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Null-aware operators

Learn ??, ?. and ??= operators.

Null-aware operators

Dart offers three short operators for null handling. ?? null coalescing: name ?? 'Guest' — when name is non-null it returns name, otherwise 'Guest'.

?. null-aware access: name?.length — when name is non-null it returns the length, otherwise null. This avoids the name.length error.

??= null-aware assignment: name ??= 'Guest'; — if name is null it assigns 'Guest', otherwise leaves it. All three provide null safety concisely.

Safe access with ?.

The ?. operator safely accesses a member of a nullable value. name?.length returns the length when name is non-null, otherwise null. No exception is thrown.

This replaces name.length, which crashes when name is null. With ?. you say 'leave it null if it is null'.

The result is nullable: int? len = name?.length;. The next steps still need null-aware thinking. Chaining works too: user?.address?.city.

Practice Print with ??

Declare a String? name (starting as null). Print it with print(name ?? 'Misafir').

Starter code

main.dart
void main() {
  // declare a nullable variable (null)

  // print with a default
}

Sample solution

main.dart
void main() {
  String? name;
  print(name ?? 'Misafir');
}

Expected output

Misafir

Questions and answers

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

1 The ?? operator

Question 
void main() {
  String? name;
  print(name ____ 'Misafir');
}
Answer 
void main() {
  String? name;
  print(name ?? 'Misafir');
}

Explanation: ?? returns the right side when the left is null.

2 Match the operators

Term Meaning
?? Default value
. Safe access
??= Assign if null
? Nullable type

3 What does name ?? 'Misafir' return?

main.dart
String? name;
print(name ?? 'Misafir');
  • null if name is null
  • 'Misafir' if name is null Correct answer
  • Always name
  • Always 'Misafir'

Explanation: ?? returns the right side when the left is null.

All lessons in Beginner

1 Dart basics

  1. What is Dart?
  2. Your first program
  3. dart run
  4. Comments
  5. String interpolation
  6. Summary - Dart basics

2 Variables

  1. var and type
  2. final
  3. const
  4. late
  5. Null safety
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. bool and String
  4. Type inference
  5. dynamic
  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. Null-aware operators
  6. Summary - Operators

6 Strings

  1. Creating strings
  2. String interpolation
  3. length and methods
  4. substring
  5. contains and indexOf
  6. Summary - Strings

7 Conditionals

  1. The if statement
  2. else and else if
  3. switch
  4. Pattern matching
  5. Ternary
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. for-in
  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 Lists

  1. Creating lists
  2. List access
  3. add and remove
  4. length and contains
  5. List methods
  6. Summary - Lists

11 Sets and Maps

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

12 Functions

  1. Defining functions
  2. Return values
  3. Arrow functions
  4. Optional parameters
  5. First-class functions
  6. Summary - Functions

13 Null safety in depth

  1. Nullable types
  2. Null-aware operators
  3. The ! operator
  4. late
  5. Null check
  6. Summary - Null safety

14 Class basics

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

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. split and join
  2. replace
  3. trim and pad
  4. RegExp
  5. StringBuffer
  6. Summary - Advanced strings

17 Enums

  1. Defining enums
  2. Enum values
  3. Enhanced enums
  4. switch with enum
  5. Enum and patterns
  6. Summary - Enums

18 Exceptions

  1. try/catch
  2. throw
  3. finally
  4. The Exception class
  5. on clause
  6. Summary - Exceptions

19 Async basics

  1. Future
  2. async/await
  3. then
  4. Future.error
  5. Future.wait
  6. Summary - Async basics

20 Beginner final

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