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?, !! and ?:

Learn safe call, non-null assertion and Elvis operators.

Safe call, non-null assertion and Elvis

Three operators work with null. ?. is the safe call: if the receiver is null the result is null. !! is the non-null assertion: if the receiver is null it throws NullPointerException. ?: is the Elvis operator: if the left is null it returns the right side.

val name: String? = null
name?.length       // null
name!!.length      // Exception!
name ?: "Guest"    // Guest

These three operators manage null-risk in one line. Which one you choose depends on what you want to do when the value is null.

Usage examples of the three operators

name?.length safely fetches the length; if name is null the result is null, no exception. name!!.length is for those who say "I am sure it is not null"; if you are wrong, the app crashes.

val len = name?.length     // Int?
val forced = name!!.length  // Int (or Exception)
val display = name ?: "Guest"   // String

The Elvis operator is especially handy when you want to give a default: name ?: "Guest" returns "Guest" when name is null, otherwise name itself.

Practice Handle name with safe call and Elvis

Define a val name: String? = null. Print the safe length with println(name?.length) and the default name with println(name ?: "Guest") on separate lines.

Starter code

Main.kt
fun main() {
    val name: String? = null
    // Print the safe length and the default name on separate lines
}

Sample solution

Main.kt
fun main() {
    val name: String? = null
    println(name?.length)
    println(name ?: "Guest")
}

Expected output

null
Guest

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Match operator to behavior

Term Meaning
?. Safe call (null if null)
!! Non-null assertion (exception if null)
?: Elvis (default if null)
String? Nullable type

2 val len = name?.length — what is len if name is null?

  • 0
  • null Correct answer
  • -1
  • Throws an exception

Explanation: The safe call (?.) returns null on a null receiver; no exception.

3 Fill in the blank: Elvis default

Question 
val display = name ____ "Guest"
Answer 
val display = name ?: "Guest"

Explanation: name ?: "Guest" gives a default when null.

All lessons in Beginner

1 Kotlin basics

  1. What is Kotlin?
  2. Your first program
  3. The main function
  4. Comments
  5. String templates
  6. Summary - Kotlin basics

2 Variables

  1. val and var
  2. Type inference
  3. const
  4. lateinit
  5. by lazy
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. Boolean and Char
  4. String
  5. Any and Unit
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Elvis operator
  6. Summary - Operators

6 Strings

  1. Creating strings
  2. String templates
  3. String methods
  4. substring
  5. split and replace
  6. Summary - Strings

7 Conditionals

  1. The if statement
  2. else and else if
  3. if expression
  4. when
  5. when expression
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. Range
  5. break and continue
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Arrays

  1. Creating arrays
  2. Array access
  3. Array size
  4. Array loop
  5. Array methods
  6. Summary - Arrays

11 Lists

  1. Creating lists
  2. List operations
  3. List access
  4. List iteration
  5. List methods
  6. Summary - Lists

12 Maps and Sets

  1. Creating sets
  2. Set operations
  3. Creating maps
  4. Map operations
  5. Map iteration
  6. Summary - Maps and Sets

13 Functions

  1. Defining functions
  2. Return values
  3. Expression body
  4. Default parameters
  5. Named arguments
  6. Summary - Functions

14 Null safety

  1. Nullable types
  2. ?, !! and ?:
  3. let
  4. Null check
  5. lateinit
  6. Summary - Null safety

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Advanced string operations

  1. Regex
  2. matches and find
  3. replace
  4. StringBuilder
  5. trim and pad
  6. Summary - Advanced strings

17 Enums

  1. enum class
  2. Enum values
  3. Enum properties
  4. when with enum
  5. Enum and range
  6. Summary - Enums

18 Exceptions

  1. try/catch
  2. throw
  3. finally
  4. try expression
  5. Custom exceptions
  6. Summary - Exceptions

19 Class basics

  1. class
  2. Constructor
  3. Properties
  4. Methods
  5. this
  6. Summary - Class basics

20 Beginner final

  1. Project design
  2. Classes
  3. Collections
  4. Error handling
  5. Putting it together