HashCode in class classes in Scala

Question

I read that the Scala 'a case class construct automatically generates an equals and hashCode . What does the generated code look like?

Answer 1

As my professor said, only the code tells the truth! So just take a look at the code that is created for:

 case class A(i: Int, s: String) 

We can instruct the Scala compiler to show us the generated code after different phases, here after typechecker:

 % scalac -Xprint:typer test.scala [[syntax trees at end of typer]]// Scala source: test.scala package <empty> { @serializable case class A extends java.lang.Object with ScalaObject with Product { .. override def hashCode(): Int = ScalaRunTime.this._hashCode(A.this); ... override def equals(x$1: Any): Boolean = A.this.eq(x$1).||(x$1 match { case (i: Int,s: String)A((i$1 @ _), (s$1 @ _)) if i$1.==(i).&&(s$1.==(s)) => x$1.asInstanceOf[A].canEqual(A.this) case _ => false }); override def canEqual(x$1: Any): Boolean = x$1.$isInstanceOf[A]() }; } 

So you can see that the calculation of the hash code has been delegated to ScalaRunTime._hashCode , and equality depends on the equality of the case 'class.

Answer 2

The generated hashCode simply calls scala.runtime.ScalaRunTime._hashCode , which is defined as:

 def _hashCode(x: Product): Int = { val arr = x.productArity var code = arr var i = 0 while (i < arr) { val elem = x.productElement(i) code = code * 41 + (if (elem == null) 0 else elem.hashCode()) i += 1 } code } 

So you get elem1 * 41**n + elem2 * 41**(n-1) .. elemn * 1 , where n is the arity of your case class, and elemi are the members of this case class.

Answer 3

Remember that the previous answers on this question are a bit outdated in the hashCode part.

In scala 2.9 hashCode , the case classes use MurmurHash : reference .

MurmurHash produces a good avalanche effect, good distribution and supports the CPU .

Answer 4

Everything seems to have changed; using the example Mirko case class A(i: Int, s: String) I get:

 override <synthetic> def hashCode(): Int = { <synthetic> var acc: Int = -889275714; acc = scala.runtime.Statics.mix(acc, i); acc = scala.runtime.Statics.mix(acc, scala.runtime.Statics.anyHash(s)); scala.runtime.Statics.finalizeHash(acc, 2) }; 

and

 override <synthetic> def equals(x$1: Any): Boolean = A.this.eq(x$1.asInstanceOf[Object]).||(x$1 match { case (_: A) => true case _ => false }.&&({ <synthetic> val A$1: A = x$1.asInstanceOf[A]; A.this.i.==(A$1.i).&&(A.this.s.==(A$1.s)).&&(A$1.canEqual(A.this)) })) };