Map of with object references as keys?

Question

I have an object with stores information about specific instances. For that, i would like to use a Map, but as the keys are not by-reference (they aren\'t, rig

Answer 1

EDIT: Based on clarifications to the question, you can create your own Map implementation, and override elemEquals().

The original implementation (in HashMap)

protected def elemEquals(key1: A, key2: A): Boolean = (key1 == key2)

Change this to:

protected def elemEquals(key1: A, key2: A): Boolean = (key1 eq key2)

class MyHashMap[A <: AnyRef, B] extends scala.collection.mutable.HashMap[A, B] {
  protected override def elemEquals(key1: A, key2: A): Boolean = (key1 eq key2)
}

Note that to use eq, you need to restrict the key to be an AnyRef, or do a match in the elemEquals() method.

case class Foo(i: Int)
val f1 = new Foo(1)
val f2 = new Foo(1)
val map = new MyHashMap[Foo, String]()
map += (f1 -> "f1")
map += (f2 -> "f2")
map.get(f1) // Some(f1)
map.get(f2) // Some(f2)

-- Original answer

Map works with hashCode() and equals(). Have you implemented equals() correctly in your obejcts? Note that in Scala, == gets translated to a call to equals(). To get the same behaviour of == in Java, use the Scala operator eq

case class Foo(i: Int)
val f1 = new Foo(1)
val f2 = new Foo(1)
f1 == f2 // true
f1.equals(f2) // true
f1 eq f2 // false

val map = new MyHashMap (f1 -> "f1", f2 -> "f2")
map.get(f1) // Some("f2")
map.get(f2) // Some("f2")

Here, the case class implements equals() to be object equivalence, in this case:

f1.i == f1.i

You need to override equals() in your objects to include object equality, i.e something like:

override def equals(o: Any) = { o.asInstanceOf[AnyRef] eq this }

This should still work with the same hashCode().