Collections#synchronizedList - what am I doing wrong?

[Stephen Neal]

Can someone tell me why the code below throws a ConcurrentModificationException (at least under Java 6)? It was my understanding that because the code takes a lock on the synchronized list prior to iterating the remove inside that block should not be allowed but when I run this it is (and the print out shows 2 items in the list) and the next iteration then throws a ConcurrentModificationException.

There are two flavours, one using the for loop and one with an iterator. Both produce the same result. I am scratching my head here. I would expected a deadlock which itself is not desirable but at this stage I am just confused about the remove being allowed.

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class AnyTest {

    /**
     * Testing ground for anything
     * 
     * @param args
     */
    public static void main(String[] args) {
        List<Object> synchedList = Collections.synchronizedList(new ArrayList<Object>());
        synchedList.add(new Object());
        synchedList.add(new Object());
        synchedList.add(new Object());
        synchronized (synchedList) {
            for (Object o : synchedList) {
                synchedList.remove(o);
                System.out.println("size=" + synchedList.size());
            }
        }
        System.out.println("done");
    }

    // /**
    // * Testing ground for anything
    // *
    // * @param args
    // */
    // public static void main(String[] args) {
    // List<Object> synchedList = Collections.synchronizedList(new ArrayList<Object>());
    // synchedList.add(new Object());
    // synchedList.add(new Object());
    // synchedList.add(new Object());
    // synchronized (synchedList) {
    // Iterator<Object> itr = synchedList.iterator();
    // while (itr.hasNext()) {
    // Object next = itr.next();
    // synchedList.remove(next);
    // System.out.println("size=" + synchedList.size());
    // }
    // }
    // System.out.println("done");
    // }
}

[Stephen Neal]

Just answered my own question. Because its executing within the synchronized block.

[shishir guptaa]

what i can see the problem is

list.remove(o);

You should always use iterator to modify the list, like as below

ListIterator itr = list.listIterator();

while(itr.hasNext()){
iitr.remove();
System.out.println(list.size());
}

This will work for you.

[Stephen Neal]

Thanks for the reply but you have misunderstood my query.

As per my previous post I have found the answer.