Scala collections: util.Map[String, AnyRef] - Map[String, String]

Question

I am getting started with Scala and I am replacing the deprecated JavaConversions library with JavaConverters. I have the following code:

import scala.collection.JavaConversions._

new AMQP.BasicProperties.Builder()
  .contentType(message.contentType.map(_.toString).orNull)
  .contentEncoding(message.contentEncoding.orNull)
  .headers(message.headers) //<<<<--------------- I SEE THE ERROR ON THIS LINE (datatype of message.heads is Map[String, String]
  .deliveryMode(toDeliveryMode(message.mode))
  .priority(..)
  .correlationId(..)
  .replyTo(..)
  .expiration(..)
  .messageId(..)
  .timestamp(..)
  .`type`(..)
  .userId(..)
  .appId(..)
  .build()

}

When I replace the import for JavaConversions to JavaConverters (or, just comment out the import altogether), I get the compilation exception:

Type mismatch expected: util.Map[String, AnyRef], actual Map[String, String]

What am I missing?

you're missing .asJava obviously util.Map[String, AnyRef] is a Java collection Map[String, String] is a Scala collection — dk14
– dk14, Commented Oct 10, 2017 at 18:36
@dk14 can you please elaborate? Where do I need to add the .asJava? — Darth.Vader
– Darth.Vader, Commented Oct 10, 2017 at 18:38

dk14 · Accepted Answer · 2017-10-10 20:36:21Z

1

You're missing .asJava obviously - explicit conversion is the whole point of using JavaConverters. util.Map[String, AnyRef] is a Java collection, Map[String, String] is a Scala collection. You need at least

.headers(message.headers.asJava.asInstanceOf[java.util.Map[String, AnyRef]])

or better to do type-cast safely before calling asJava:

val params: Map[String, AnyRef] = message.headers
...
.headers(params.asJava)

P.S. The reason you've got a second error after just doing asJava isn't Scala or JavaConvertors related, it's just that V in java.util.Map[K,V] isn't covariant (it's invariant, unlike in Scala's Map[K, +V]). Actually compiler messages explains it:

Note: String <: AnyRef, but Java-defined trait Map is invariant in type V.

edited Oct 10, 2017 at 20:36

answered Oct 10, 2017 at 18:38

dk14

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2 Comments

Darth.Vader Over a year ago

now I see: Type mismatch expected: util.Map[String, AnyRef], actual util.Map[String, String]

dk14 Over a year ago

@Darth.Vader this isn't related to Scala or converters - java's map isn't contra-variant on second argument, so you have to do it java style (typecast). Basically compiler message suggests it: "Note: String <: AnyRef, but Java-defined trait Map is invariant in type V."

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