Using ConcurrentHashMap

Question

I'm new to threading in Java and I need to access data structure from few active threads. I've heard that java.util.concurrent.ConcurrentHashMap is threading-friendly. Do I need to use synchronized(map){} while accessing ConcurrentHashMap or it will handle locks itself?

Answer 1

It handles the locks itself, and in fact you have no access to them (there is no other option)

You can use synchronized in special cases for writes, but it is very rare that you should need to do this. e.g. if you need to implement your own putIfAbsent because the cost of creating an object is high.

Using syncrhonized for reads would defeat the purpose of using the concurrent collection.

Answer 2

ConcurrentHashMap is suited only to the cases where you don't need any more atomicity than provided out-of-the-box. If for example you need to get a value, do something with it, and then set a new value, all in an atomic operation, this cannot be achieved without external locking.

In all such cases nothing can replace explicit locks in your code and it is nothing but waste to use this implementation instead of the basic HashMap.