is, the default variable is not empty , Non empty type variables cannot be assigned null value
  
• Kotlin Calling functions on nullable type values... Is not allowed , Unless you actively accept safety management
  

Security call operators

Safe calls allow functions to be called on nullable types , The compiler sees a security call security symbol , If you encounter null value , Just skip function calls , Instead of going back null

use let Security call for

When the input is not empty , call let function

    var letter = readLine()?.let {
    
        if (it.isNotBlank()) {
    
            it.capitalize()
        } else {
    
            "butterfly"
        }
    }

Non empty assertion operators !!

When the value of the variable is null when , A null pointer exception will be thrown

Null merge operator ?:

If the left evaluates to null, Just use the result value on the right

Two 、 abnormal

Custom exception

class MyException : IllegalArgumentException(" Improper operation ")

Throw an exception

throw Exception("Hi There!")

exception handling

try {
    
    //  Some codes 
}
catch (e: SomeException) {
    
    //  The handler 
}
finally {
    
    //  Optional  finally  block 
}

Nothing type

• stay Kotlin in throw Is an expression , So you can use it （ such as ） As Elvis Part of an expression ：

  val s = person.name ?: throw IllegalArgumentException("Name required")
  

• throw The type of expression is a special type Nothing. The type has no value , It's used to mark code locations that can never be reached . In your own code , You can use Nothing To mark a function that will never return ：

  fun fail(message: String): Nothing {
        
      throw IllegalArgumentException(message)
  }
  

Prerequisite function

The prerequisite function defines the prerequisite , If the conditions are met , Object code can only be executed

fun check(letter : String?): String{
    
    val s = checkNotNull(letter) {
     "something  Not good. " }
    return s
}