Fast-fail - Exception only happens when adding an Element not when removing

Question

Fast-Fail : meaning that if they detect that the collection has changed since iteration began, they throw the unchecked ConcurrentModificationException.

I have written a test example to demonsterate this:

    String hi = "Hi";
    list.add(hi);
    list.add("Buy");
    System.out.println("list before: " + list);
    for (Iterator<String> iterator = list.iterator(); iterator.hasNext();) {
        String string = iterator.next();
        list.add("Good");
    }

the output is:

list before: [Hi, Buy]
Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:859)
    at java.util.ArrayList$Itr.next(ArrayList.java:831)
    at thread.CollectionTest.main(CollectionTest.java:19)

which is expected. However, when an element is removed, the exception is not thrown:

    List<String> list = new ArrayList<>();

    String hi = "Hi";
    list.add(hi);
    list.add("Buy");
    System.out.println("list before: " + list);
    for (Iterator<String> iterator = list.iterator(); iterator.hasNext();) {
        String string = iterator.next();
        list.remove(hi);
    }

Output:

list before: [Hi, Buy]
list after: [Buy]

Why is that? both cases the list is modified.

Answer 1

The point is that it is not hasNext() that checks for modification, but next(). In your "remove" scenario, you suppress the next call that would throw because there is no next element.

If you had 3 elements in the beginning, removing one would result in hasNext to be 'true'. Then the following next will throw the expected exception.

The JavaDoc points out that the "fail-fast" functionality works based on "best effort" and that it shall be used merely for detecting bugs rather than really depending on it for the program to behave correctly. Obviously because of side-effects like this one.