How to use CopyOnWriteArraySet in Java with Example

CopyOnWriteArraySet is little brother of CopyOnWriteArrayList class. These are special purpose collection classes which was added on JDK 1.5, along with their most popular cousin ConcurrentHashMap. They are part of concurrent collection framework and reside in java.util.concurrent package. CopyOnWriteArraySet is best suited as read-only collection whose size is small enough to copy if some mutative operation happens, for example you can use CopyOnWriteArraySet to store object at start-up of application and let multiple application thread access them during application life time. If an new condition or object comes up during that time, it can also be added into this Set, with incurring cost of creating a new array. One of the most important thing to know about CopyOnWriteArraySet is that it is backed by CopyOnWriteArrayList, which means it also share all basic properties of CopyOnWriteArrayList. Another important thing to remember is that Iterators of this collection class doesn't support remove() operation, trying to remove an element while iterating will result in UnSupportedOperationException. This is done to ensure speed during traversal, traversing this set implementation using Iterator is fast and cannot encounter interference from other threads. Iterators actually rely on unchanging snapshots of the array at the time the iterators were constructed. In short, use CopyOnWriteArraySet if set is small enough to copy on add, set or remove, and main purpose is to read data with occasional updates. Also if you want to remove elements during iteration, don't use this Set implementation because its iterator doesn't support remove(), and throws java.lang.UnsupportedOperationException as shown below :