Traits

Similar to interfaces in Java, traits are used to define object types by specifying the signature of the supported methods. Unlike Java, Scala allows traits to be partially implemented; i.e. it is possible to define default implementations for some methods. In contrast to classes, traits may not have constructor parameters. Here is an example:

trait Similarity {
  def isSimilar(x: Any): Boolean
  def isNotSimilar(x: Any): Boolean = !isSimilar(x)
}

This trait consists of two methods isSimilar and isNotSimilar. While isSimilar does not provide a concrete method implementation (it is abstract in the terminology of Java), method isNotSimilar defines a concrete implementation. Consequently, classes that integrate this trait only have to provide a concrete implementation for isSimilar. The behavior for isNotSimilar gets inherited directly from the trait. Traits are typically integrated into a class (or other traits) with a mixin class composition:

class Point(xc: Int, yc: Int) extends Similarity {
  var x: Int = xc
  var y: Int = yc
  def isSimilar(obj: Any) =
    obj.isInstanceOf[Point] &&
    obj.asInstanceOf[Point].x == x
}
object TraitsTest extends Application {
  val p1 = new Point(2, 3)
  val p2 = new Point(2, 4)
  val p3 = new Point(3, 3)
  println(p1.isNotSimilar(p2))
  println(p1.isNotSimilar(p3))
  println(p1.isNotSimilar(2))
}

Here is the output of the program:

false
true
true

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