ModCount in map and list

Question

While Debugging the collections in eclispse I just Inspect that there is thing called modCount for example if we debug list we will see while inspecting in debugging what this modCount represents..!!please advise

Answer 1

See the javadoc

The number of times this list has been structurally modified. Structural modifications are those that change the size of the list, or otherwise perturb it in such a fashion that iterations in progress may yield incorrect results.

This field is used by the iterator and list iterator implementation returned by the iterator and listIterator methods. If the value of this field changes unexpectedly, the iterator (or list iterator) will throw a ConcurrentModificationException in response to the next, remove, previous, set or add operations. This provides fail-fast behavior, rather than non-deterministic behavior in the face of concurrent modification during iteration.

Use of this field by subclasses is optional. If a subclass wishes to provide fail-fast iterators (and list iterators), then it merely has to increment this field in its add(int, E) and remove(int) methods (and any other methods that it overrides that result in structural modifications to the list). A single call to add(int, E) or remove(int) must add no more than one to this field, or the iterators (and list iterators) will throw bogus ConcurrentModificationExceptions. If an implementation does not wish to provide fail-fast iterators, this field may be ignored.

Answer 2

It's a counter used to detect modifications to the collection when iterating the collection: iterators are fail fast, and throw an exception if the collection has been modified during the iteration. modCount is used to track the modifications.

FYI, the sources of the standard classes are part of the JDK, and you may read them to understand how the standard classes work.