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

问题描述

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

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()

}

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

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]

我错过了什么?

推荐答案

您显然缺少 .asJava - 显式转换是使用 JavaConverters 的全部意义.util.Map[String, AnyRef] 是一个 Java 集合，Map[String, String] 是一个 Scala 集合.你至少需要

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]])

或者在调用 asJava 之前安全地进行类型转换:

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

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

附言在执行 asJava 后出现第二个错误的原因与 Scala 或 JavaConvertors 无关，只是 java.util.Map[K] 中的 V,V] 不是协变的(它是不变的，与 Scala 的 Map[K, +V] 不同).实际上编译器消息解释了它:

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:

注意:String <: AnyRef，但 Java 定义的 trait Map 在类型 V 中是不变的.

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

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