private fun testElvisReturn(commands: List<String>?) {
    //如果值不符合预期(null)，直接返回
    val firstCommand = commands?.firstOrNull() ?: return

    when(firstCommand) {
        "ADD" -> "Add something"
        "DELETE" -> "Delete something"
        "UPDATE" -> "Update something"
        else -> "Find something"
    }
}