How to handle lists of multiple entities being born, drawn and killed simultaneously without conflicts?

I'll try to keep it short, these is the situation:

• Creature Class : x y coordinates, moves around randomly and shoots Projectiles

• Projectile Class : x y coordinates, moves in a straight line and dies when impacting Creatures

• Arena Class : JPanel which iterates over the lists of creatures and projectiles and calls their draw() method.

If you dont't wish to read any further this is my main question: What is the best way to handle Projectiles being created and inserted in the projectile list of the JPanel? Is having such a list is even a good idea in the first place?

Should I create a mutex semaphore which acquires a permit whenever the JPanel is iterating over the projectile list and releases the permit once it's done? I'm running into trouble when the creatures decide to insert a new bullet while iterating over the list. If this is the case, what is the best way of letting the creature class access this semaphore?

Also I would like to add that I usually use this tactic to avoid concurrent modifications in similar situations:

1. List of items ready to be added
2. List of "active" items
3. List of items ready to be removed

And then:

if (!toBeAdded.isEmpty()) { activeItems.addAll(toBeAdded); activetoBeAdded.clear();
}
if(!toBeRemoved.isEmpty()) { activeItems.removeAll(toBeRemoved); toBeRemoved.clear();
} 

But this of course doesn't work in this particular situation because I am just "moving" the problem from the active list to the toBeAdded one, I still get a concurrent modification if my creatures try and add bullets to the toBeAdded list while it's being used.