Question

why following code throwing ConcurrentModificationException? Josh Bloch can avoid ConcurrentModificationException.

ArrayList<Integer> list=new ArrayList<Integer>();
list.add(100);
list.add(200);
list.add(300);
list.add(400);
for(Integer field : list) {    
    list.remove(field);
    list.add(200);
}       

Answer 1

You can’t use remove on the list while using the “for each” loop. Instead, you can use this to call remove on the iterator:

Iterator<Integer> iterator = list.iterator();
while(iterator.hasNext()) {
    Integer integer = iterator.next();

    // ...check if you want to remove this one...

    iterator.remove();
}

If you actually want to replace every value with “200”, or replace with some other value, it might make more sense to build up a new list:

List<Integer> newList = new ArrayList<Integer>();

Iterator<Integer> iterator = list.iterator();
while(iterator.hasNext()) {
    Integer integer = iterator.next();

    newList.add(integer);

    iterator.remove();
}