Scala 2.12.7中字符串的奇怪错误[重复]

Question

在我的一个 Scala 应用程序中处理字符串时遇到了一个非常奇怪的错误，我无法解释！我所拥有的是一个看起来像这样的观察者：

class MqttObserver[String] extends Observer[String] {

  private val logger = Logger.of[MqttObserver[String]]

  override def onNext(elem: String): Future[Ack] = {
    logger.info(s"Got a message from Mqtt broker $elem")
    Continue
  }

  override def onError(ex: Throwable): Unit = {
    logger.error(s"Stream error happened ${ex.getMessage}")
  }

  override def onComplete(): Unit = {
    logger.info(s"Stream ended")
  }
}

然后我从

调用 onNext(elem: String)

这是错误：

[error]  found   : java.lang.String
[error]  required: String(in class MqttObservable)
[error]       mqttObserver.onNext(message.getPayload.toString)

这就是我打电话的方式！

  val callback: MqttCallback = new MqttCallback {
    override def connectionLost(cause: Throwable): Unit = {
      logger.info(cause)
    }
    override def deliveryComplete(token: IMqttDeliveryToken): Unit = {

    }
    override def messageArrived(topic: Predef.String, message: MqttMessage): Unit = {
      mqttObserver.onNext(message.getPayload.toString) // FAILS HERE!!!!
      logger.info("Using Default Console Callback --> Receiving Data, Topic : %s, Message : %s".format(topic, message))
    }
  }

Answer 1

替换

class MqttObserver[String] extends Observer[String]

与

class MqttObserver[S] extends Observer[S]

否则类型参数String隐藏标准类型java.lang.String。