Why the ConcurrentModificationException?

Question

Hello my Java friends.
When executing the following code:
143        // Create the Clique's medium.
144        this.medium.addAll(ownerIdentity.getConnection().getMediaProfileNode().flattenMediaProfile());
145        for(MI i:candidateIdentities){
146            Set<SourceMediaProfile>memberMedium=i.getConnection().getMediaProfileNode().flattenMediaProfile();
147
148            for(SourceMediaProfile ownerMediumMediaProfile:this.medium){
149                if(memberMedium.contains(ownerMediumMediaProfile)){
150                    continue;
151                }
152                this.medium.remove(ownerMediumMediaProfile);
153            }
165        }I get the following excetption:
java.util.ConcurrentModificationException
        at java.util.HashMap$HashIterator.nextEntry(HashMap.java:810)
        at java.util.HashMap$KeyIterator.next(HashMap.java:845)
        at cliquespace.core.agentdevice.source.SourceClique.<init>(SourceClique.java:148)The flattenMediaProfile method has the following code:
114    public Set<SourceMediaProfile>flattenMediaProfile() {
115        Set<SourceMediaProfile>mps=new HashSet<SourceMediaProfile>();
116        mps.add(this);
117
118        MPN mpn=null;
119        try{
120            mpn=(MPN)this;
121            Collection<SourceMediaProfile<CS,?extends MediaProfileIdentifier,MPN,C,I,MI,P,OP,CL>>mpc=mpn.getParentMediaProfiles();
122            for(SourceMediaProfile p:mpc){
123                mps.addAll(p.flattenMediaProfile());
124            }
125        }catch(ClassCastException cce){/*Do nothing.*/}
126
127        return mps;
128    }Now, I can probably create arrays to do what I'm trying to do with these collections, but I think I might be doing something wrong with the collection objects, and, rather than remaining ignorant, I would like to take this opportunity to find out what that might be.
Your considered advice would be well received.
Thanks,
Owen.

Answer 1

Owen Thomas wrote:
jverd wrote:
Owen Thomas wrote:
One must assume that no response implies that there is no substantial difference, That's a pretty silly assumption.No it isn't.Yes, as a matter of fact it's totally ridiculous to think that the only reason someone doesn't respond to you is because you were correct. Believe it or not, people do have lives outside this forum, and proving something to you is not the center of our respective universes.
and that matters pertaining to whether one chooses to use an iterator in a while loop or a copy of a collection in a foreach loop are... trivial matters of style.There's also the fact that one creates a copy of the collection and the other does not. That could be a significant cost in time and space.Surely an iterator must also have a collection over which it iterates. Yeah, the collection that already exists. What you're doing is creating an additional one beyond that, which is totally unnecessary here.
If not, how does the creation of an iterator enable me to "iterate" over the collection when the size and content of the collection may change?Um, that's kind of the point of ConcurrentModificationException. If a modification occurs other than via the Iterator, then the Iterator's invariants may not hold, so that Iterator may not have an accurate picture of the state of the collection, which is why the exception is thrown.
Providing me with a link to an article that discusses this would be appreciated.The javadocs for Collection and Iterator would be a start. You'll notice they say that the Iterator iterates over the Collection, and does not say anything about making a copy of it.