Kotlin Programming: The Big Nerd Ranch Guide, 2nd Edition

For the More Curious: Defining Structural Comparison

Imagine a Weapon class that has name and type properties:

    open class Weapon(val name: String, val type: String)

Suppose you would like two individual weapon instances to be considered structurally equal, using the structural equality operator (==), if the values of their names and types are structurally equal. By default, as we said earlier in this chapter, == checks referential equality for objects, so this expression would evaluate as false:

    open class Weapon(val name: String, val type: String)

    Weapon("Mjolnir", "hammer") == Weapon("Mjolnir", "hammer")  // False

You saw in this chapter that data classes provide a solution to this problem – an implementation of equals that ...