rspec/rules/S2097/kotlin/rule.adoc

== Why is this an issue?

The `Any#equals(other: Any?)` method is used to compare two objects to see if they are equal.

The `other` parameter's type is `Any?`, this means that an object of any type, as well as `null`, can be passed as an argument to this method.

Any class overriding `Any#equals(other: Any?)` should respect this contract, accept any object as an argument, and return `false` when the
argument's type differs from the expected type. The `other` parameter's type can be checked using the `is` operator or by comparing the `javaClass` field:

[source,kotlin]
----
override fun equals(other: Any?): Boolean {
  // ...
  if (other?.javaClass != this.javaClass) {
    return false
  }
  // ...
}
----

However, it is an issue to assume that the `equals` method will only be used to compare objects of the same type. Casting the `other` parameter without a prior test will throw a `ClassCastException` instead of returning false.
[source,kotlin]
----
class MyClass {
  override fun equals(other: Any?): Boolean {
    val that = other as MyClass // may throw a ClassCastException
    // ...
  }
  // ...
}
----

This rule raises an issue when `other` parameter's type has not been tested before a cast operation.

== How to fix it

Ensure the `other` parameter's type is checked by comparing `other?.javaClass` and `this.javaClass`, or use the `is` operator to test `other`'s type.

=== Code examples

==== Noncompliant code example
[source,kotlin,diff-id=1,diff-type=noncompliant]
----
class MyClass {
  override fun equals(other: Any?): Boolean {
    if (this === other) {
        return true
    }
    val that = other as MyClass // Noncompliant, may throw a ClassCastException
    // ...
  }
  // ...
}
----

==== Compliant solution
[source,kotlin,diff-id=1,diff-type=compliant]
----
class MyClass {
  override fun equals(other: Any?): Boolean {
    if (this === other) {
        return true
    }
    if (other?.javaClass != this.javaClass) {
      return false
    }
    val that = other as MyClass // Compliant, other's type is MyClass
    // ...
  }
  // ...
}
----

== Resources

=== Documentation

* https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-any/equals.html[Kotlin Standard Library - Any#equals(other: Any?)]