java - magic number of ConcurrentModificationException

Question

I am testing ConcurrentModificationException of collections using code bellow:

public static void main(String[] args) {
    ArrayList<String> list = new ArrayList<String>();
    list.add("a");
    list.add("b");
    list.add("c");

    for (String s : list) {
     // if (s.equals("a")) { // ConcurrentModificationException!
        if (s.equals("b")) { // -->> Magic number, NO Exception, Why? 
     // if (s.equals("c")) { // ConcurrentModificationException!
            list.remove(s);
        }
    }
    System.out.println(list);
}

I don't understand why remove "b" is OK, but others NG?

Answer 1

The first thing to know is that (as stated in the JLS) the following enhanced for loop:

for (String s : list) {
    // Do something with s
}

Is equivalent to:

for (Iterator<String> it = list.iterator(); it.hasNext();) {
    String s = it.next();
    // Do something with s
}

If you have a look at the implementation of the iterator in AbstractList, you will see that:

• hasNext() doesn't check for concurrent modification, and just check if we are at the end of the list, using its size:

  public boolean hasNext() {
          return cursor != size();
  }
  

• the first thing done by next() is to call checkForComodification() to see if the list was modified while we are iterating:

  public E next() {
          checkForComodification();
      try {
      E next = get(cursor);
      lastRet = cursor++;
      return next;
      } catch (IndexOutOfBoundsException e) {
      checkForComodification();
      throw new NoSuchElementException();
      }
  }
  
  final void checkForComodification() {
      if (modCount != expectedModCount)
      throw new ConcurrentModificationException();
  }
  

So, when you are iterating and removing the penultimate element of your list, the next instruction will be a call to hasNext(), which will return false, because removing one element caused the size of the list to be one less, and your iteration will stop without calling next() and throwing an Exception.

By the way, all of this is just an implementation detail, you should not rely on it as it can change, and use it.remove() to remove an element from the list while you are iterating.