The ConcurrentMap represents a Map which is capable of handling concurrent access (puts and gets) to it.
The ConcurrentMap has a few extra atomic methods in addition to the methods it inherits from its superinterface, java.util.Map.
ConcurrentHashMap
The ConcurrentHashMap is very similar to the java.util.HashTable class, except that ConcurrentHashMap offers better concurrency than HashTable does. ConcurrentHashMap does not lock the Map while you are reading from it. Additionally, ConcurrentHashMap does not lock the entire Map when writing to it. It only locks the part of the Map that is being written to, internally.
ConcurrentMap concurrentMap = new ConcurrentHashMap();
concurrentMap.put("key", "value");
Object value = concurrentMap.get("key");