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Summary - Null safety

Combine ?, !!, ?: and let to write a null-safe function.

Summary: ?, !!, ?: and let together

In this summary you will combine the null-safe tools. A nullable value can be processed with ?.let and given a fallback with ?:. !! is used only at points where you say "I am sure".

val name: String? = null
name?.let { println("Hello $it") } ?: println("Guest")
// Guest

This chain prints "Guest" when null; otherwise it greets. So one line manages both branches.

Practice Greet with let and Elvis together

Define a val name: String? = null. Print a null-safe greeting with name?.let { println("Hello $it") } ?: println("Guest").

Starter code

Main.kt
fun main() {
    val name: String? = null
    // If name is null print "Guest", else print "Hello $it" (use let ?:)
}

Sample solution

Main.kt
fun main() {
    val name: String? = null
    name?.let { println("Hello $it") } ?: println("Guest")
}

Expected output

Guest

Practice Print length with if (name != null)

Define a val name: String? = "Ava". Inside an if (name != null) block, print the length with println(name.length).

Starter code

Main.kt
fun main() {
    val name: String? = "Ava"
    // If name is not null, print its length (use if != null)
}

Sample solution

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

Expected output

3

Questions and answers

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

1 Which uses let and Elvis together?

  • if (name == null) println(name)
  • name?.let { println(it) } ?: println("Guest") Correct answer
  • name!! + name?.length
  • println(name)

Explanation: ?.let runs when not null, ?: gives a default when null.

2 Match operator to role

Term Meaning
?. Safe call
!! Non-null assertion
?: Elvis default
?.let { } Null-safe block

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