Question

ConcurrentHashMap was introduced in 1.5 as a part java java.util.concurrent package. Before that the only way to have a threadsafe map was to use HashTable or Collections.synchronizedMap(Map).

For all the practical purpose (multithread environment),ConcurrentHashMap is sufficient to address the needs except one case wherein a thread needs a uniform view of the map.

My question is, apart from having a Uniform View of the map, are there any other scenarios wherein ConcurrentHashMap is not an option ?

Answer 1

This is a stretch but I will give it as a use case.

If you needed a thread-safe Map implementation which you can do some extra compound operation on which isn’t available via ConcurrentMap. Let’s say you want to ensure two other objects don’t exist before adding a third.

Hashtable t = new Hashtable();

synchronized(t){
   if(!t.contains(object1) && !t.contains(object2)){
      t.put(object3,object3);
   }
}

Again this is a stretch, but you would not be able to achieve this with a CHM while ensuring atomicity and thread-safety. Because all operations of a Hashtable and its synchronizedMap counter part synchronize on the instance of the Map this ensures thread-safety.

At the end of the day I would seldom, if ever, use a synchronizedMap/Hashtable and I suggest you should do the same.