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Null-aware operators

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

Null-aware operators

Dart, with null safety, offers special operators to handle null: ??, ?. and ??=.

?? returns the right side if the left is null: name ?? 'Guest' — if name is null, returns 'Guest'; otherwise the value of name.

?. accesses a member only if not null: user?.name — if user is null, returns null; otherwise calls name. ??= assigns only if null.

Practice Default with ??

Declare String? name = null;. Print with print(name ?? 'Guest'). Since name is null you should see 'Guest'.

Starter code

main.dart
// Use the ?? operator

Sample solution

main.dart
void main() {
  String? name = null;
  print(name ?? 'Guest');
}

Expected output

Guest

Difference between ?? and ??=

?? returns an expression: name ?? 'Guest' gives a value but does not change name. name is still null.

??= does assignment: name ??= 'Guest'; — if name is null, it sets it to 'Guest'. So if you read name later, you see 'Guest'.

Summary: ?? gives a default when reading; ??= assigns a default when writing. You choose based on your need.

Questions and answers

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

1 Default operator

Question 
String? name; print(name ____ 'Guest');
Answer 
String? name; print(name ?? 'Guest');

Explanation: ?? gives the right side if null.

2 What does the ?. operator do?

  • Deletes the variable
  • Forces null
  • Skips the call if null Correct answer
  • Throws an error

Explanation: ?. skips the member call if null; returns null.

3 Null-aware operators

Term Meaning
?? Default
?. Safe call
??= Assign if null
? Nullable

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