What is the difference between the keywords final and const in Dart?

final and const are used to create variables whose values cannot be changed after initialization.

Main differences:

• Initialization time:

  • final is initialized at runtime upon first access.
  • const is initialized at compile-time.

• Type of values:

  • final variable can hold a value computed at runtime.
  • const variable must hold a value that can be computed at compile-time (literals, results of constant expressions).

• Constancy:

  • final: The variable is constant, but the value it references (if it is an object) can be mutable.
  • const: The variable is constant, and the value it references must also be constant (immutable).

Examples:

// final - initialized at runtime
final DateTime now = DateTime.now(); // Value known only at runtime

// const - initialized at compile-time
const double pi = 3.14; // Value known at compile time

// Using const for lists and maps makes them immutable
const List<int> constantList = [1, 2, 3];
// constantList[0] = 10; // Error! List is immutable

// Using final for lists and maps makes the variable immutable,
// but the contents of the object can be changed (if the object is mutable)
final List<int> finalList = [1, 2, 3];
finalList[0] = 10; // Allowed, since the List itself is mutable

// final with a constant value
final int a = 5; // Allowed, but const is more appropriate

	Initialization	Value requirements	Mutability of the object (if referencing an object)
final	At runtime	Can be computed at runtime	Can be mutable if the object itself is mutable
const	At compile-time	Must be known at compile-time	Must be immutable