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

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

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

last updated: 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()
}

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

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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 project in IntelliJ IDEA for study 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 ?

Class

• What is class in Kotlin? Example: class MyBook { ... }

• 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> { ... }

Collections and Arrays

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

null

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

Lambda function

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