UserDataSource.kt

interface UserDataSource {
    fun save(user: User)
}

class RemoteUserDataSource : UserDataSource {
    override fun save(user: User) {
        println("Calling save in remote data source.")
    }
}

class FakeUserDataSource : UserDataSource {
    override fun save(user: User) {
        println("Calling save in fake data source.")
    }
}