Diff btw final & const

Overview

Both

final

const

final — Runtime Constant

final

dartCopy
final name = 'Alice';              // Set once — cannot reassign
final DateTime now = DateTime.now(); // OK — runtime value
final list = [1, 2, 3];           // Reference is final, but list is mutable!

list.add(4);     // ✅ Works — list content can change
// list = [5];   // ❌ Error — can't reassign the reference

// In a class
class User {
  final String id;         // Must be set in constructor
  final String name;

  User(this.id, this.name);
}

const — Compile-Time Constant

const

dartCopy
const pi = 3.14159;
const appName = 'MyApp';
const maxRetries = 3;

// const list — deeply immutable
const colors = ['red', 'green', 'blue'];
// colors.add('yellow'); // ❌ Error — const collections are immutable!

// ❌ Cannot use runtime values
// const now = DateTime.now(); // Error! Runtime value

In Widgets — const Constructors

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// const widget — created at compile time, cached
// Flutter reuses the same widget instance → better performance
const Text('Hello World');     // ✅ const widget
const SizedBox(height: 16);   // ✅ const widget
const Padding(padding: EdgeInsets.all(8), child: Text('Hi')),

// final inside StatelessWidget
class MyWidget extends StatelessWidget {
  final String title; // Runtime — set by parent
  const MyWidget({required this.title}); // Constructor is const
}

Key Differences

final

const

dartCopy
// final can hold runtime values
final time = DateTime.now();    // ✅
final user = fetchUser();      // ✅ (Future result)

// const cannot
const time = DateTime.now();   // ❌ Compile error

Rule: Prefer

textCopy

const

everywhere possible (better performance). Use

textCopy

final

when the value is only known at runtime (like from API, user input, DateTime).