classの引数でずらずらーと書きたくないでござる！つhttp://d.hatena.ne.jp/alpha_neet/20120213/1329157666

01_class.scala

case class Idenshi(id: Int)

class Animal(val i: Idenshi)
class Dog(i: Idenshi) extends Animal(i)
class Tiwawa(i: Idenshi) extends Dog(i)

val a = new Tiwawa(Idenshi(3))
println(a.i)

/**
output:
Idenshi(3)
*/

02_trait.scala

case class Idenshi(id: Int)  

trait Animal { val i: Idenshi }
trait Dog extends Animal
trait Tiwawa extends Dog

val a = new Tiwawa { val i = Idenshi(3) }
println(a.i)

/**
output:
Idenshi(3)
*/

03_cloneable.scala

case class Idenshi(id: Int)

@cloneable
trait Animal {
  val i: Idenshi
  
  override def hashCode = i.hashCode
  override def equals(that: Any): Boolean = that match {
    case x: Animal => i == x.i
    case _ => false
  }
  override def clone: this.type = super.clone.asInstanceOf[this.type]
  override def toString = i.toString
}

trait Dog extends Animal
trait Tiwawa extends Dog

def create(id: Int): Tiwawa = new Tiwawa { val i = Idenshi(id) }

val animals = scala.collection.mutable.Set[Animal]()

val a = create(3)
val a2: Tiwawa = a.clone
assert(a == a2)
assert(a == create(3))
println(a.i)

// 同じと認識されるので何度 id=3 を足しても増えない
animals += create(3)
animals += create(3)
animals += create(4)
animals += create(3)
animals += create(3)

println(animals)
assert(animals.size == 2)

animals -= create(3)
println(animals)
assert(animals.size == 1)

/**
output:
Idenshi(3)
Set(Idenshi(4), Idenshi(3))
Set(Idenshi(4))
*/