Contents

prefer_final_fields

Contents

Private field could be final.

This rule is available as of Dart 2.0.

Rule sets: recommended, flutter

This rule has a quick fix available.

Details

#

From Effective Dart:

DO prefer declaring private fields as final if they are not reassigned later in the library.

Declaring fields as final when possible is a good practice because it helps avoid accidental reassignments and allows the compiler to do optimizations.

BAD:

dart
class BadImmutable {
  var _label = 'hola mundo! BadImmutable'; // LINT
  var label = 'hola mundo! BadImmutable'; // OK
}

BAD:

dart
class MultipleMutable {
  var _label = 'hola mundo! GoodMutable', _offender = 'mumble mumble!'; // LINT
  var _someOther; // LINT

  MultipleMutable() : _someOther = 5;

  MultipleMutable(this._someOther);

  void changeLabel() {
    _label= 'hello world! GoodMutable';
  }
}

GOOD:

dart
class GoodImmutable {
  final label = 'hola mundo! BadImmutable', bla = 5; // OK
  final _label = 'hola mundo! BadImmutable', _bla = 5; // OK
}

GOOD:

dart
class GoodMutable {
  var _label = 'hola mundo! GoodMutable';

  void changeLabel() {
    _label = 'hello world! GoodMutable';
  }
}

BAD:

dart
class AssignedInAllConstructors {
  var _label; // LINT
  AssignedInAllConstructors(this._label);
  AssignedInAllConstructors.withDefault() : _label = 'Hello';
}

GOOD:

dart
class NotAssignedInAllConstructors {
  var _label; // OK
  NotAssignedInAllConstructors();
  NotAssignedInAllConstructors.withDefault() : _label = 'Hello';
}

Usage

#

To enable the prefer_final_fields rule, add prefer_final_fields under linter > rules in your analysis_options.yaml file:

analysis_options.yaml
yaml
linter:
  rules:
    - prefer_final_fields

Unless stated otherwise, the documentation on this site reflects Dart 3.6.0. Page last updated on 2024-07-03. View source or report an issue.