Equals method for data class in Kotlin

Question

I have the following data class

data class PuzzleBoard(val board: IntArray) {
    val dimension by lazy { Math.sqrt(board.size.toDouble()).toInt() }
}
         


        

Answer 1

For Data classes in Kotlin, hashcode() method will generate and return the same integer if parameters values are same for both objects.

val user = User("Alex", 1)
val secondUser = User("Alex", 1)
val thirdUser = User("Max", 2)

println(user.hashCode().equals(secondUser.hashCode()))
println(user.hashCode().equals(thirdUser.hashCode()))

Running this code will return True and False as when we created secondUser object we have passed same argument as object user, so hashCode() integer generated for both of them will be same.

also if you will check this:

println(user.equals(thirdUser))

It will return false.

As per hashCode() method docs

open fun hashCode(): Int (source)

Returns a hash code value for the object. The general contract of hashCode is:

Whenever it is invoked on the same object more than once, the hashCode method 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.

For more details see this discussion here