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.

Exercise Declare with final

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

Show example solution

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

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.

Test yourself

Runtime constant
```
____ pi = 3.14;
```
Show answer
```
final pi = 3.14;
```
Can a final variable be reassigned later?
- Yes, same type
- Yes, always
- No, never
- Only on first use
Show answer

No, never — final is assigned once; it never changes after.
final concepts

final

Runtime constant
const

Compile-time constant
var

Mutable
int

Integer type

Next lesson: const

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