I have created some of the examples of the JCF (Java Collections Framework).One thing that had confused me is that
When we are iterating a Colection say ArrayList using the iterator and during the iteration if we try to modify the collection by by calling the ArrayList's add method then it will give me the exception
But i did the iterations and try to add the new element by using the listiterator's add method then it is not giving me such exception.
My Question is why is such a different behaviour is there? Why java allowed to modify the collection using the iterator's own methods when iterating and why not through the arraylist method ?
After all both ways we are going to add the element to the ArrayList .
Please any help would be appreciated.
Note that it depends on the implementation of the iterator and the collection class whether you get a ConcurrentModificationException or not - it's not guaranteed that you get that exception. The API documentation of ConcurrentModificationException explains it:
This exception may be thrown by methods that have detected concurrent modification of an object when such modification is not permissible.
For example, it is not generally permissible for one thread to modify a Collection while another thread is iterating over it. In general, the results of the iteration are undefined under these circumstances. Some Iterator implementations (including those of all the general purpose collection implementations provided by the JRE) may choose to throw this exception if this behavior is detected. Iterators that do this are known as fail-fast iterators, as they fail quickly and cleanly, rather that risking arbitrary, non-deterministic behavior at an undetermined time in the future.
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