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This technique is also called client-side locking \[[Goetz 06|AA. Java References#Goetz 06]\], because the class holds a lock on an object that presumably might be accessible to other classes. Client-side locking should not be used when the underlying class does not commit to its locking strategy. For more information see [CON31-J. Avoid client-side locking when using classes that do not commit to their locking strategy]. 

Although expensive, {{CopyOnWriteArrayList}} and {{CopyOnWriteArraySet}} classes are sometimes used to create copies of the core {{Collection}} so that iterators do not fail with a runtime exception when some data in the {{Collection}} is modified. However, any updates to the {{Collection}} are not immediately visible to other threads. Consequently, their use is limited to boosting performance in code where the writes are fewer (or non-existent) as compared to the reads  \[[JavaThreads 04|AA. Java References#JavaThreads 04]\]. In all other cases they must be avoided (see [MSC13-J. Do not modify the underlying collection when an iteration is in progress]).