UserBuilderGeneralImplementation.kt

class User private constructor(
    firstName: String?,
    // remaining properties…
) {
    val firstName: String
    // other properties...

    init {
        // validations, if any

        // set values
        this.firstName = firstName
        // remaining properties ...
    }

    class Builder {
        private var firstName: String? = null

        Other Properties

        Public Setters for the builder

        // build() that returns the User object.
        fun build(): User {...}
    }
}