ConcurrentModificationException (Java)

Question

Exception in thread "main" java.util.ConcurrentModificationException
Squash the PC dirties the room Violet. The room's state is now dirty
Lily the animal growls
The Animal Lily left the room and goes to Green through the west door.
        at java.util.HashMap$HashIterator.nextEntry(HashMap.java:793)
        at java.util.HashMap$KeyIterator.next(HashMap.java:828)
        at homework5.Room.critReactRoomStateChange(Room.java:76)
        at homework5.PC.play(PC.java:121)
        at homework5.Main.main(Main.java:41)
Java Result: 1

That is the error I receive.

My method looks like

public void critReactRoomStateChange(String command, PC pc) {
    Creature temp = null;
    Iterator iterator = getCreatures().keySet().iterator();
    while (iterator.hasNext()) {
        String names = iterator.next().toString();
        if (!(getCreatures().get(names) instanceof PC)) {
            temp = getCreatures().get(names);
            if (temp != null) {
                temp.reactStateChange(command, pc);
                temp.checkNewRoom();
            }
        }
    }
} 

So what I understand is this means I'm changing the size of the iterator before it is finished and this is the error you get. This is true as one of the reactStateChange is for an object to be removed out of the hashMap. How do I do this safely so that when I remove something it lets the Iterator know ahead of time so I can avoid this error. Thanks in advance. If more details are needed I'd be glad to meet your requests.

Answer 1

The only safe way to remove an element from an underlying collection and continue the iteration is to use the remove() method of the Iterator. This removes the last element returned by the next() method of the Iterator.

In your case, it appears that this would require passing the Iterator to the method that performs the modification (or make it an instance field, like the Map object is already).

Answer 2

You remove it using iterator.remove().

Answer 3

Another option is to use ConcurrentHashMap which doesn't have this issue. You can use this as a drop in replacement and you don't need to change the rest of the code.