null value, use the symbol ? and ?. and !! and ?: and !!. and ?. in Kotlin

Компьютер, программы

→

Kotlin

null value, use the symbol ? and ?. and !! and ?: and !!. and ?. in Kotlin

посмотрели 2320 раз

обновлено: 2 February 2025

? null To specify an empty value in a variable

Add the ? character to the type and assign the empty value null

Kotlin

var bookPrice:Int? = null

!! Take a value from a variable with an exception

Add !! to take the value.
If we use !! and the value is null, then a exception is thrown.

Kotlin

var bookPrice1:Int? = 10
var bookPrice2:Int? = null
var bookPrice3:Int? = 7

var allPrice:Int = bookPrice1!! + bookPrice2!! + bookPrice3!!

Note!

In my example, allPrice is not calculated because bookPrice2!! will throw an exception.

?: Take a value from a variable without exception

Add ?: to take the value.
If we use ?: and the value is null, then we set the value ourselves.

Kotlin

var bookPrice1:Int? = 10
var bookPrice2:Int? = null
var bookPrice3:Int? = 7

var allPrice:Int = (bookPrice1?:0) + (bookPrice2?:0) + (bookPrice3?:0)

In my example, allPrice = 17

!!. Call a class method with an exception

Add !!. to call the method.
If we use !!. and the value is null, then a exception is thrown.

Kotlin

class MyBook
{
     fun Show()
     {
          println("Hello")
     }
}

fun main() {
     var b1:MyBook? = null

     b1!!.Show()
}

In my example, the Show method won't be called because b1!!. will throw an exception.

?. Call a class method without exception

Add ?. to call the method.
If we use ?. and the value is null, then the method will not be called. There will be no exception.

Kotlin

class MyBook
{
     fun Show()
     {
          println("Hello")
     }
}

fun main() {
     var b1:MyBook? = null

     b1?.Show()
}

← Previous topic
Declare the variable and set the value in Kotlin | val, var, lateinit, constant: const val, global variable: companion object

Next topic →
Integer numbers in Kotlin are: Byte, UByte, Short, UShort, Int, UInt, Long, ULong

Your feedback ... Comments ...

Объявления

• What is Kotlin?

• Why is IntelliJ IDEA the most popular development environment for Kotlin?

• Download and install IntelliJ IDEA to explore Kotlin

• Create a new console project to explore Kotlin | IntelliJ IDEA

Setting the value

• Declare the variable and set the value in Kotlin | val, var, lateinit, constant: const val, global variable: companion object

null

• null value, use the symbol ? and ?. and !! and ?: and !!. and ?. in Kotlin

Int, Float, Boolean, Char ...

• Integer numbers in Kotlin are: Byte, UByte, Short, UShort, Int, UInt, Long, ULong

• Decimal numbers in Kotlin are: Float, Double

• A flag with values of true or false in Kotlin is: Boolean

• The symbol in Kotlin is: Char

• Convert number to text in Kotlin | Int → String

String

• The string, the text in Kotlin is: String

• Interpolate strings in Kotlin. Example: val address:String = "${street}, ${country}"

• What is the difference between String and StringBuilder ?

Enum

• What is an enumeration (enum) in Kotlin ?

• How do I find enum by value in Kotlin ?

Collections and Arrays

• What are collections in Kotlin?

• Create collections list, set, map and an array array in Kotlin

Function

• fun() { ... } is a function in Kotlin. The function is needed to perform an action.

Extension function

• Extension function in Kotlin. Allows you to add a new function to the system (List, ..) or my (Book, ...) class

Lambda function

• Lambda function in Kotlin. Example 1: var myFunc1 : (a:Int, b:Int) -> Int = { p1, p2 -> p1 + p2 };

Class

• What is class in Kotlin? Example: class MyBook { ... } | Primary and secondary constructor | block init

• lateinit is a late initialization for the class field | Kotlin

• class that inherits from interface in Kotlin | Example: class MyBook : IBook { ...}

• Unnamed class that inherits from interface in Kotlin | Example: val book1 = object : IBook { ...}

• Generic, template class in Kotlin | Example: class MyBook<T> { ... }

Flow

• flow is the data flow in Kotlin. These are the substitution of LiveData in Android Kotlin and the substitution of Observable in RxJava

Writing a program

• The Kotlin console app works like Android Jetpack Compose