[HELP] Removing objects from an ArrayList!

[bpx95]

I've had this issue before with other programs, but I've never found a solution.

for(Bullet bu: buArray){
			bu.moveX();
			bu.moveY();
			if(!bu.getVisible()){
				buArray.remove(buArray.indexOf(bu));
			}
		}

This is the part of my code where I have an issue. Everything works fine up until I try to remove the bu object from buArray. Even if I replace "buArray.remove(buArray.indexOf(bu));" with a System.out.println("test") test, it works perfectly. But when I do as shown above, I get this:

Exception in thread "AWT-EventQueue-0" java.util.ConcurrentModificationException
	at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372)
	at java.util.AbstractList$Itr.next(AbstractList.java:343)
	at Panel.actionPerformed(Panel.java:68)
	at javax.swing.Timer.fireActionPerformed(Timer.java:291)
	at javax.swing.Timer$DoPostEvent.run(Timer.java:221)
	at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
	at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:678)
	at java.awt.EventQueue.access$000(EventQueue.java:86)
	at java.awt.EventQueue$1.run(EventQueue.java:639)
	at java.awt.EventQueue$1.run(EventQueue.java:637)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
	at java.awt.EventQueue.dispatchEvent(EventQueue.java:648)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:296)
	at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:211)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:201)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:196)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:188)
	at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

I googled the error, but couldn't find anything that I could understand and helps me.

[sunde887]

I could be wrong and if I am, perhaps I will learn something as well. What happens if you use a standard for loop instead of a for each loop?

Here is the page on that exception: ConcurrentModificationException (Java 2 Platform SE 5.0)

It says it can be caused by a fail fast iterator which I think the for each loop may be using. I could be wrong but this is an easy change to check.

[bpx95]

Whoa, thanks! It worked.

[pbrockway2]

The for loop "for(Bullet bu: buArray){" walks along the array. And in the course of this you remove part of the array which you are traversing. This is a bit like removing the floor on which you are walking!

Both ArrayList and LinkedList (and maybe others) provide the following caution: "The iterators returned by this class's iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future."

The for loop uses such an iterator, hence the exception. What you should do is obtain your own iterator - with the iterator() method of the array - and use it to walk along the array. When you want to remove an element use the iterator's remove() method. (Note that this remove() method is actually optional.)

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class RemoveEg {
	public static void main(String[] args) {
		List<String> list = new ArrayList<String>();
		list.add("");
		list.add("foo");
		list.add("");
		list.add("bar");
		System.out.println(list);
		
			// remove the empty strings
		//for(String str :list) {
		//	if(str.isEmpty()) {
		//		list.remove(str);
		//	}
		//}
		Iterator<String> it = list.iterator();
		while(it.hasNext()) {
			String str = it.next();
			if(str.isEmpty()) {
				it.remove();
			}
		}
		System.out.println(list);
	}
}

[Edit] Slow ;(

[sunde887]

Your explanation was good pbrock, mine was more of an assumption that turned out to be correct, yours verified my thoughts and I did learn something, thanks. I always thought for each loop used iterators but wasnt entirely sure.

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