Page History

According to the Java API documentation \[[API 06|AA. Java References#API 06]\] for {{Iterator.remove()}} method:

...

It is possible that in a multithreaded program, one thread iterates over a collection while another concurrently modifies the Collection. This can lead to result in unspecified behavior. Many implementations throw a ConcurrentModificationException when such a condition is detected.

According to the Java API documentation \[[API 06|AA. Java References#API 06]\] for {{ConcurrentModificationException}}:

... 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.

Note that fail-fast behavior cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast operations throw ConcurrentModificationException on a best-effort basis. Consequently, it would be wrong to write a program that depended on this exception for its correctness: ConcurrentModificationException should be used only to detect bugs.

...

// ...
iter.remove();
// ...

Exceptions

EX1: The Iterator.remove() method can be used to modify the underlying collection when an iteration is in progress. This is also shown in the compliant solution.

Risk Assessment

Modifying a Collection while iterating over it can lead to nondeterministic behavior unless the Iterator.remove() method is used.

...