Question

I’m having trouble converting a java SortedMap into a scala TreeMap. The SortedMap comes from deserialization and needs to be converted into a scala structure before being used.

Some background, for the curious, is that the serialized structure is written through XStream and on desializing I register a converter that says anything that can be assigned to SortedMap[Comparable[_],_] should be given to me. So my convert method gets called and is given an Object that I can safely cast because I know it’s of type SortedMap[Comparable[_],_]. That’s where it gets interesting. Here’s some sample code that might help explain it.

// 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

Firstly, to clarify your error:

// 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
//       ^

You can write an implicit to treat Comparable[T] as Ordering[T] as follows.

// 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]]

However, if you really don’t know the type of the key, I don’t think you can create the Ordering in terms of Comparable#compareTo, at least without 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)