Question

When debugging Java code using Eclipse, for collection variables, I saw the modcount member. What does it mean?

Answer 1

Many of Java’s collections produce iterators that are “fail-fast”, which means that if the collection is changed after an iterator is created, the iterator will be invalidated and throw a ConcurrentModificationException as soon as it can. (As compared to failing later or returning invalid data.)

In order to support this functionality, the collection has to keep track of whether it has been modified. Each time the collection is changed, it increments modcount. When the collection produces an iterator, the iterator stores the value of modcount from when it was created. Then whenever you try to use the iterator, it checks to see if its saved modcount is different from the parent collection’s current modcount; if it is, the iterator fails with a ConcurrentModificationException.

(An exception to this rule is that modifications to the collection made through the iterator itself (like the iterator’s remove method) do not invalidate the iterator.)