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Nullable types

Learn String? and nullable types.

Nullable types: String?

In Kotlin a type does not accept null by default. val name: String = null is a compile error. For a variable to be null, the type gets a ? suffix: String?.

val name: String? = null   // ok
val city: String = null   // compile error

The ? mark tells the compiler the type is nullable; null-risk is tracked at compile time. This prevents most runtime NullPointerException crashes.

Why use nullable variables?

Sometimes a value is genuinely absent: a field not yet fetched from the database, an input the user left blank, or a lookup with no target. null states this "no value" case explicitly.

val nickname: String? = null
val realName: String? = "Ava"

A nullable type tells the programmer to manage the null-risk consciously. The ?., !! and ?: operators you will see in the next lessons are the tools for this management.

Practice Print a nullable name set to null

Define a val name: String? = null and print it with println(name).

Starter code

Main.kt
fun main() {
    // Define name as String? set to null and print it
}

Sample solution

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

Expected output

null

Questions and answers

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

1 What happens with val name: String = null?

  • It is a compile error Correct answer
  • name becomes null
  • A runtime exception is thrown
  • It returns Unit

Explanation: String (no ? suffix) cannot hold null; it is a compile error.

2 Fill in the blank: nullable type

Question 
val name: ____ = null
Answer 
val name: String? = null

Explanation: String? is the type that can hold null.

3 Match expression to meaning

Term Meaning
String cannot be null
String? can be null
? Nullable mark
null No value

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