collections - Java Modcount (ArrayList)

Question

In Eclipse, I see that ArrayList objects have a modCount field. What is its purpose? (number of modifications?)

Answer 1

It allows the internals of the list to know if there has been a structural modification made that might cause the current operation to give incorrect results.

If you've ever gotten ConcurrentModificationException due to modifying a list (say, removing an item) while iterating it, its internal modCount was what tipped off the iterator.

The AbstractList docs give a good detailed description.