== Why is this an issue?

In Kotlin, `==` means structural equality and `!=` structural inequality and both map to the left-side term's `equals()` function.
It is, therefore, redundant to call `equals()` as a function.
Also, `==` and `!=` are more general than `equals()` and `!equals()` because it allows either of both operands to be `null`.

Developers using `equals()` instead of `==` or `!=` is often the result of adapting
styles from other languages like Java, where `==` means reference equality and `!=` means reference inequality.

=== What is the potential impact?

==== Wrong logic

`==` and `!=` allow either of both operands to be `null`, while `equals()` doesn't.

==== Readability and Understanding

The `==` and `!=` operators are a more concise and elegant way to test structural equality than calling a function.

== How to fix it

Replace `a.equals(b)` with `a == b`.
Replace `!a.equals(b)` with `a != b`.

=== Code examples

==== Noncompliant code example

[source,kotlin]
----
data class Person(
    val name: String,
    val firstName: String,
    val age: Int,
    val address: String
)
----

[source,kotlin,diff-id=1,diff-type=noncompliant]
----
fun checkEquality() {
    val personA = Person("Claus", "Santa", 200, "North Pole")
    val personB = Person("Nicholas", "Saint", 1700, "Myra")
    if (personA.name.equals(personB.name)) { // Noncompliant, should use `==` instead
        // ...
    }
    if (!personA.equals(personB)) { // Noncompliant, should use `!=` instead
        // ...
    }
}
----

==== Compliant solution

[source,kotlin,diff-id=1,diff-type=compliant]
----
fun checkEquality() {
    val personA = Person("Claus", "Santa", 200, "North Pole")
    val personB = Person("Nicholas", "Saint", 1700, "Myra")
    if (personA.name == personB.name) { // Compliant
        // ...
    }
    if (personA != personB) { // Compliant
        // ...
    }
}
----

== Resources

=== Documentation

* https://kotlinlang.org/docs/operator-overloading.html#equality-and-inequality-operators[Kotlin Docs, Equality and inequality operators]