MutableSet don't prevent duplicated content when using sealed class - kotlin

Question

If I use MutableSet with sealed class, the MutableSet accept all duplicated content.
Sample:
sealed class LoginSavedCommand {
class Login(val email: String, val password: String) : LoginSavedCommand()
class SaveData(val email: String, val password: String) : LoginSavedCommand()
}
fun main(args: Array<String>) {
val mSet: MutableSet<LoginSavedCommand> = hashSetOf()
mSet.add(LoginSavedCommand.Login("oba", "pass"))
mSet.add(LoginSavedCommand.Login("faiii", "blabla"))
if (mSet.add(LoginSavedCommand.Login("oba", "pass"))) {
println("don't")
} else {
println("do")
}
}
I passed the same values to LoginSavedCommand.Login, but the MutableSet keep accepting add the same value (on the sample println print "don't", and I need to print "do", because I need to prevent duplicated content using this selaed class)

Answer 1

A MutableSet checks whether it contains an element by using the elements' equals checks and, depending on the implementation, hashCode. A HashSet, for instance, uses hashCode to store and quickly lookup the elements in a hash table.
The two subclasses of the sealed class in your example don't override the equals function and therefore provide the default equality check implementation, which is identity equality (i.e. an object is only equal to itself, and different objects are never equal even if their properties are equal).
To achieve uniqueness of LoginSavedCommand items within a MutableSet, you need to ensure the subclasses provide proper equality check implementation.
A simple way to do that is to make both subclasses data classes, so that the compiler generates the equals and hashCode implementations based on the properties:
sealed class LoginSavedCommand {
data class Login(val email: String, val password: String) : LoginSavedCommand()
data class SaveData(val email: String, val password: String) : LoginSavedCommand()
}
(runnable sample)
Alternatively, override the equals and hashCode functions manually in the subclasses.
Important: when overriding these functions, ensure that the implementations follow the contracts of the functions described in the API reference for equals and hashCode.
For example:
sealed class LoginSavedCommand {
class Login(val email: String, val password: String) : LoginSavedCommand() {
override fun equals(other: Any?): Boolean {
if (this === other) return true
if (javaClass != other?.javaClass) return false
other as Login
if (email != other.email) return false
if (password != other.password) return false
return true
}
override fun hashCode(): Int {
var result = email.hashCode()
result = 31 * result + password.hashCode()
return result
}
}
class SaveData(val email: String, val password: String) : LoginSavedCommand() {
/* ... */
}
}
These implementations where generated by IntelliJ IDEA, using the Generate... → equals() and hashCode() action within the class body.