Scala semantics of equals/hashCode for case classes with traits

Question

I am a newcomer to Scala. In 2.7.7, the following code

abstract class C
case class CC() extends C

trait T

val c1 = CC()
val c2 = new CC() with T
println(c1.hashCode == c2.hashCode,c1 equals c2)

prints

(false,true)

whereas I would have expected

(false,false)

What am I missing? Thanks in advance.

Answer 1

This behaviour is expected for equals since the Scala compiler overrides the equals method for case classes.

I am however unsure why the hashCode is different in Scala 2.7.7. Your example results in (true, true) using Scala 2.8.

Answer 2

Case class equality (particularly in Scala 2.8) equality and hash codes are based upon tuple and/or product equality, and do not currently take the class into account. There was a recent discussion on this matter on the scala-debate mailing list here: http://old.nabble.com/Possible-Collision-Issue-with-Product.hashCode-td27026790.html

For what it's worth, here's what it currently looks like in 2.8:

Welcome to Scala version 2.8.0.Beta1-RC6 (Java HotSpot(TM) Client VM, Java 1.6.0_16).
Type in expressions to have them evaluated.
Type :help for more information.

scala> abstract class C
defined class C

scala> case class CC() extends C
defined class CC

scala> trait T
defined trait T

scala> val c1 = CC()
c1: CC = CC()

scala> val c2 = new CC() with T
c2: CC with T = CC()

scala> println(c1.hashCode == c2.hashCode,c1 equals c2)
(true,true)