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final

Learn to declare immutable variables with final.

The final keyword

With final you declare a variable that is assigned once and cannot be changed afterwards. For example final pi = 3.14;; then pi = 3; is a compile error.

final is a runtime constant. The value can be computed when the program runs; what matters is that the first value is given. After that it does not change.

final is commonly used for config data, fixed lists and 'set once' logic. Writing final instead of var prevents accidental changes.

Practice Declare with final

Declare a name variable with final (set to Alex) and print it with print.

Starter code

main.dart
// Declare a final variable

Sample solution

main.dart
void main() {
  final name = 'Alex';
  print(name);
}

Expected output

Alex

Difference between final and const

Both final and const create immutable variables, but there is a difference. final sets the value at runtime; const sets it at compile time.

So final now = DateTime.now(); works (the time is read at runtime) but const now = DateTime.now(); is an error; DateTime.now() cannot be known at compile time.

We will see const in detail in the next lesson. For now, know that final is flexible and const is strict.

Questions and answers

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

1 Runtime constant

Question 
____ pi = 3.14;
Answer 
final pi = 3.14;

Explanation: final is a runtime constant.

2 Can a final variable be reassigned later?

  • Yes, same type
  • Yes, always
  • No, never Correct answer
  • Only on first use

Explanation: final is assigned once; it never changes after.

3 final concepts

Term Meaning
final Runtime constant
const Compile-time constant
var Mutable
int Integer type

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