ConcurrentModificationException in a loop

Question

I'm running this code on a separate Thread (not the UI Thread)

strains = new ArrayList<Strain>();
for (Breeder b : breeders) {
   for (Strain s : b.getStrains()) {
        strains.add(s);
    }
}

It sometime causes ConcurrentModificationException. I know that i can't add or remove object from a Collection that i'm iterating, but in this snippet I'm not iterating on strains. Where i'm wrong? Thanks

poitroae · Accepted Answer · 2013-03-10 16:04:58Z

1

Synchronize your access. This kind of freezes the current breeder object, so that it will not be modified in any other thread. Methods that try to do this will be blocked. Blocked means that they just wait (like System.sleep(x)), until the synchronized block in the other thread has been processed.

synchronized(breeders) {
    for (Breeder b : breeders) {
       for (Strain s : b.getStrains()) {
            strains.add(s);
        }
    }
}

Make sure that you also put synchronized around the other threads' access to breeders.

answered Mar 10, 2013 at 16:04

poitroae

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2 Comments

JB Nizet Over a year ago

That could be the issue. Or it could be the loop over the breeder strains. But synchronizing the access won't change anything unless all the threads do the same thing every time they access the collection. It should be encapsulated inside a class.

TheModularMind Over a year ago

I tried synchronizing the breedes acces, but no help.. The exception is fired here "for (Breeder b : breeders)"

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