Constants in Dart: final and const

What are Constants?

A constant is a variable whose value cannot be changed after it is set. Using constants makes your code safer (prevents accidental modification), more readable (meaningful names), and sometimes more performant (compile-time constants).

The final Keyword

A final variable can be set only once, at runtime. Its value is determined when the program runs.

final String name = 'Alice';
final int year = DateTime.now().year;  // runtime value OK

// name = 'Bob';  ← ERROR: can't change a final variable

final List<int> numbers = [1, 2, 3];
numbers.add(4);  // ✅ OK — the list itself can be modified
// numbers = [5, 6];  ← ERROR — can't reassign the reference

The const Keyword

A const variable is a compile-time constant. Its value must be known at compile time (cannot use runtime values like DateTime.now()).

const double pi = 3.14159;
const int maxItems = 100;
const String appName = 'MyApp';

// const int year = DateTime.now().year;  ← ERROR (runtime value)

const List<int> primes = [2, 3, 5, 7, 11];
// primes.add(13);  ← ERROR — const list is immutable

final vs const

const Objects & Lists

// const list — deeply immutable
const List<String> colors = ['red', 'green', 'blue'];
// colors.add('yellow');  ← ERROR: Unsupported operation

// const map
const Map<String, int> statusCodes = {
  'ok': 200,
  'notFound': 404,
  'error': 500,
};
print(statusCodes['ok']);  // 200

Full Example

void main() {
  const double pi = 3.14159;
  final double radius = 5.0;   // could be user input
  final double area = pi * radius * radius;

  print('Pi: $pi');
  print('Radius: $radius');
  print('Circle area: $${area.toStringAsFixed(2)}');

  const String appVersion = '1.0.0';
  final DateTime launchDate = DateTime.now();

  print('App: $appVersion launched at $launchDate');
}

Pi: 3.14159
Radius: 5.0
Circle area: 78.54
App: 1.0.0 launched at 2024-01-15 10:30:00.000

⚠️ Common Mistakes

• Using const with a runtime value like DateTime.now() — use final instead.
• Trying to modify a const list — all elements are immutable.
• Confusing "the reference can't change" (final) with "the object can't change" (const).

🧠 Quiz

Summary

Use final for variables set once at runtime, and const for compile-time constants. final allows mutable objects (you can modify a final list's contents), while const makes everything deeply immutable. Prefer const when the value is known at compile time for better performance.