Re: Collection implementations and fail-fast iterator problems.

Patricia Shanahan <>
Sat, 03 Nov 2007 19:01:28 -0700
Daniel Pitts wrote:

Patricia Shanahan wrote:

Daniel Pitts wrote:

Roedy Green wrote:

On Fri, 02 Nov 2007 14:06:09 -0700, Daniel Pitts
<> wrote, quoted or indirectly
quoted someone who said :

I'd like to avoid having to keep track of "to-be-deleted" and
"to-be-added" elements, but I don't see an elegant way to handle
both those cases without getting a ConcurrentModificationError.


The problem is that the element to remove isn't necessarily the
element that the iterator is pointing to. For example.
class ItemHolder {
  Collection<Item> items;
  public void doAllSomething() {
   for (Item item: items) {

class Item {
  ItemHolder parent;
  public void doSomething() {
    for (Item item: parent.items) {
       if (item.shouldBeRemovedNow()) {
    if (shouldAddNewItems()) {

This is the gist of what happens. As you can see, there are multiple
iterators to deal with.

A few questions:

1. Is the underlying Collection large? (That affects whether it is
reasonable to make a working copy).

2. Does it have to work with arbitrary Collections?

3. How should added items be handled? Should they be processed in later
inner iterations of the same outer loop? Should they be processed in the
same run of the outer loop?

4. Similar questions for deleted items, but that is a simpler problem
because of the option of marking an item to indicate it is not really


The lists aren't huge, but they are processed continuously. Basically,
the doAllSomething is called around 30-60 times a second.

Now that I think about it, items added in this case should be processed
on the next iteration only.

I suppose the best approach is to have a list of to-be-added, and a
marker for to-be-deleted. Then items.addAll(toBeAdded) can be used
after a look to delete the to-be-deleted.

As an alternative to actually adding the short-lived entries to the
collection, consider having your own Iterator. It would scan both the
underlying Collection, and the supplemental elements. It would have
support for putting things in the added list.

[You could wrap the Collection in a subclass that only overrides Iterator]


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