scala - Scala似乎无法将可变映射转换为Java Map

Question

我在Scala应用程序中使用了Java POJO:

public class AppRuntimeContext {
  // Lots of stuff...
  public Map<Object,Object> contextMap;

  // Getters & setters, ctors, etc.  
}

在我的Scala应用中:

val ctx : AppRuntimeContext = new AppRuntimeContext()
val ctxMap : Map[String,Fizz] = Map()
// Some code that populates 'ctxMap'
ctx.setContextMap(ctxMap)

这会在setter方法上产生编译器错误:

type mismatch; found : scala.collection.mutable.Map[String,com.me.myapp.Fizz] required: java.util.Map[Object,Object]

因此，我尝试通过添加以下import语句将 ctxMap转换为 java.util.Map:

import collection.JavaConversions._

然后通过将setter调用更改为:

ctx.setContextMap(mapAsJavaMap(ctxMap))

但是，当我这样做时，仍然会出现编译器错误:

type mismatch; found : scala.collection.mutable.Map[String,com.me.myapp.Fizz] required: scala.collection.Map[Object,Object] Note: String <: Object, but trait Map is invariant in type A. You may wish to investigate a wildcard type such as _ <: Object. (SLS 3.2.10)

任何想法导致此错误的原因和解决方法是什么？

Answer 1

一种选择是将 map 转换为Map[Object, Object]:

ctx.setContextMap(mapAsJavaMap(ctxMap.asInstanceOf[mutable.Map[Object, Object]]))

更好的方法可能是首先将 ctxMap声明为 Map[Object, Object]，尽管这将允许放置不符合预期 [String, Fizz]类型的对象:

val ctxMap : mutable.Map[Object, Object] = mutable.Map()
// Some code that populates 'ctxMap'
ctxMap.put("k", Fizz(1))

ctx.setContextMap(mapAsJavaMap(ctxMap))