Reposting: Question regarding iteration over synchronized list

Question

[added proper markup]
I've got an A-Life program I work on for fun. I thought I understood (sorta) synchronized collections, but this puzzles me. The method body is as follows:
     * Create new animals from existing animals
     * @return The cohort of newborn animals
    public List<IAnimal> regenerate() {
        List<IAnimal> children = new ArrayList<IAnimal>();
        synchronized (animalList) {
            Iterator<IAnimal> it = animalList.iterator();          // must be in synchro block according to JDK notes
             while (it.hasNext()) {
                 IAnimal a = it.next();
                 if (a.isPregnant() && ((GeneModel.getInstance().getTicks() % getEnvironment().getAnimal().getReproCycle()) == 0)) {
                     IAnimal child = a.reproduce();
                     children.add(child);                    // newborns
//                     animalList.add(child);                    // the whole population  //THROWS CME!
             animalList.addAll(children);                                     // does not throw CME
        return children;
    }Animal list is a synchronized list (backed by and ArrayList). Note that I've synchronized on the list, yet adding children individually throws a ConcurrentModificationException (I did overwrite the add() method on the backing list, but I wouldn't think that would be a problem; it might be in that it iterates over itself... but synchronizing that doesn't help, either).
Anyhow, doing an addAll outside of the synchronization block works fine (also works if I loop through the children and add them at this point).
Is it my override of the add() method? From what I can see, the synchronized wrapper just adds a mutex to each access method in the backing list. Here's the method override:
      List<IAnimal> backingList =
            new LinkedList<IAnimal>() {
                 * Checks that we're not putting one animal on top of another. If we are, we generate a new animal and
                 * try again.
                 * @param   animal  The animal to be added
                 * @return  true
                public boolean add(IAnimal animal) {
                    boolean added = false;
outer:
                    do {
                        // synchronized (this) {                // DIDN'T HELP
                        Iterator<IAnimal> iAnimals = iterator();
                        while (iAnimals.hasNext()) {
                            //FIXME: this algorithm assumes square animals
                            Point existingAnimalLocation = iAnimals.next().getLocation();
                            double distance = existingAnimalLocation.distance(animal.getLocation());
                            if (distance < animal.getSize().getWidth()) {
                                animal = new Animal();
                                continue outer;
                        //}  //end unhelpful synchro block
                        super.add(animal);
                        added = true;
                    while (!added);
                    return added;
                } // end method add
            };

Answer 1

Jefferino wrote:
spoon_ wrote:
By the way that Iterators in Java are designed, you are not allowed to modify a list as you iterate over it.
Not true: Iterator.remove().
Edited by: Jefferino on Mar 20, 2008 2:24 AMOh yeah, you can modify it using the iterator's methods; but not using any of the collection's methods, like add().