Difference between CopyOnWriteArrayList and synchronizedList

Question

As per my understanding concurrent collection classes preferred over synchronized collections because the concurrent collection classes don\'t take a lock on the complete co

Answer 1

For write (add) operation, CopyOnWriteArrayList uses ReentrantLock and creates a backup copy of the data and the underlying volatile array reference is only updated via setArray(Any read operation on the list during before setArray will return the old data before add).Moreover, CopyOnWriteArrayList provides snapshot fail-safe iterator and doesn't throw ConcurrentModifficationException on write/ add.

But when I checked add method of CopyOnWriteArrayList.class, we are acquiring lock on complete collection object. Then how come CopyOnWriteArrayList is better than synchronizedList. The only difference I see in add method of CopyOnWriteArrayList is we are creating copy of that array each time add method get called.

1. No, the lock is not on the entire Collection object. As stated above it is a ReentrantLock and it is different from the intrinsic object lock.
2. The add method will always create a copy of the existing array and do the modification on the copy and then finally update the volatile reference of the array to point to this new array. And that's why we have the name "CopyOnWriteArrayList" - makes copy when you write into it.. This also avoids the ConcurrentModificationException