Scala case class with cached hashCode

Question

I was under the impression that the hashCode of a Scala case class was solely determined by its fields. Consequently, I thought that caching a hashCode was safe for immutable case classes.

Seems like I am wrong:

case class Foo(s: String) {
  override val hashCode: Int = super.hashCode()
}

val f1 = Foo("foo")
val f2 = Foo("foo")

println(f1.hashCode == f2.hashCode) // FALSE

Could anyone explain what's going on here, please?

Addendum – Just for comparison:

case class Bar(s: String)

val b1 = Bar("bar")
val b2 = Bar("bar")

println(b1.hashCode == b2.hashCode) // TRUE

Answer 1

Not sure about the value of this, but you can inline the implementation of ScalaRuntime._hashCode:

case class Foo(s: String) {
  override val hashCode: Int = scala.util.hashing.MurmurHash3.productHash(this)
}

Answer 2

Not sure what you mean by "cached hasCode", but ... You've override hashCode with custom solution that built from Object, that is why you're getting false. Remove this override and you'll get expected value.