rspec/rules/S1206/dart/rule.adoc

== Why is this an issue?

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

=== Noncompliant code example

[source,dart]
----
class A {
  final int value;
  A(this.value);

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

=== Compliant solution

[source,dart]
----
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

* https://dart.dev/tools/linter-rules/unnecessary_overrides[Dart Lint rule]