Dart Defined Constant using final and const keyword

In Dart, both final and const are used for defining constants, but they behave differently:

1. final:

   • final variables can only be set once and cannot be changed afterwards.

   • They are initialized the first time they are used.

   • Their value can be determined at runtime.

   • They are not implicitly const and can hold values computed at runtime.

   • Example:

       final int x = 10;
       final String name = "John";
     

2. const:

   • const variables are implicitly final but are compile-time constants.

   • They must be initialized with a constant value, known at compile-time.

   • They are eagerly initialized at compile-time.

   • They can hold literals and constant expressions, but not variables.

   • Example:

       const int y = 20;
       const double pi = 3.14;
     

Example illustrating the difference:

final int a = 10;
const int b = 20;

void main() {
  final int c = a + 5; // Valid, a is a runtime constant
  // const int d = b + 5; // Invalid, b is a compile-time constant, but expression involves runtime calculation
}

In summary, use final when the value of the variable might change at runtime but is set only once, and use const when the value is known at compile-time and will never change.