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Nullable types

Learn String? and nullable types.

Nullable types

In Dart 3 a type is non-nullable by default. String name; requires name to hold a value. To allow null, append ?: String? name;.

Now name is either a String or null. The question mark says 'this variable may be empty'. Assigning null to a non-nullable type is a compile error.

Nullable types describe 'maybe a value, maybe not' cases. A form field may be left blank, so its type becomes String?. The compiler then warns you about the null risk.

Assigning null

You can directly assign null to a nullable variable: String? name = null;. Leaving it unassigned also defaults to null: String? name; equals String? name = null;.

Be careful when using a nullable variable. Calling name.length throws if name is null. That is why null-aware operators and null checks matter; later lessons cover them.

Once you assign a non-null value, it is safe to use: name = 'Ayşe'; print(name.length);. Still, the compiler may warn about the nullable type; ! or a null check then helps.

Practice Declare nullable and assign

Declare a String? name (starting as null). Then assign name = 'Ayşe' and print it with print(name).

Starter code

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

  // assign 'Ayşe' and print
}

Sample solution

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

Expected output

Ayşe

Questions and answers

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

1 Match the types

Term Meaning
String Text, never null
String? Text, may be null
int Number, never null
int? Number, may be null

2 Which is nullable?

  • String? Correct answer
  • int
  • String
  • double

Explanation: String? is a nullable type.

3 Nullable declaration

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

Explanation: the variable name is name.

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