scala - Java SortedMap 到 Scala TreeMap

Question

我在将 java SortedMap 转换为 scala TreeMap 时遇到问题。 SortedMap来自反序列化，在使用之前需要转换为scala结构。

出于好奇，一些背景是序列化结构是通过 XStream 编写的，并且在反序列化时我注册了一个转换器，它表示可以分配给 SortedMap[Comparable[_],_] 的任何内容应该给我。因此，我的 Convert 方法被调用，并被赋予一个可以安全地转换的 Object，因为我知道它的类型为 SortedMap[Comparable[_],_]。这就是有趣的地方。这里有一些示例代码可能有助于解释它。

// a conversion from comparable to ordering
scala> implicit def comparable2ordering[A <: Comparable[A]](x: A): Ordering[A] = new Ordering[A] {
     |     def compare(x: A, y: A) = x.compareTo(y)
     |   }
comparable2ordering: [A <: java.lang.Comparable[A]](x: A)Ordering[A]

// jm is how I see the map in the converter. Just as an object. I know the key
// is of type Comparable[_]
scala> val jm : Object = new java.util.TreeMap[Comparable[_], String]()        
jm: java.lang.Object = {}

// It's safe to cast as the converter only gets called for SortedMap[Comparable[_],_]
scala> val b = jm.asInstanceOf[java.util.SortedMap[Comparable[_],_]]
b: java.util.SortedMap[java.lang.Comparable[_], _] = {}

// Now I want to convert this to a tree map
scala> collection.immutable.TreeMap() ++ (for(k <- b.keySet) yield { (k, b.get(k))  })
<console>:15: error: diverging implicit expansion for type Ordering[A]
starting with method Tuple9 in object Ordering
       collection.immutable.TreeMap() ++ (for(k <- b.keySet) yield { (k, b.get(k))  })

Answer 1

首先，澄清您的错误:

// The type inferencer can't guess what you mean, you need to provide type arguments.
// new collection.immutable.TreeMap  
// <console>:8: error: diverging implicit expansion for type Ordering[A]
//starting with method Tuple9 in object Ordering
//       new collection.immutable.TreeMap
//       ^

您可以编写一个隐式将 Comparable[T] 视为 Ordering[T]，如下所示。

// This implicit only needs the type parameter.
implicit def comparable2ordering[A <: Comparable[A]]: Ordering[A] = new Ordering[A] {
   def compare(x: A, y: A) = x.compareTo(y)
}

trait T extends Comparable[T]

implicitly[Ordering[T]]

但是，如果您确实不知道 key 的类型，我认为您无法根据 Comparable#compareTo 创建 Ordering，位于至少没有反射(reflection):

val comparableOrdering = new Ordering[AnyRef] {
  def compare(a: AnyRef, b: AnyRef) = {
    val m = classOf[Comparable[_]].getMethod("compareTo", classOf[Object])
    m.invoke(a, b).asInstanceOf[Int]
  }
}
new collection.immutable.TreeMap[AnyRef, AnyRef]()(comparableOrdering)