Re: iterators

Tom Anderson <>
Thu, 6 Aug 2009 13:51:57 +0100
On Wed, 5 Aug 2009, Daniel Pitts wrote:

Eric Sosman wrote:

Daniel Pitts wrote:

What I would *love* is an iterator that can be made smart enough to not
throw ConcurrentModificationException if the modification can be proven to
be non-conflicting (such as appending to a list, or removing a node from a
linked-list, which is not any node being pointed to by the iterator.)

    Can you give some examples of situations where you've wished you
had such a thing?

I have a simulation involving robots which can shoot at each other. Once a
robot is destroyed, it is removed from the list. At the time that damage is
dealt, I am already iterating through that list.

This means that I must go through the list afterward and remove the dead
robots, instead of removing them as they die.

This is a simplified example. The list itself may contain other objects (such
as missiles, mines, etc...) each of which may cease to exist and/or inflict
damage at any time.

I had exactly that problem many years ago, only it was spaceships instead
of robots.

How about something like:

Collection<Thing> things; // robots, missiles, mines, etc

void carryOutATurn() {
  List thingsToDo = new LinkedList(things);
  while (!thingsToDo.isEmpty()) {
  Thing next = thingsToDo.remove(0);
  Collection<Thing> casualties = next.takeTurn();
  if (!casualties.isEmpty()) {

This does involve creating and throwing away a linked list of everything
in the universe on every turn, and potentially a lot of little casualty
lists too - although these can be emptySet or singleton sets from
Collections, which are very cheap.

It would be straightforward to extend this to handle new things (a
newly-fired missile, etc) as well.

An alternative to the casualty list would be to create and pass in a
little callback object for deleting things:

class Undertaker {
  public void kill(Thing t) {


If it ain't broke, open it up and see what makes it so bloody special.

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