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ConcurrentHashMap uses multiple buckets to store data. This avoids read locks and greatly improves performance over a HashTable. Both are thread safe, but there are obvious performance wins with ConcurrentHashMap.
When you read from a ConcurrentHashMap using get(), there are no locks, contrary to the HashTable for which all operations are simply synchronized. HashTable was released in old versions of Java whereas ConcurrentHashMap is a java 5+ thing.
HashMap is the best thing to use in a single threaded application.
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License : cc by-sa 3.0
http://stackoverflow.com/questions/12646404/concurrenthashmap-and-hashtable-in-java
