型パラメータを使うとだめぽ

MessagePackWritable.scala

import java.io.{DataInput, DataOutput}
import org.apache.hadoop.io.{WritableComparable, WritableComparator}
import org.msgpack.ScalaMessagePack

trait MessagePackWritable extends WritableComparable[MessagePackWritable] {
  type T

  var instance:T

  def get = instance

  def getRawBytes = ScalaMessagePack.write(instance)

  def write(out: DataOutput) {
    val raw = ScalaMessagePack.write(instance)
    assert(raw != null)
    out.writeInt(raw.length)
    out.write(raw, 0, raw.length)
  }

  def readFields(in: DataInput) {
    val size = in.readInt()
    if (size > 0) {
      val raw = new Array[Byte](size)
      in.readFully(raw, 0, size)
      instance = unpack(raw)
      assert(instance != null, "unpacked null")
    }
  }

  def compareTo(other: MessagePackWritable) = {
    val raw1 = ScalaMessagePack.write(instance)
    val raw2 = ScalaMessagePack.write(other.instance)
    WritableComparator.compareBytes(raw1, 0, raw1.length, raw2, 0, raw2.length)
  }

  def unpack(raw: Array[Byte]): T
}

class ListMessagePackWritable(l: List[Int]) extends MessagePackWritable {
  var instance = l

  type T = List[Int]

  def this() = this(null)

  def unpack(raw: Array[Byte]) = ScalaMessagePack.read[T](raw)
}