hash_and_equals

Always override hashCode if overriding ==.

DO override hashCode if overriding == and prefer overriding == if overriding hashCode.

Every object in Dart has a hashCode. Both the == operator and the hashCode property of objects must be consistent in order for a common hash map implementation to function properly. Thus, when overriding ==, the hashCode should also be overridden to maintain consistency. Similarly, if hashCode is overridden, == should be also.

BAD:

class Bad {
  final int value;
  Bad(this.value);

  @override
  bool operator ==(Object other) => other is Bad && other.value == value;
}

GOOD:

class Better {
  final int value;
  Better(this.value);

  @override
  bool operator ==(Object other) =>
      other is Better &&
      other.runtimeType == runtimeType &&
      other.value == value;

  @override
  int get hashCode => value.hashCode;
}

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

linter:
  rules:
    - hash_and_equals

If you're instead using the YAML map syntax to configure linter rules, add hash_and_equals: true under linter > rules:

linter:
  rules:
    hash_and_equals: true