Traits and Composition

Mixing Traits

Combine two traits into one class.

multiple-traits A class can mix in several traits with `extends` and `with`. It then offers the methods from every trait it combines.

Mixing Traits

extra
MultipleTraits.scala
Replay: real traced execution (multi-file project)
trait Named {
  def label: String = "item"
}

trait Counted {
  def count: Int = 1
}

class Box extends Named with Counted {
  override def toString: String = "[box]"
}

object Main {
  def main(args: Array[String]): Unit = {
    val extra = 2
    val box = new Box()
    val total = box.count + extra

    println("label=" + box.label)
    println("count=" + box.count)
    println("total=" + total)
  }
}

trait Named {
  def label: String = "item"
}

trait Counted {
  def count: Int = 1
}

class Box extends Named with Counted {
  override def toString: String = "[box]"
}

object Main {
  def main(args: Array[String]): Unit = {
    val extra = 0
    val box = new Box()
    val total = box.count + extra

    println("label=" + box.label)
    println("count=" + box.count)
    println("total=" + total)
  }
}

trait Named {
  def label: String = "item"
}

trait Counted {
  def count: Int = 1
}

class Box extends Named with Counted {
  override def toString: String = "[box]"
}

object Main {
  def main(args: Array[String]): Unit = {
    val extra = 5
    val box = new Box()
    val total = box.count + extra

    println("label=" + box.label)
    println("count=" + box.count)
    println("total=" + total)
  }
}

  1. extra ← 2, box ← [box], total ← 3

    13object Main {14  def main(args: Array[String]): Unit = {15    val extra→ 2 = 2 //@extra=0, 516    val box→ [box] = new Box()17    val total→ 3 = box[box].count + extra21819    println("label=" + box.labelitem)20    println("count=" + box.count1)21    println("total=" + total3)22  }23}
    outputlabel=item
count=1
total=3

  1. extra ← 0, box ← [box], total ← 1

    13object Main {14  def main(args: Array[String]): Unit = {15    val extra→ 0 = 016    val box→ [box] = new Box()17    val total→ 1 = box[box].count + extra01819    println("label=" + box.labelitem)20    println("count=" + box.count1)21    println("total=" + total1)22  }23}
    outputlabel=item
count=1
total=1

  1. extra ← 5, box ← [box], total ← 6

    13object Main {14  def main(args: Array[String]): Unit = {15    val extra→ 5 = 516    val box→ [box] = new Box()17    val total→ 6 = box[box].count + extra51819    println("label=" + box.labelitem)20    println("count=" + box.count1)21    println("total=" + total6)22  }23}
    outputlabel=item
count=1
total=6