java ArrayList中的ConcurrentModificationException

Question

I have the following code running on the only thread:

ArrayList<Ball> allBalls; 
ArrayList<Ball> toRemove = new ArrayList<Ball>();

for (Ball ball:allBalls) { //Exception thrown here
    ball.move();

    if (shouldRemove(ball)) {
        toRemove.add(ball);
    }
}               
allBalls.removeAll(toRemove);       
toRemove.removeAll(toRemove);

Sometimes it throws

java.util.ConcurrentModificationException
  at java.util.ArrayList$ArrayListIterator.next(ArrayList.java:576)

But when I replace this line

for (Ball ball:allBalls) {

by this

for (int i=0; i<allBalls.size();i++) {
    Ball ball = allBalls.get(i);

No exception is thrown.

Why?

Answer 1

Only reason you are getting this exception is that your collection is getting modified in loop/iteration (I think move method is modifying object which will get reflected in list).

To explain why you are getting exception in foreach but not in for :

foreach loop uses iterator internally. And collections are fail fast (immediately report failure), as a result of which next() method of iterator will throw exception if you collection is getting modified in the loop. However normal for loop does not use iterator , so that is reason you are not getting exception in that loop but there be other side effects of modifying in for loop like size getting changed or iteration becoming infinite.