What is difference between const and final keyword in flutter

In Flutter (and Dart), both const and final are used to declare immutable variables, but there are key differences in when and how they are initialized.

Summary of Differences

Feature	final	const
Immutability	Single assignment only	Single assignment only
When Initialized	At runtime	At compile time
Usage	When value is known at runtime	When value is known at compile time
Memory	New instance each time	Canonicalized (same instance reused)
Can be used with	API responses, dynamic values	Static values like hard-coded literals
In Classes	Use final for instance fields	Use const for compile-time constants

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Detailed Explanation

final

• Initialized only once
• Value is set at runtime
• Useful when you don’t know the value until runtime (e.g., API results, user input, DateTime)

final name = 'John';
final currentTime = DateTime.now(); // Allowed

const

• Compile-time constant
• Must be initialized with a value known at compile time
• Uses canonicalization – same object is reused in memory

const pi = 3.14;
const list = [1, 2, 3];

const time = DateTime.now(); //  Error: not allowed (runtime value)

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const with Widgets

In Flutter UI code, you often use const for widgets to improve performance:

const Text('Hello'); // Widget is created at compile time and reused

This avoids unnecessary rebuilds and improves efficiency.

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When to Use What?

Use case	Keyword to use
Value doesn’t change and known early	const
Value doesn’t change but known later	final
Value changes	No final/const