rspec/rules/S1206/dart/rule.adoc

== Why is this an issue?

In Dart, you must override either both or neither of the operator `==` and the `hashCode()` method.

That is required to keep the contract between the two:

* whenever the `hashCode` method is invoked on the same object more than once, it must consistently return the same integer, provided no information used in `==` comparisons on the object is modified
* if two objects are equal according to the `==` operator, then calling the `hashCode` method on each of the two objects must produce the same integer result

By overriding only one of the two methods with a non-trivial implementation, this contract is almost certainly broken.

=== Exceptions

The rule does not raise in mixins, because they are not supposed to be instantiated directly, but rather included in other classes.

The rule raises, however, on mixin classes, since they can also be used as normal classes, and instantiated directly.

=== Noncompliant code example

[source,dart,diff-id=1,diff-type=noncompliant]
----
class A {
  final int value;
  A(this.value);

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

=== Compliant solution

[source,dart,diff-id=1,diff-type=compliant]
----
class A {
  final int value;
  A(this.value);

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

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

== Resources

* Dart Docs - https://dart.dev/tools/linter-rules/hash_and_equals[Dart Linter rule - hash_and_equals]
* Dart Docs - https://dart.dev/language/mixins[Language - Classes & objects - Mixins]

ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

* Missing a corresponding override of 'hashCode'.
* Missing a corresponding override of '=='.

=== Highlighting

The identifier of the overridden method: either `==` or `hashCode`.

'''
== Comments And Links
(visible only on this page)

include::../comments-and-links.adoc[]

endif::env-github,rspecator-view[]