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lateinit

Learn deferred initialization with lateinit var.

lateinit

Sometimes you cannot give a variable its value right away; for example, a configuration might load later. With lateinit var you declare the variable now and assign its value later.

lateinit works only with var and only for reference types (e.g., String, class instances); it cannot be used for primitive types (Int, Double).

lateinit var config: String

fun main() {
    config = "active"
    println(config)
}

Practice Initialize lateinit

At the top level, declare lateinit var config: String. Inside fun main(), first assign config = "active", then print with println(config).

Sample solution

Main.kt
lateinit var config: String

fun main() {
    config = "active"
    println(config)
}

Expected output

active

lateinit safety

If you access a lateinit variable before assigning a value, Kotlin throws UninitializedPropertyAccessException. To prevent this, you can check whether it has been initialized with ::config.isInitialized.

This is especially useful in dependency-injection frameworks, where objects are wired later during setup. So lateinit is common in the pattern of declaring first, initializing after injection.

Questions and answers

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

1 lateinit: match the terms

Term Meaning
lateinit Assigned later
var Mutable
val Immutable
const val Compile-time

2 Which can lateinit NOT be used with?

  • String
  • Int Correct answer
  • A class instance
  • List

Explanation: lateinit cannot be used with primitive types like Int or Double.

3 lateinit: fill in the missing part

Question 
lateinit var config: ____
Answer 
lateinit var config: String

Explanation: lateinit works with reference types like String.

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