HashMap is not synchronized, which means you cannot use it on multi-threaded Java program without external synchronization. In another word, if you share one instance of HashMap between multiple thread, each is either adding, removing or updating entries then it’s possible that HashMap loss its structure and not behave as expected. If you have read my earlier article about HashMap, you know that during re-sizing its possible that HashMap exposed to multiple thread, may end up in infinite loop. In order to avoid this, usually one HashMap instance is used by one thread, sharing of HashMap instance is not allowed, but if you have to share HashMap and there is no option to avoid that, you can always synchronize HashMap in Java. Ofcourse this will affect the performance and probably reduce the speed of HashMap as synchronized method is always slower than non-synchronized one. In this tutorial, we will learn how we can synchornize HashMap in Java.
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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 :