Difference Between final and const in Dart

In Dart, both final and const are used to create variables whose values cannot be changed after initialization (immutable). However, there are important differences between them.

1. Time of Value Assignment

• final → Value is determined at runtime (run-time constant). It can come from calculations or inputs known only when the program runs.
• const → Value must be known at compile-time (compile-time constant). Cannot depend on values computed at runtime.

//  Allowed: runtime value
final currentTime = DateTime.now();

//  ERROR: must be compile-time constant
const currentTimeConst = DateTime.now();

2. Level of Immutability

• final → The variable itself cannot be reassigned, but the content of the object it references can be changed if it's mutable.
• const → The value and the object are deeply immutable. Const objects are also canonicalized (all const objects with the same value point to the same memory instance).

final listFinal = [1, 2, 3];
listFinal.add(4); //  Allowed, list is mutable
// listFinal = [5, 6]; //  ERROR

const listConst = [1, 2, 3];
// listConst.add(4); //  ERROR, immutable

3. Usage in Classes

• final → Can be used for instance variables set once, often via constructor.
• const → Only allowed for:
  • static const (class-level constants)
  • const constructors for immutable objects

class Person {
  final String name; // set via constructor
  static const maxAge = 150; // compile-time constant
  Person(this.name);
}

4. Quick Comparison Table

Aspect	final	const
Evaluation Time	Runtime	Compile-time
Mutable Object Content	Yes (if object is mutable)	No
Usage in Classes	Instance variables	Static const / const constructors
Canonicalization	No	Yes

5. Flowchart: When to Use final vs const

If Blogger supports Mermaid.js, you can embed this diagram:

flowchart TD
    A[Do you know the value at compile-time?] -->|Yes| B[Use const]
    A -->|No| C[Use final]
    B --> D[Value never changes & deeply immutable]
    C --> E[Value set at runtime, but variable cannot be reassigned]

If Blogger does not support Mermaid.js, here’s the simplified logic:

• If the value is known before the program runs → Use const
• If the value is known only when the program runs → Use final