hashCode in case classes in Scala

问题:

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

回答1:

As my professor used to say, only the code tells the truth! So just take a look at the code that is generated for:

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

We can instruct the Scala compiler to show us the generated code after the different phases, here after the 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 is delegated to ScalaRunTime._hashCode and the equality depends on the equality of the case class' members.

回答2:

The generated hashCode just 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 what you get is elem1 * 41**n + elem2 * 41**(n-1) .. elemn * 1, where n is the arity of your case class and elemi are the members of that case class.

回答3:

Please be aware that the previous answers on this question are a bit outdated on the hashCode part.

As of scala 2.9 hashCode for case classes uses MurmurHash: link.

MurmurHash produces good avalanche effect, good distribution and is CPU friendly.

回答4:

Looks like things have changed; using Mirko's example 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))
    }))
  };