Scala: Is my `equals` method correct?

Question

I wrote an equals method for class A .

class A(x: Int, s: String) {
  override def equals(that: Any) = that match {
    case a: A => this.x == a.x && this.s == a.s
    case _    => false
  } 
}

Is it correct?

Answer 1

Yes that is correct. However you should also override the hashCode method to reflect equality. That is, if you have two instances a and b where a == b , you should ensure that a.hashCode == b.hashCode .

The simplest way to achieve the two is to use a case-class:

case class A(private val x: Int, private val s: String)

This gives you correct equals and hashCode for "free".

---

If you plan for the possibility that there are sub-classes of A , you may look at Odersky's idea of canEqual (also here ).

Answer 2

a better approach is defining on companion object of A an Ordering[A] instance.

object A{
    implicit object aOrdering extends Ordering[A]{
        override def compare(x:A,y:A):Int = ......
    }
}

than on you equals method you should write:

override def equals(that: Any) = that match {
  case a: A => {
        val ordering = implicitly[Ordering[A]] 
        ordering.compare(this,a) == 0 
 }
  case _    => false
} 

this way you can reuse your code ie scala collections and other places that you need Ordering for this class.