HashSet.remove() and Iterator.remove() not working

Question

I\'m having problems with Iterator.remove() called on a HashSet.

I\'ve a Set of time stamped objects. Before adding a new item to the Set, I loop through the set, i

Answer 1

It's almost certainly the case the hashcodes don't match for the old and new data that are "equals()". I've run into this kind of thing before and you essentially end up spewing hashcodes for every object and the string representation and trying to figure out why the mismatch is happening.

If you're comparing items pre/post database, sometimes it loses the nanoseconds (depending on your DB column type) which can cause hashcodes to change.

Answer 2

You should all be careful of any Java Collection that fetches its children by hashcode, in the case that its child type's hashcode depends on its mutable state. An example:

HashSet<HashSet<?>> or HashSet<AbstaractSet<?>> or HashMap variant:

HashSet retrieves an item by its hashCode, but its item type is a HashSet, and hashSet.hashCode depends on its item's state.

Code for that matter:

HashSet<HashSet<String>> coll = new HashSet<HashSet<String>>();
HashSet<String> set1 = new HashSet<String>();
set1.add("1");
coll.add(set1);
print(set1.hashCode()); //---> will output X
set1.add("2");
print(set1.hashCode()); //---> will output Y
coll.remove(set1) // WILL FAIL TO REMOVE (SILENTLY)

Reason being is HashSet's remove method uses HashMap and it identifies keys by hashCode, while AbstractSet's hashCode is dynamic and depends upon the mutable properties of itself.

Answer 3

I'm not up to speed on my Java, but I know that you can't remove an item from a collection when you are iterating over that collection in .NET, although .NET will throw an exception if it catches this. Could this be the problem?

Answer 4

Have you tried something like

boolean removed = allResults.remove(oldData)
if (!removed) // COMPLAIN BITTERLY!

In other words, remove the object from the Set and break the loop. That won't cause the Iterator to complain. I don't think this is a long term solution but would probably give you some information about the hashCode, equals and equalsData methods