Object Equality

Comparing objects — class instances — in Kotlin is a little different than Java, and very similar to Scala.

Key points

• == calls equals under the hood (structural equality)
• === is used to test reference equality
• Classes don’t have equals or hashCode methods by default, you need to implement them
• Data classes have automatically generated equals and hashCode methods

== and ===

This code shows how object equality works when a class does not have an equals method (Person) and when a class does have an equals method (PersonWithEquals):

class Person(var name: String)

class PersonWithEquals(var name: String) {
    override fun equals(that: Any?): Boolean {
        if (that == null) return false
        if (that !is PersonWithEquals) return false
        if (this.name == that.name) {
            return true
        } else {
            return false
        }
    }
}

fun main(args: Array<String>) {

    // Person (without `equals`)
    val p1 = Person("Joe")
    val p2 = Person("Joe")
    println(p1 == p2)   //false
    println(p1 === p2)  //false
    println(p1 === p1)  //true

    // PersonWithEquals
    val p1a = PersonWithEquals("Joe")
    val p2a = PersonWithEquals("Joe")
    println(p1a == p2a)   //true
    println(p1a === p2a)  //false
    println(p1a === p1a)  //true

}

Notes:

• You can write the equals function differently; I’m just trying to be obvious about the tests
• In the real world you should implement hashCode
• As you’ll see in the Data Classes lesson, a data class implements equals and hashCode for you
• See the “nullability” lessons for the meaning of Any?