Question

I was reading the official Oracle documentation about Concurrency in Java and I was wondering what could be the difference between a Collection returned by

public static <T> Collection<T> synchronizedCollection(Collection<T> c);

and using for example a

ConcurrentHashMap. I’m assuming that I use synchronizedCollection(Collection<T> c) on a HashMap. I know that in general a synchronized collection is essentially just a decorator for my HashMap so it is obvious that a ConcurrentHashMap has something different in its internals. Do you have some information about those implementation details?

Edit: I realized that the source code is publicly available:
ConcurrentHashMap.java

Answer 1

I would read the source of ConcurrentHashMap as it is rather complicated in the detail. In short it has

• Multiple partitions which can be locked independently. (16 by default)
• Using concurrent Locks operations for thread safety instead of synchronized.
• Has thread safe Iterators. synchronizedCollection’s iterators are not thread safe.
• Does not expose the internal locks. synchronizedCollection does.