Question

我开始使用Scala，我正在用JavaConversions替换已弃用的JavaConverters库。我有以下代码：

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()

}

当我将JavaConversions的导入替换为JavaConverters（或者只是完全注释掉导入）时，我得到了编译异常：

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

我错过了什么？

Answer 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.

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

1 个答案: