Scala HashMap.contains() returns false, although key already exists

Question

I use a mutuable Scala Map and check with contains() if a key already exists.

In one instance it returns false, although the key (with the same hashCode(), verified it in the debugger) already exists. The objects in the map are deserialized by using Java's default serialization method.

My bet is that it's somehow related to the serialization/deserialization, since without it, I don't have the issue.

What further steps can I do, to find the root cause ?

HashMap

 private val knowledgeRecords = collection.mutable.Map[Entity, KnowledgeRecord]().empty

Test if key already exists:

if (knowledgeRecords.contains(entity))

HashCode function of Entity

override def hashCode(): Int = {
if (id == null)
  0
else
  id.hashCode

} }

equal functions of Entity

  def canEqual(other: Any): Boolean = other.isInstanceOf[Entity]

  override def equals(other: Any): Boolean = other match {
   case that: Entity =>
    (that canEqual this) && id == that.id
   case _ => false

}

Id is defined in the constructor of Entity

class Entity(var id: String)

Edit: If I call after the deserilization

val cloneValuesOfMap = knowledgeRecords.values.toList
knowledgeRecords.clear()
cloneValuesOfMap.foreach(item=>knowledgeRecords.put(item.entity, item))

then contains() works. I don't know why.

I've debugged a litte the contains function. In HashTable there is the function

private[this] def findEntry0(key: A, h: Int): Entry = {
var e = table(h).asInstanceOf[Entry]
while (e != null && !elemEquals(e.key, key)) e = e.next
e

The keys are in the bucket table(0). But in the 'deserialization' usecase this function is called with h = 2, thus e gets null.

please show a [Minimal, Complete, and Verifiable example](https://stackoverflow.com/help/mcve) - edit the post to include a code example. — Tzach Zohar, Dec 01 '17 at 17:08
@janos that does not help. Scala differs here from Java, see https://stackoverflow.com/questions/26297473/how-to-compare-two-strings-in-scala — Marco, Dec 01 '17 at 17:54
Just because the hash code is the same doesn't mean the objects are the same, it just means they'll go in the same "bin" in the hash map. Check the equals of the two objects. — ameer, Dec 01 '17 at 18:43
Hmm, the fact that `id` is mutable makes me suspicious. Relying on mutable fields for hashcode and equality is dangerous. — Yuval Itzchakov, Dec 01 '17 at 18:44
@Hawk66 can you add a more complete example with how you're using these methods — ameer, Dec 01 '17 at 18:45
@YuvalItzchakov it's to facilitate cloning. The id does not change in the deserialization process. — Marco, Dec 01 '17 at 19:34
@ameer maybe my edit helps. It is a little tough to show a complete example. — Marco, Dec 01 '17 at 19:34
@Hawk66 Does it change afterwards? Maybe something updates the `id` after storing it in the hashset? — Yuval Itzchakov, Dec 02 '17 at 07:06

Scala HashMap.contains() returns false, although key already exists

0 Answers0