Why Iterator.next() throws ConcurrentModificationException [duplicate]

Question

Strangly enough, this small piece of code throws the above mentioned Exception. Also, looking at code posted around the web this seems to be correct:

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

public class IteratorTest {

    ArrayList<Integer> arr = new ArrayList<Integer>();

    Iterator i = arr.iterator();

    public void show() {
        arr.add(2);
        arr.add(5);
        arr.add(9);

        while(i.hasNext()){
            System.out.println(i.next());
        }
    }
}

Any advice? Thanks

Answer 1

This call:

Iterator i=arr.iterator();

should be after you've done all the writes into your ArrayList.

So in your code do this just before you start iterating like this:

Iterator i=arr.iterator();
while(i.hasNext()) {
...
}

Answer 2

It's because you've modified the backing list between getting the Iterator via iterator() and calling next().

The typical usage of an Iterator is:

for (Iterator<Integer> iter=arr.iterator(); iter.hasNext(); ) {
    Integer element = iter.next();
}

Or better yet, use the new for-each loop:

for (Integer element: arr) {
}

Make sure to to perform additions to the Collection outside of the loop.