`::` head and tail deconstruction of a java collection in scala

`::` head and tail deconstruction of a java collection in scala

I am trying to write a "deconstructor" for java collections in scala.
With most scala collections, I can use the :: case class to deconstruct
the collection in head/tail:
scala> val (a :: b) = Seq(1,2)
a: Int = 1
b: List[Int] = List(2)
scala> val (a :: b :: Nil) = Seq(1,2)
a: Int = 1
b: Int = 2
And even more complicated cases (e.g. summing the two first elements of
the inside lists):
scala> val m = Map("a" -> Seq(1,2,3,4), "b" -> Seq(2,3,4,5))
scala> m.collect { case (k, a :: b :: _) => k ->(a+b)}
res5: scala.collection.immutable.Map[java.lang.String,Int] = Map(a -> 3, b
-> 5)
but I can't work out how to have this work for java collections without
extraneous code:
Let's say that I get from an external library something like this:
m: java.util.Map[java.lang.String,java.util.List[Int]] = {a=[1, 2, 3, 4],
b=[2, 3, 4, 5]}
With scala<=>java collection conversions, I can convert the map to a scala
map, and work on the inside lists, which are still java ones:
m.asScala.collect { case (k, jl) => jl.asScala.toList match { case (a :: b
:: _) => k->(a+b) } }
or
m.asScala.map{
case (k, v) => k -> v.asScala.toList
}.collect {
case (k, a :: b :: _) => k ->(a+b)
}
I have found the unapplySeq matcher trick, but this only works when I know
the size of the collection:
object JavaCollection {
import scala.collection.JavaConverters._
def unapplySeq[T](array: java.util.Collection[T]): Option[Seq[T]] =
Option(array).map(_.asScala.toIndexedSeq)
}
m.asScala.collect { case (k, JavaCollection(a,b,c,d)) => k ->(a+b)}
How do I get the :: deconstruction to work directly on a Java typed
collection without going through the explicit conversion? I have tried
building my own case class ::[T](head:T, tail: java.util.Collection[T])
class, but that doesn't seem to be enough.