Dart - 3 - Constants

Constants in Dart are values that remain unchanged during the execution of a program. They play a crucial role in ensuring the immutability and consistency of data, which is essential in many programming scenarios. In this discussion, we’ll explore the concept of constants in Dart, how they differ from variables, and various types of constants available in the language.

Declaring Constants

Dart offers two primary ways to declare constants: final and const.

1. Final Constants

The final keyword is used to declare variables as constants. These variables can be assigned a value only once, and that value remains unchanged throughout the program’s execution.

final int maxAttempts = 3;
final String appName = 'MyApp';

In the example above, maxAttempts and appName are declared as final constants, and their values cannot be modified once assigned.

2. Const Constants

The const keyword, on the other hand, is used to create compile-time constants. These constants are known at compile time and are useful for values that will never change.

const double pi = 3.14159;
const List<int> primeNumbers = [2, 3, 5, 7, 11];

In this example, pi and primeNumbers are const constants, ensuring that their values remain constant and are known at compile time.

Differences Between `final` and `const`

While both final and const create constants, they have notable differences:

final constants are initialized when accessed, allowing for different values in different executions, whereas const constants are evaluated at compile time and have the same value across all program executions.
const is more restrictive than final. While final can have its value determined at runtime, const requires values to be known at compile time.
const can be used to create constant expressions, whereas final is more flexible and can be used with variables whose values are determined later but remain constant once set.

Use Cases for Constants

Constants are useful in various programming scenarios:

1. Configuration Values

Constants are often used for storing configuration values that should remain unchanged throughout the program’s execution, such as application settings, URLs, or API keys.

final String apiUrl = 'https://api.example.com';
final int maxRetries = 3;

2. Mathematical Constants

Mathematical constants like π (pi) or the speed of light are typically declared as const values in Dart to ensure their immutability.

const double pi = 3.14159;
const double speedOfLight = 299792458; // meters per second

3. Enumerations

Constants are commonly used to define enumerations, representing a finite set of values.

enum Color { red, green, blue }
final Color primaryColor = Color.blue;

Example: Constants in Action

Here’s an example that demonstrates the use of constants in Dart to create a simple configuration object:

class AppConfig {
  final String appName;
  final String apiUrl;

  AppConfig({required this.appName, required this.apiUrl});
}

void main() {
  const config = AppConfig(appName: 'MyApp', apiUrl: 'https://api.example.com');
  print('Application Name: ${config.appName}');
  print('API URL: ${config.apiUrl}');
}

In this example, appName and apiUrl are declared as final constants within the AppConfig class, ensuring that they remain constant once set.

Conclusion

Constants play a vital role in Dart programming by ensuring the immutability and consistency of data values. Whether used for configuration settings, mathematical constants, or enumerations, constants enhance code readability, maintainability, and reliability. It’s essential for developers to choose the appropriate type of constant, final or const, based on the nature of the data and the program’s requirements. By leveraging constants effectively, Dart programmers can create robust and predictable applications.

Dart – 3 – Constants