rspec/rules/S1206/kotlin/rule.adoc

== Why is this an issue?

In Kotlin, you must override either both or neither of the `equals(Any?)` and `hashCode()` methods 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 `equals` comparisons on the object is modified.
- If two objects are equal according to the `equals` method, 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,kotlin]
----
class MyClass {    // Noncompliant - should also override "hashCode()"

  override fun equals(other: Any?): Boolean {
    /* ... */
  }

}
----

=== Compliant solution

[source,kotlin]
----
class MyClass {    // Compliant

  override fun equals(other: Any?): Boolean {
    /* ... */
  }

  override fun hashCode(): Int {
    /* ... */
  }

}
----

== Resources

* https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-any/hash-code.html[`hashCode` - kotlinlang.org]