# 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:
        
        ```dart
        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:
        
        ```dart
        const int y = 20;
        const double pi = 3.14;
        ```
        

Example illustrating the difference:

```dart
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.