Question

ConcurrentHashMap is fully interoperable with Hashtable in programs that rely on its thread safety but not on its synchronization details.

I am confused with Thread Safety vs Synchronization details, can any one tell me an example?

Answer 1

Hashtable has every method synchronized, and it’s a publicly available information. For example, you can inherit from Hashtable, add more synchronized methods—and know you’re doing mostly OK as you use the same synchronisation mechanism.

Moreover, your code can use synchronized (myHashTable) block, effectively ensuring no myHashTable methods are called from other threads while you’re in this block.

That’s all “synchronization details” which are available to you and which you are free (while discouraged) to use.

Not so with ConcurrentHashMap: it’s as thread safe (and even more so in some sense, see e.g. the answer by Peter Lawrey) as Hashtable, but you’re not told how this thread safety is achieved. As a result, you cannot abuse or extend it as you like: you’re supposed to use it as it is.