What are the pros of using traits over abstract classes?

Can someone please explain traits in Scala? What are the advantages of traits over extending an abstract class?

Solution 1:

The short answer is that you can use multiple traits -- they are "stackable". Also, traits cannot have constructor parameters.

Here's how traits are stacked. Notice that the ordering of the traits are important. They will call each other from right to left.

class Ball {
  def properties(): List[String] = List()
  override def toString() = "It's a" +
    properties.mkString(" ", ", ", " ") +
    "ball"
}

trait Red extends Ball {
  override def properties() = super.properties ::: List("red")
}

trait Shiny extends Ball {
  override def properties() = super.properties ::: List("shiny")
}

object Balls {
  def main(args: Array[String]) {
    val myBall = new Ball with Shiny with Red
    println(myBall) // It's a shiny, red ball
  }
}

Solution 2:

This site gives a good example of trait usage. One big advantage of traits is that you can extend multiple traits but only one abstract class. Traits solve many of the problems with multiple inheritance but allow code reuse.

If you know ruby, traits are similar to mix-ins

Solution 3:

package ground.learning.scala.traits

/**
 * Created by Mohan on 31/08/2014.
 *
 * Stacks are layered one top of another, when moving from Left -> Right,
 * Right most will be at the top layer, and receives method call.
 */
object TraitMain {

  def main(args: Array[String]) {
    val strangers: List[NoEmotion] = List(
      new Stranger("Ray") with NoEmotion,
      new Stranger("Ray") with Bad,
      new Stranger("Ray") with Good,
      new Stranger("Ray") with Good with Bad,
      new Stranger("Ray") with Bad with Good)
    println(strangers.map(_.hi + "\n"))
  }
}

trait NoEmotion {
  def value: String

  def hi = "I am " + value
}

trait Good extends NoEmotion {
  override def hi = "I am " + value + ", It is a beautiful day!"
}

trait Bad extends NoEmotion {
  override def hi = "I am " + value + ", It is a bad day!"
}

case class Stranger(value: String) {
}

Output :

List(I am Ray
, I am Ray, It is a bad day!
, I am Ray, It is a beautiful day!
, I am Ray, It is a bad day!
, I am Ray, It is a beautiful day!
)