Remove elements from an Arraylist if they are present in another one without raising ConcurrentModificationException

Question

Here is the code:

Ledger obj = null;
MyUtilPojo obj1 = null;
Iterator it = toList.iterator();
while (it.hasNext()) {
    obj = (Ledger) it.next(); //after first iteration next here produce an error
    Iterator it1 = moreToList.iterator();
    while (it1.hasNext()) {
        obj1 = (MyUtilPojo) it1.next();
        if (obj.getId() == obj1.getKey()) {
            toList.remove(obj);                                
        }
    }
}

This raise an error ConcurrentModificationException , can someone help?

Answer 1

Something like that should work (constructs the new content for toList in a temp list):

final List<Ledger> target = new ArrayList<Ledger>();
for (final Ledger led : toList) {
    for (final MyUtilPojo mup : moreToList) {
        if (led.getId() != mup.getKey()) { // beware of !=
            target.add(led);
        }
    }
}

toList = target;

Answer 2

ConcurrentModificationException occurs when you modify the list (by adding or removing elements) while traversing a list with Iterator.

    Ledger obj = null;
    MyUtilPojo obj1 = null;
    List thingsToBeDeleted = new ArrayList();

    Iterator it = toList.iterator();
    while (it.hasNext()) {
        obj = (Ledger) it.next();
        Iterator it1 = moreToList.iterator();
        while (it1.hasNext()) {
            obj1 = (MyUtilPojo) it1.next();
            if (obj.getId() == obj1.getKey()) {
                thingsToBeDeleted.add(obj);    // put things to be deleted                            
            }
        }
    }
    toList.removeAll(thingsToBeDeleted);