Compare two objects for equality in Kotlin

This article explores different ways to compare two objects for equality in Kotlin.

The recommended option to compare two objects in Kotlin is using the == operator, which gets compiled to the equals() function. However, simply calling the == operator might not work as expected, as shown below:

class TV(var company: String, var model: String, var warranty: Int)

fun main() {

val tv1 = TV("Vu", "Premium 4K", 2)

val tv2 = TV("Vu", "Premium 4K", 2)

println(tv1 == tv2) // false

}

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This post provides an overview of some of the available alternatives to compare two objects for equality using the == operator.

1. Overriding `equals()` and `hashCode()` function

Every class should override the equals and hashCode function to specify the equivalence relation between objects. A typical implementation of this approach would look like:

class TV(private var company: String, private var model: String, private var warranty: Int) {

override fun equals(other: Any?): Boolean {

if (this === other) return true

if (javaClass != other?.javaClass) return false

other as TV

if (company != other.company) return false

if (model != other.model) return false

if (warranty != other.warranty) return false

return true

}

override fun hashCode(): Int {

var result = company.hashCode()

result = 31 * result + model.hashCode()

result = 31 * result + warranty

return result

}

fun main() {

val tv1 = TV("Vu", "Premium 4K", 2)

val tv2 = TV("Vu", "Premium 4K", 2)

println(tv1 == tv2) // true

}

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2. Using Data class

Instead of overriding the equals() and hashCode() functions, you can use a data class that automatically derives the equals() and hashCode().

data class TV(private var company: String, private var model: String, private var warranty: Int)

fun main() {

val tv1 = TV("Vu", "Premium 4K", 2)

val tv2 = TV("Vu", "Premium 4K", 2)

println(tv1 == tv2) // true

}

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3. Using custom routine

If you prefer not to use data class or override the equals() and hashCode() function, create a custom routine to check for equality. This is demonstrated below:

class TV(var company: String, var model: String, var warranty: Int)

fun isEqual(tv1: TV, tv2: TV?): Boolean {

return if (tv1 === tv2) {

true

} else if (tv2 == null || tv1.javaClass != tv2.javaClass) {

false

} else {

tv1.warranty == tv2.warranty && tv1.company == tv2.company &&

tv1.model == tv2.model

}

fun main() {

val tv1 = TV("Vu", "Premium 4K", 2)

val tv2 = TV("Vu", "Premium 4K", 2)

println(isEqual(tv1, tv2)) // true

}

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That’s all about comparing two objects for equality in Kotlin.

Compare two objects for equality in Kotlin

1. Overriding equals() and hashCode() function

2. Using Data class

3. Using custom routine

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1. Overriding `equals()` and `hashCode()` function