use of the scala.Equals trait

equals symmetry is not respected.scala

trait Car {
  val doorsNb: Int

  override def equals(a: Any) = {
    a match {
      case c: Car => doorsNb == c.doorsNb
      case other  => false 
    }
  }
}

trait Ferrari extends Car {
  val doorsNb = 2
  val speed = 300

  override def equals(a: Any) = {
    a match {
      case c: Ferrari => super.equals(c) && speed == c.speed 
      case other  => false 
    }
  }
}

val c = new Car { val doorsNb = 2 }
val f = new Ferrari {}

// true
c == f
// false, meaning that equals is not symmetric
f == c

equals symmetry is respected.scala

trait Car extends scala.Equals {
  val doorsNb: Int

  // define the canEqual method
  def canEqual(a: Any) = a.isInstanceOf[Car]

  override def equals(a: Any) = {
    a match {
      // make sure we can compare the 2 objects
      case c: Car => c.canEqual(this) && doorsNb == c.doorsNb
      case other  => false 
    }
  }
}

trait Ferrari extends Car {
  val doorsNb = 2
  val speed = 300

  // redefine the canEqual method so that only equality between
  // Ferraris is allowed
  override def canEqual(a: Any) = a.isInstanceOf[Ferrari]

  override def equals(a: Any) = {
    a match {
      case c: Ferrari => c.canEqual(this) && super.equals(c) && speed == c.speed 
      case other  => false 
    }
  }
}

val c = new Car { val doorsNb = 2 }
val f = new Ferrari {}

// false
c == f
// false as well
f == c

// true (just a check...)
f == f