Variables, final, const and Basic Types

Values and types

Declare variables with var (type inferred, reassignable), an explicit type (int count = 0), final (assigned once, at run time) or const (a compile-time constant, deeply immutable). Prefer final for values that do not change, and const for values known at compile time, which Dart canonicalises: identical const objects share one instance, an important optimisation for Flutter widgets. Everything is an object, including numbers and functions, and every type is a subtype of Object?. Core types: int and double (both num), String (UTF-16, with interpolation '$name' and '${expr}', and multi-line triple quotes), bool, List, Set, Map, Record, Function and Null. dynamic turns off static checking for a value and should be rare; Object? is the safe way to say "anything". Dart has no implicit conversions between int and String: use int.parse, int.tryParse and toString(). late declares a non-nullable variable that will be initialised before use, checked at run time.

var, final, const and conversions

const values are compile-time constants; final values are set once at run time.

void main() {
  var quantity = 3;                 // int, reassignable
  final createdAt = DateTime.now();  // set once at run time
  const gstRate = 0.18;              // compile-time constant
  const sizes = ['S', 'M', 'L'];     // deeply immutable const list

  final unitPrice = 49.5;
  final total = quantity * unitPrice * (1 + gstRate);
  print('Total: ${total.toStringAsFixed(2)}');

  final parsed = int.tryParse('42') ?? 0;   // null if the text is not a number
  final label = 'Qty: ' + quantity.toString();

  // sizes.add('XL');                // runtime error: cannot modify a const list
  // createdAt = DateTime.now();     // compile error: final cannot be reassigned

  print(identical(const [1, 2], const [1, 2]));   // true: canonicalised
}

final versus const

final means "this variable is assigned once"; the object may still be mutable (a final List can be added to). const means the value itself is a compile-time constant and deeply immutable. Use const wherever possible in Flutter widget trees.

Quick check: What is the difference between final and const in Dart?

• There is none
• const can be reassigned
• final is set once at run time; const must be a compile-time constant and is deeply immutable
• final only works with numbers