Java Modcount (ArrayList)

Question

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

Answer 1

protected transient int modCount = 0;
is the property declared at public abstract class AbstractList,
to identify total number of structural modification made in this collection.

Means if there is a add/remove there will be an increment in this counter for both operation. Hence this counter always get incremented for any modification. So not useful for size computation.

This will be useful to throw ConcurrentModificationException.
ConcurrentModificationException will be thrown while iterating the collection by one thread and there is a modification in the collection by another thread. This is achieved like whenever iterator object is created modCount will be set into expectedCount, and each iterator navigation expectedCount will be compared with modCount to throw ConcurrentModificationException when there is a change.

private class Itr implements Iterator {
    ...
    ...
    /**
     * The modCount value that the iterator believes that the backing
     * List should have.  If this expectation is violated, the iterator
     * has detected concurrent modification.
     */
    int expectedModCount = modCount;

    public E next() {
        checkForComodification();
    ...
    ...
    }

    final void checkForComodification() {
        if (modCount != expectedModCount)
            throw new ConcurrentModificationException();
    }
    ...
    ...

}

size() api won't suits here; since if there is two operation (add and remove) happened before next() called still size will show the same value; hence not able to detect the modification happened on this collection using size() api while iteration. Hence we need modification_increment_counter which is modCount.