Traits or classes with abstract type members are often used in combination with anonymous class instantiations. To illustrate this, we now look at a program which deals with a sequence buffer that refers to a list of integers:

trait Buffer {
  type T
  val element: T
}

abstract class SeqBuffer extends Buffer {
  type U
  type T <: Seq[U]
  def length = element.length
}

abstract class IntSeqBuffer extends SeqBuffer {
  type U = Int
}

object AbstractTypeTest1 extends App {
  def newIntSeqBuf(elem1: Int, elem2: Int): IntSeqBuffer =
    new IntSeqBuffer {
         type T = List[U]
         val element = List(elem1, elem2)
       }
  val buf = newIntSeqBuf(7, 8)
  println("length = " + buf.length)
  println("content = " + buf.element)
}

The return type of method newIntSeqBuf refers to a specialization of trait Buffer in which type Uis now equivalent to Int. We have a similar type alias in the anonymous class instantiation within the body of method newIntSeqBuf. Here we create a new instance of IntSeqBuffer in which type T refers to List[Int].