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Null check

Learn null checking with if (x != null).

Null check: if (x != null)

When you check a nullable variable with if (x != null), the compiler smart-casts x to its non-null type inside the block. You do not need !!.

val name: String? = "Ava"
if (name != null) {
    println(name.length)  // smart cast: String
}

Outside the block name is still String?; the null-risk remains. This keeps the check valid only inside the block.

Null check and early return

The if (x == null) return pattern handles the null case early at the top of a function, so the rest of the code can be written as non-null. This avoids needless nesting.

fun greet(name: String?) {
    if (name == null) return
    println("Hello $name")
}

After the early return, name is now String; no ?. or !! is needed. This technique is especially useful when validating at the top of a function.

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 Outside the if (name != null) block, what is the type of name?

  • String (smart cast)
  • Any
  • Unit
  • String? Correct answer

Explanation: Outside the block name is still String?; smart cast applies only inside.

2 Match expression to role

Term Meaning
if (x != null) Inside, x is non-null
if (x == null) Inside, x is null
smart cast Type narrows automatically
return Early exit from block

3 Fill in the blank: null check

Question 
if (name ____ null) { println(name.length) }
Answer 
if (name != null) { println(name.length) }

Explanation: Inside the if (name != null) block, name is non-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