Question

Suppose a map contains integer keys, and a list of strings as its value. Then, I CAN’T do this :

for (Map.Entry<Integer, List<String>> entry : map.entrySet()){
    for (String string : entry.getValue()){
        if (string.startsWith("a")){
           entry.getValue().remove(string);
        }
    }
}

It throws ConcurrentModificationException. But if I do the following :

for (Map.Entry<Integer, List<String>> entry : map.entrySet()){
    entry.setValue(new ArrayList<String>());
}

This works perfectly. Aren’t we modifying the underlying map even now ?

Answer 1

The problem has nothing to do with the Map, only the way you’re using the value list. The following will fail with any ArrayList:

for (String string : list){
    if (string.startsWith("a")){
       list.remove(string);
    }
}

The reason for this is discussed in the Javadoc of ArrayList:

The iterators returned by this class’s iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator’s own remove or add methods, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

(To put it another way: if you remove the element from the list, the iterator might not be pointing to the right index in the underlying array anymore. So instead of allowing you to use a potentially corrupted iterator, it throws a ConcurrentModificationException as a courtesy to let you know that you need to redesign your program.)

A simple fix is to use

Iterator<String> itr = entry.getValue().iterator();
while (itr.hasNext()) {
  if (itr.next().startsWith("a")) {
    itr.remove();
  }
}