Question

I have a map. Lets say:

Map<String, Object> map = new HashMap<String, Object>();

Multiple threads are accessing this map, however each thread accesses only its own entries in the map. This means that if thread T1 inserts object A into the map, it is guaranteed that no other thread will access object A. Finally thread T1 will also remove object A.

It is guaranteed as well that no thread will iterate over the map.

Does this map need to be synchronized? If yes how would you synchronize it? (ConcurrentHashMap, Collections.synchronizedMap() or synchronized block)

Answer 1

Yes, you would need synchronization, or a concurrent map. Just think about the size of the map: two threads could add an element in parallel, and both increment the size. If you don’t synchronize the map, you could have a race condition and it would result in an incorrect size. There are many other things that could go wrong.

But you could also use a different map for each thread, couldn’t you?

A ConcurrentHashMap is typically faster that a synchronized HashMap. But the choice depends on your requirements.