Hi, I’d like to use data class with private constr...

# server

[Udomomo]

Hi,
I’d like to use data class with private constructor, and now searching the way to make copy method not visible from outside.
Many people provide workarounds and I found this. Seems interesting but I don’t know how it works. This uses sealed class and seems hashCode equals toString method of data class can be still used from outside, while copy method is not. Why only copy method is hidden by this way?

// Original data class with private constructor
data class User private constructor(val name: String, val id: Int){
    companion object{
        fun of(name:String, id: Int): User{
            return User(if(name.isEmpty()) "Unknown" else name, id)
        }
    }
}

// Workaround with sealed class
sealed class User {
    companion object {
        fun of(name: String, id: Int): User = UserData(if (name.isEmpty()) "Unknown" else name, id)
    }

    abstract val name: String
    abstract val id: Int

    private data class UserData(override val name: String, override val id: Int) : User()
}

[ephemient]

because the data class is private, its methods (such as copy) are not accessible

[ephemient]

there are other options such as https://github.com/AhmedMourad0/no-copy or https://github.com/drewhamilton/Poko

[Udomomo]

because the data class is private, its methods (such as copy) are not accessible

So equals and hashCode in this case don’t belong to data class too?

[ephemient]

they are methods of Any, which every type inherits

[ephemient]

similarly you have val name: String and val id: Int on the sealed class in the example you pasted above, so those are available even if the implementation is hidden

[ephemient]

but copy only exists on the data class which isn't accessible, so the method isn't accessible either

[ephemient]

the sealed class + private impl is an easy hack but it has its failings (e.g. toString uses the wrong name, component1 etc. functions aren't accessible meaning no destructuring)

[ephemient]

if you really need to prevent copy, I would either manually write the POJO

class User private constructor(val name: String, val id: Int) {
    override fun equals(other: Any?): Boolean = other is User && name == other.name && id == other.id
    override fun hashCode(): Int = name.hashCode() * 31 + int.hashCode()
    override fun toString(): String = "User(name=$name, id=$id)"

    operator fun component1(): String = name
    operator fun component2(): Int = id

    companion object {
        fun of(name: String, id: Int): User = User(name.ifEmpty { "Unknown" }, id)
    }
}

or use one of the plugins I mentioned previously

[Udomomo]

they are methods of Any, which every type inherits

Oh That’s what I overlooked. Thanks!