Strange Error with String in Scala 2.12.7

Question

I'm facing a very strange error dealing with Strings in one of my Scala application which I cannot reason about! What I have is an Observer that looks like this:

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")
  }
}

I'm then calling the onNext(elem: String) from the

Here is the error:

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

This is how I make the call!

  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))
    }
  }

https://stackoverflow.com/questions/10178377/java-generics-the-type-parameter-string-is-hiding-the-type-string https://stackoverflow.com/questions/32820133/the-type-parameter-g-is-hiding-the-type-g — Dmytro Mitin, Jun 14 '19 at 12:41
And what is `mqttObserver` then? How did you instantiate it? I'd prefer not to reopen it only so it can be re-closed for lacking an [mcve]. — Andrey Tyukin, Jun 14 '19 at 12:49

Dmytro Mitin · Accepted Answer · 2019-06-14T12:31:42.627

3

Replace

class MqttObserver[String] extends Observer[String]

with

class MqttObserver[S] extends Observer[S]

Otherwise type parameter String hides standard type java.lang.String.

edited Jun 14 '19 at 12:31

answered Jun 14 '19 at 12:29

Dmytro Mitin

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Could you please explain why? – joesan Jun 14 '19 at 12:31
@sparkr Type parameter `String` hides standard type `java.lang.String`. – Dmytro Mitin Jun 14 '19 at 12:32
Is there another way to fix this? I cannot change the MqttObserver signature – joesan Jun 14 '19 at 12:33
1

@sparkr Names of type parameters are not part of the signature, they can be replaced arbitrarily. Just use some less confusing name (one which is not `String`). – Andrey Tyukin Jun 14 '19 at 12:40
@AndreyTyukin It's possible that `MqttObserver` is a third-party class. – Dmytro Mitin Jun 14 '19 at 12:42
@sparkr Here `message.getPayload.toString` you created standard Java string and you try to use it where `MqttObservable`'s type is expected (maybe it should be inferred). So don't. I can't say better since you didn't provide MCVE. What error is if you put `message.getPayload` there? – Dmytro Mitin Jun 14 '19 at 12:45
@sparkr What is the type of `mqttObserver`? I guess it should be `MqttObserver[...]`. So in `mqttObserver.onNext(???)` you should substitute something of the same type `...` – Dmytro Mitin Jun 14 '19 at 12:52
mqttObserver: MqttObserver[String] – joesan Jun 14 '19 at 13:09
@sparkr What is `String` in `mqttObserver: MqttObserver[String]`? Is it `java.lang.String`? – Dmytro Mitin Jun 14 '19 at 13:13
It is of type String = java.lang.String It's the one standard from the Scala library – joesan Jun 14 '19 at 13:13
1

@sparkr Weird. Prepare MCVE. – Dmytro Mitin Jun 14 '19 at 13:15

Strange Error with String in Scala 2.12.7

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